Mineralogical Inventories of all 62 HED meteorite Falls
Last Updated: 2nd Jul 2016By Lon Clay Hill
EXPANDED TITLE:
Mineralogical Inventories of the Mineral Groups and Series, Minerals, mineral 'varieties', and other indispensable terms for all 62 Witnessed Falls of the Howardites, Eucrites and Diogenites [HED meteorites] as currently posted with MinDat ('mindat.org).
ABSTRACT
A combined essay and mineralogical inventory of the 62 witnessed HED meteorites falls. Currently 61 mineralogical important items ('Mineralites') are listed at 'mindat.org' Location Sites for one or more HED meteorite falls. The HED meteorites (Howardites, Eucrites, Diogenites) are achondritic or differentiated meteorites whose original homeworlds were large asteroidal bodies. The number of hosted mineralogical items for a single HED meteorite ranges from 5 to 29 (median 13); the number of hosted IMA-defined minerals ranges from 1 to 21 (median 6.5). In addition to presenting linked and annotated lists of the 62 HED witnessed falls and the 61 listed mineralites, the essay contains two pedagogical sections (Sections I & II, Introduction and Overview) and two more technical sections (Sections III & IV, critical evaluations of both the listed mineral Groups and the 45 IMA-defined minerals, respectively. Critical discussion of expected and unexpected differences in the mineralogical assemblages of Eucrites, Howardites, and Diogenites is included. A Bibliography with notes is also included. Present research on HED meteorites is governed by the dawning realization that most — but not all — HED meteorites are indeed fragments of the asteroid 4 Vesta. This essay-and-inventory does not attempt to resolve the issues involved in assigning individual HED meteorites to an emerging 'Main Group' of HED meteorites whose Original Parent Body (OPB) was Vesta or in assigning 'anomalous' HED meteorites to other (presently unknown) homeworlds. The essay is also limited in that only brief attention here is given to important HED 'Finds' — HED meteorites which were recovered at some unknown time after their unobserved fall. We have chosen to concentrate on the falls because they provide 'fresh' extraterrestrial material relatively free of the weathering effects which beset and complicate our ability to understand those exotic meteoritic minerals (e. g., sulfides, sulfates, phosphides,unoxidized metal…) which are most easily altered by crustal oxygen and water. Within these stated limitations, this essay attempts to provide publicly available information indicating where one can investigate the mineralogical evidence which constitutes a critical component of this continuing inquiry.
INDISPENSABLE TERMS:
Achondrite, Breccia, Clinopyroxene, Diogenite, Fe-Ni metal, Eucrite, Falls, Finds, HED meteorite, Homeworld, Howardite, Kamacite, Meteorite, Meteoritic Iron, Meteoroid, Mineral, Mineral Group, Mineralite, Mineralite Number, Orthopyroxene, Plagioclase, Stony meteorite, Taenite, Vesta. Neologisms are occasionally employed.
Mostly for reference purposes. Most terms which are not directly linked to Mindat mineralogical entries are defined in the text, as needed, and again in the Glossary.
Nota bene: The author often uses idiosyncratic spellings in words with unspoken Germanic gutturals(e.g., 'gh', 'ght') when no ambiguity is involved
Section I: Introduction
The Howardites, Eucrites, and Diogenites are three closely related classes of achondrite meteorites — collectively known as the HED meteorites. They are normally loosely conglomerated stony meteorites formed on moderately large asteroidal worlds, mixed together by (usually) multiple impact events, and eventually ejected into space by sufficiently violent impacts. Since their fundamental components — their primary minerals — were formed on ancient asteroids over 4.5 billion years ago, collisions, chance, circumstance, and gravitational perturbations have brought a small fraction of the ejecta into the inner solar system where they presently constitute approximately 5% of all witnessed and recovered meteorite falls. Their falls are often dramatic events — accompanied by spectacular fireballs, sonic booms, and/or whizzing sounds — but the recovered meteorites are often unspectacular, usually consisting of a few fragile stones and, perhaps, numerous small fragments. Their loud, brite arrivals and their thin black fusion crusts are often the only obvious sign to the uninitiated that these stones are actually skyrocks — visitors from distant worlds somewhere beyond the earth. Truth to tell, once we can identify a fallen stone as an HED meteorite, we now can usually say something more about them. First, like all well studied meteorites, they are bona fide members of the solar system. Secondly, most or all of them are derived from the 'Asteroid belt' which lies between the orbits of Mars and Jupiter. Third, most — but not all — of these rocks were once part of the asteroid, 4 Vesta, the britest asteroid and the closest of the larger asteroids. But, these, are conclusions — let us turn to the rocks themselves and see how these conclusions have emerged from considerations of their constituents and, most especially, their mineralogy.
Stipulations:
We add some fundamental stipulations to place these skyrocks within the framework of meteoritical research.
One, HED meteorites are called 'achondrites' because — unlike most meteorites — they do not contain small and minute silicate grains called 'chondrules' which are almost never found in earth rocks. However, that very label means they are in many ways much more like earth rocks than most meteorites, especially mineralogically.
Two. Of course, Eucrites, Howardites, and Diogenites are not simply earth rocks — they are extra-terrestrial meteorites. Our missions to the moon have helped us to understand that lunar rocks and HED meteorites are usually 'breccia' (or, at least, 'brecciated'). They are fragmental aggregates altered by countless small and large meteorite impacts. In addition, both HED meteorites and lunar rocks originated on airless and water-free worlds where ferric iron (Fe+++) and hydrated phyllosilicates are virtually non-existent.
Three. In untangling the mix of familiar and exotic minerals present in extraterrestrial materials, it has been gradually discovered that after the formation of a world and subsequent core-formation a number of isotopic and chemical ratios are exceedingly resistant to any further substantial changes. These ratios — especially, O-isotope ratios, Fe/Mn, and Rare Earth Element (REE) ratios — have helped to establish that certain meteorites are from the Moon and others are from Mars. In similar fashion, with additional aid from astronomical observations, it is now very clear that most — but not all — HED meteorites are fragments of the asteroid 4 Vesta. These last developments have also made geochemical considerations unusually important in almost all types of meteoritic research, including mineralogical reports. As we consider our meteorite mineralogical inventories we will find that the character of a 'mineral inventory' is influenced by the underlying queries which have motivated the research.
PRESENTATION ORDER: HED CLASSES; EUCRITES; DIOGENITES; HOWARDITES; VESTA
Prelude: The Skyrocks labelled 'Eucrites,' 'Howardites' and 'Diogenites' — What's in and not in a classificatory name.
Before we even begin our inquiry into the three classes of meteorites labelled 'Eucrites', 'Diogenites', and 'Howardites' and 'Diogenites' it would be best to step back briefly and speak about these particular meteorites simply as stones. For several decades it was customary to speak of the Eucrites and Howardites as basaltic meteorites because, like terrestrial basalts, they were compositionally and mineralogically dominated by various aggregates and intergrowth of pyroxene and plagioclase. The Diogenites, especially the witnessed falls, are compositionally dominated by pyroxene alone and are thus properly considered 'pyroxenites.' After the Apollo missions, it was discovered that the Eucrites and Howardites, in particular, were even closer in many ways to lunar rocks brought back from the moon. The constituent minerals of the Eucrites and Howardites, like those in the moon rocks, were formed in airless, anhydrous lavas where both water and oxygen were — by terrestrial standards — extremely scarce. And after crystallization on or near the surface of airless homeworlds, most moonrocks, Eucrites, and Howardites have been subject to meteorite bombardment and fracturing. All Eucrites, Howardites, and Diogenites are thus, to some extent, breccias. To some extent, observers simply take this into account and do not highlight this brecciation unless there are unusual lithic fragments (clasts), pervasive veining, glass and/or others noteworthy features due to impact shock. Brecciation features — like those sometimes beautiful fusion crusts formed during a meteorite's brief passage thru the atmosphere — are there, but they are (almost always) not the main mineralogical feature. Thus, when we speak about the HED meteorites — meteorites which are either Howardites, Eucrites, or Diogenites — we are speaking about meteorites, normally partially or highly fractured, whose primary minerals and common accessory minerals are, almost always, utterly familiar to terrestrial geologists.
During the 1970's and continuing into following decades, however, several developments occurred. One, both chemistry and mineralogy revealed that:
(1) Eucrites were rocks formed near or moderately near the surface [a few kilometers depth at most] of an original homeworld(s).
(2) the pyroxene-rich Diogenites were plutonic rocks formed well below the surface (~ 10-25 [?] kilometers) surface of an original homeworld(s).
(3) Howardites were mixtures of Eucritic and Diogenitic material seemingly accumulated on the surface of, perhaps, a single homeworld.
Simultaneously, it was also slowly beginning to dawn upon meteoriticists, that there were a number of other meteorites — also rich in basalts and/or other familiar igneous silicates — which were almost certainly formed on other worlds than those one or more worlds which produced the HED meteorites. Most notable, a few basaltic meteorites turned out to be lunar meteorites while a few others turned out to be Martian rocks. Crucial in these developments were the recognition that certain isotopic and elemental ratios could aid in recognizing the determination of a specific homeworld or, 'Original Parent body' (OPB). Not quite as crucial to our discussion but lurking in the background, it is slowly becoming a near certainty that the OPB for most — but not all — HED meteorites is the nearest large asteroid 4 Vesta. (Vide infra!)
With these considerations in mind, then, we look at the mineralogies of the three HED meteorite groups thru the lens of the Meteoritical Community.
We start with the Eucrites because mineralogically they resemble familiar terrestrial basalts and, both mineralogically and texturally, they resemble the moderately familiar lunar basalts even more. Eucrites are compositionally dominated by pyroxenes and plagioclase [normally > 90 vol%]. Eucrites are named because their constituents are 'well discerned, readily distinguished' [Greek - 'eu', 'kritos']. The predominant moderate-sized pyroxene and plagioclase crystals are readily recognized with a hand lens and, except for their fusion crust, darker minerals which might obscure one's view are quite sparse. The Meteoritical Bulletin Database defines them as follows:
"Eucrites are the most abundant type of basaltic achondrite, linked by geochemical traits such as oxygen isotopic ratios and certain elemental ratios, of which Fe/Mn is the most widely cited. The main minerals in eucrites are Fe-rich pyroxene and Na-poor plagioclase. The eucrites are strongly linked with the diogenites and howardites; the three groups are collectively known as HED meteorites and may come from asteroid 4 Vesta."
We note here that the Fe-rich pyroxene is Fe-rich relative to most meteoritic pyroxenes. The Fe/Mg ratios are usually roughly between 2/1 and 1/2. The predominant pyroxene is usually either pigeonite or orthopyroxene. We also note that the plagioclase is often simply characterized as 'anorthitic plagioclase.' When referring to concentrations or averages it is common to find references to either 'anorthite' [An90—An100] or to 'bytownite' [An70—An90] or to both. The predominant pyroxene is usually either pigeonite or orthopyroxene. The pyroxene textures can be quite complex. The most obvious source of differences are rooted in contrasts in mineralogy and textures of quickly cooling rocks which formed from surface or near-surface lava and slowly cooling rocks which solidified hundreds or thousands of meters below the surface. A second consideration is that rocks can be remelted — at least partially — by powerful impacts which can heat as well as shock any nearby rocks and rock layers. In addition to the simple physics of combined heating, cooling, and shock events, there is an additional complication in regolith composition. Some of the brecciated meteorites which reach us had formed on a surface whose constituents were mostly quite local and compositionally similar. These meteorites are classified as 'monomict eucrites'. Other brecciated eucrites have significant compositional heterogeneities. These meteorites apparently were part of a surface which sampled ejecta from more distant regions with their concomitant geochemical and physical diversity. Eucrites with significant constituent diversity are classified as 'polymict eucrites'. The 'cumulate eucrites' apparently grew most slowly in moderately deep magma chambers and, when ejected, often escaped some of the drastic fracturing that befell most eucrites. It may even be the case that the cumulate eucrites were formed only a few kilometers or so above the pyroxene-rich region(s) that produced the diogenites.
Thirty one of the 35 Eucrite falls are thus further characterized or subdivided as follows:
18 monomict Eucrites (eu-mm), 9 polymict Eucrites (eu-pm); 4 Cumulate Eucrites (eu-cm).
Of the remaining four Eucrites, one is simply described as 'brecciated' and another as 'unbrecciated.' Two are simply described as 'Eucrites' without further characterization. There are a number of interesting Eucrite finds which we note only en passant. A few of them have MinDat location sites. One Eucrite (Camel Donga, a 1984 find from Australia) contains about 2% Fe-rich metal.
We now consider the Diogenites. As extraterrestrial pyroxene-rich stones they represent a rock type which is not quite as common as the basaltic Eucrites, but is still quite familiar to terrestrial geologists. All Diogenites falls and almost all Diogenite finds are compositionally dominated by orthopyroxene (85-99%). Being essentially monominerallic stones they are well-characterized as extraterrestrial plutonic pyroxenites.
Diogenites are named for Diogenes of Apollonia, a Greek philosopher (circa 424 BCE) who suggested that meteorites were from the heavens:
"With the visible stars revolve stones which are invisible, and for that reason nameless. They often fall on the ground and are extinguished, like the stone star that came down on fire at Aegospotami." [Following Wikipedia here.]
The Meteoritical Bulletin Database defines Diogenites as follows:
"Diogenites are an abundant type of achondrite, linked by geochemical traits such as oxygen isotopic ratios and certain elemental ratios, of which Fe/Mn is the most widely cited. The dominant mineral in diogenites is orthopyroxene. The diogenites are strongly linked with two other achondrite groups: eucrites and howardites; the three groups are collectively known as HED meteorites and may come from asteroid 4 Vesta."
Most Diogenite falls (8 of 11) are simply characterized as 'Diogenites'. 3 are further characterized as polymict Diogenites (diog-pm).
The pyroxene is predominantly orthopyroxene as the original rocks (or rock formations) solidified slowly while ~10 or more kilometers below the surface. Other features are consistent with original formation in a deep magma chamber as we would expect in a plutonic rock. On the basis of the record here, the Ca-rich clinopyroxenes were perhaps a little more likely to equilibrate as diopside than the augite which is found more frequently in Eucrites. (Augite lattices are a little more tolerant of 'impurities' than diopside lattices.) Chromite and ilmenite are minor, but ubiquitous components of both terrestrial plutonic rocks and the Diogenites. However, the nature of the small amounts of iron-rich metal are puzzling (kamacite, some taenite, and some very Ni-poor Iron are found). Whether these instances of unoxidized metal are residues of incomplete core-mantle separation of the original homeworld(s), exotics due to meteorite impacts, or by-products of metamorphic reduction processes with the original homeworld(s) is not at all clear at this time.
While the impacts that lifted the diogenites from their original plutonic sites deep in the original homeworld onto the surface or directly into space would have been relatively powerful, components that are sufficiently distant are sometimes hoisted into space with only moderated fracturing and disruption. Furthermore, being blasted into space with after as a single impact or two may give a solitary plutonic rock a better chance to hide from impact damage than a surface rock which is near the epicenter of multiple large and small impacts. In any case, only a relatively small number of Diogenite falls are classified as 'polymict Diogenites'. This is true of Diogenite finds as well. While it is outside the scope of this study, it should be mentioned that olivine - a common terrestrial plutonic mineral — is a relatively minor component of the Diogenites falls. However, about 6% (10 of 159) of all diogenites - all finds are classified as olivine-rich diogenites. [At least one olivine-rich diogenite, Miller Range 03443, has a Mindat location site.] It may be a merely stochastic feature of the present epoch that all 11 diogenite falls collected in the past two centuries were pyroxenites. A few millennia or hundred megayears from now the fraction of olivine-rich meteorites from the Diogenite homeworld(s) may be significantly different.
While Howardites are the initial component of the HED acronym, there are two good reasons for treating them after discussing the Eucrites and Diogenites. First, Howardites have been created by the commingling of eucritic & diogenitic material by impact — a process which in itself creates additional complexities. Secondly, there are additional components in Howardites which are very rare in the other two types. Compositionally odd eucritic and diogenitic clasts appear to be from different lithologies within the HED homeworld(s) than those represented by the Eucrites and Diogenites in our collection. Furthermore, Howardites are more likely to harbor exotic clasts which are residues of meteoroidal impacts from other non-HED meteorites (e.g., carbonaceous and ordinary chondrites).
Howardites are named after Edward Charles Howard (1774-1816), a British Chemist who helped establish the extra-terrestrial nature of several Fe-Ni metal containing meteorites :
The Meteoritical Bulletin Database defines Howardites as follows:
"Howardites are an abundant group of polymict-breccia achondrites that appear to represent mixtures of eucrites + diogenites (these three linked groups are collectively known as HED meteorites and may come from asteroid 4 Vesta). The main minerals in howardites are pyroxene (largely orthopyroxene) and Na-poor plagioclase. A minority of howardites are rich in solar-wind noble gases and thus inferred to be regolith breccias."
All 16 Howardite falls are simply characterized as 'Howardites'. In actual fact, the working definition for a howardite is that it contains at least 10% Eucritic material and at least 10% Diogenitic material. Whether the mix is 10-90 or 90-10 Eucrite: Diogenite does not affect the definition, but obviously it would affect the mineralogical and geochemical ratios. More difficult are borderline cases with 5-10% exotic components (CM2 material, in particular, may be present at the several % level). As all Diogenites are by definition 2-or-more component breccias, sampling problems can also result in different classifications by different observers.
To a first order one can understand Howardites as mechanical mixtures of Eucrites rich in pigeonite and plagioclase and Diogenites rich in orthopyroxene. An indeed, one finds individual clasts in Howardites that still retain their Eucritic or Diogenitic identity. However, there are several features that require some additional explanations. For one, the range of pyroxene compositions in individual Howardites exceeds the total range observed in both Eucrites and Diogenites. Thus the Howardite Kapoeta has some pyroxene components that are more Mg-rich than those observed in Eucrites and Howardites. (The Howardite find, Monticello, also has some unusually Mg-rich pyroxene). It appears that the Howardites have been enriched by fragments from a wider range of surface area and, perhaps, from other deep regions than we observe in the Eucrites and Diogenites that are in our collections. Secondly, after material from diverse places was brought together on or near the surface of any original homeworld(s) significant fractions have been partially mobilized and equilibrated by additional heating. The heating may have been due of the original radioactive sources which melted significant fractions of many early solar system worlds. Or, the heating may have been due to the material being trapped under hot debris from various impacts. Finally, a significant fraction of exotic material from Carbonaceous Chondrites and other meteorite types has been found in a number of Howardites. In several meteorites it appears that at least some of the material has been involved in more than one scattering impact and in more than one impact heating event. In some cases these events have created inhomogeneities and, in other instances, equilibrating metamorphosis. These processes have created some especially complex textures in those pyroxenes that have alternately cooled and slowly equilibrated and then been shocked-heated and subsequently reequilibrated at varying rates. For example, some orthopyroxenes have two or more generations of thin augite lamellae which has exsolved out of cooling orthopyroxene. Another issue that this author will not attempt to explain is that more or less ordinary meteoritic Fe-Ni metal (kamacite) and some very meteoritically unusual Ni-poor Fe metal is found in very small amounts here and there in some Howardites.
The asteroid 4 Vesta was discovered on 29 March 1807 by Heinrich Wilhelm Olbers. Although it is the britest asteroid, it was the 4th to be discovered - hence its official designation as 4 Vesta. Occasionally barely visible to the naked eye it is the third largest astroid (diameter ~550 km). It is the britest asteroid both because it is the closest of the larger asteroids and its surface is considerably more reflective than most asteroids. And, it is also very unusual because its spectra reveals the prominent presence of ferrous and calcic silicates which are rarely observed in the vast majority of asteroidal spectra. Over four decades ago it was discovered that there were especially strong similarities between the spectra of Vesta and the reflection spectra of those meteorites which we today call Eucrites and Howardites. Approximately two decades ago it was discovered that there is a large set of much smaller asteroids (called 'Vestoids') whose spectral properties are remarkably similar to the those of Vesta and HED meteorites. The orbital properties of the Vestoids suggest that they are a family of Vesta impact ejectites and that a significant number of them are or are well placed to be perturbed into earth-crossing orbits (i.e., potential meteorite sources). During the past three decades parallel studies with oxygen isotope ratios in meteorites (and moon rocks) have proved to be powerful discriminants in narrowing possible parent bodies for a number of meteorite groups. The essential underpinning for such methods is the realization that once a planetary body is formed and roughly equilibrated thru worldwide magmatic differentiation into a core and mantle, oxygen isotope ratios are tightly constrained as changes are mostly mass dependent. Changes in O-16/O-17 ratios are roughly half any changes in O-16/O-18 ratios and vice versa. These studies have revealed — in particular — that lunar rocks and meteorites, Martian meteorites, and HED meteorites have oxygen isotope ratios that separate them from each other and from other meteorites. A number of textural, geochemical, and radiometric measures accompany these oxygen isotope groupings. Finally, the recent DAWN spacecraft mission to Vesta spent several months mapping the surface of Vesta and revealed a surface covered with properties closely matching those of Howardite meteorites, especially, with other regions more closely resembling Eucrites and Howardites. The surface even contains a small admixture of Carbonaceous Chondrite material — material presumably derived from the asteroids responsible for many of the impact features of the current Vesta surface and the brecciated nature of most HED meteorites. There is also a vary large impact crater, Rheasilvia, created somewhat over a billion years ago which may well be related to studies of Ar-40 and other radiometric isotope species which suggest that some Howardites experienced a significant energetic event at approximately the same time. One might even be tempted to consider the case closed.
Actually, the case is somewhat more complicated than suggested above. New very high resolution studies of Oxygen-isotope ratios have revealed that several HED meteorites — including three Eucrite falls — are almost certainly not derived from the same parent body as most HED meteorites. Thus, while most HED meteorites are indeed likely fragments of Vesta — establishing a definitive homeworld identification for specific HED meteorites will in most cases required a more detailed concordance of O-isotopes, Fe/Mn and REE ratios, and chronological information than that presently available. One emerging topic which may aid in establishing definitive homeworld connections is the possibility of determining 'orbital cohorts.' Different isotopic ratios of radioactive elements record different events within a rock or even within different minerals in that rock. Slowly, dating methods are confirming that some HED meteorites — or components of these meteorites — either (1) formed at approximately the same time and/or (2) experienced unusually strong impacts at approximately the same time and/or (3) were exposed to cosmic rays after subsequent impact(s) created a very small meteoroid in an earth-crossing orbit at approximately the same time. At the present time these groupings of shared history are too small in membership and are too imprecise for definitive conclusions — but that may change.
But this is overview, the main theme of our essay is to provide some perspective on the actual mineralogical entities which comprise the HED meteorites. In our conclusions we will offer some additional remarks on the HED-Vesta connection. The continuing attempts to determine which of the HED meteorites are fragments of Vesta and other asteroids is indeed one of the hot topics of the day - with quite interesting complexities. In our bibliography we also provide a number of references to the relevant astronomical and geochemical inquiries which form the backdrop for our more specifically meteoritic and mineralogical inquiry.
The 62 HED meteorites classified as witnessed falls by the Meteoritical Society before November 2014.
SECTION III (Expanded Title): PRIMARY AND ACCESSORY MINERALOGICAL GROUPS, SUBGROUPS, SERIES, etc. FOUND IN HED FALLS
Featuring: Pyroxenes, Feldspars, Silica Polymorphs, Olivine, Fe-rich Metal, Serpentine, Apatite
The Petrography and Mineralogy of HED meteorites are dominated by minerals within the Pyroxene and Feldspar Mineral Groups. The Pyroxenes include both Orthopyroxenes [Orthorhombic crystal system] and Clinopyroxenes [Monoclinic crystal system]. HED Feldspars are mostly Ca-rich Plagioclase. Both because meteoritic research must often be limited to small samples and because geochemical information is often given greater weight in meteoritical studies than in terrestrial mineralogically focused studies, reports by different research groups on the ingredients of the very same meteorite have different degrees of mineralogical specificity and are sometimes indeterminate or even seriously incomplete from a strictly mineralogical perspective. In particular, listed mineralogically important phases and assemblages of a given meteorite are often reported in terms of overlapping categories of Mineral Groups, Mineral Subgroups, Mineral Series, IMA defined Minerals, or Mineral Varieties. These categories constitute the primary labels utilized at 'mindat.org', but chemically defined terms and other more informal labels are also used. So we will begin our description by looking at the actual frequency of occurrence of some important mineralogical groups (and subgroups, etc.) found in HED meteorites. [All such groups are found in our List B which has definitional links to precise definitions of all such terms.]
In this section — unless otherwise indicated — we combine instances from overlapping categories to give actual sums based on all operative labels that apply. Later, in Section IV we examine the 45 IMA defined minerals found in the HED falls. In that section it is more sensible to adhere to the formal listings without combining items listed from overlapping categories.
A. HED pyroxenes. The Pyroxene Group; Orthopyroxene and Clinopyroxene subgroups; Pigeonite, Augite, Hypersthene, Ferroaugite, etc.
As most Diogenites are pyroxenites (~90% vol% pyroxene) and Howardites are mixtures of Diogenitic material intermingled with Eucritic material dominated by their pyroxene intergrowths, it is obvious that pyroxenes are the dominant mineral group of most HED meteorites. The only exception are those Eucrites and a few rare Howardites which contain slitely more plagioclase than pyroxene. Within the framework of this rather straightforward statement, however, a host of complexities abound. The dominant pyroxene, reported in all Diogenite falls and almost all Diogenite Falls, is mildly ferroan orthopyroxene (Fs ~26 mol%) — which is sometimes referred to as either 'hypersthene' or 'bronzite'. In Eucrites — often cooling quickly after their intrusion onto or near the surface — pigeonite is more common. Naturally enough, in Howardites the relative proportions of orthopyroxene and pigeonite vary considerably. The problem is actually much more complicated than this. In a number of HED meteorites slow re-equilibration of initially formed pyroxenes occurred — often leading to exsolution of augite lamellae and/or to partial or complete inversion of the pigeonite into orthopyroxene. Furthermore, foreign clasts of significantly different pyroxene composition are always a possibility when dealing with these brecciated rocks which have, usually, experienced multiple impacts since the initial crystallization of these rocks. We provide here, then, only a rough indication — a sample — of the embedded mineralogical narratives waiting to be further decoded.
Augite is reported in a majority of Eucrites and Howardites and in some Diogenites. Augite is a primary mineral in some instances, but is usually found as one or more generations of thin or very thin augite exsolution lamellae. In addition, to different cooling rates at different depths during the formation of parent body crust, heating episodes triggered by impact events have also been followed by subsequent exsolution events. The actual number of HED meteorites with augite lamellae is almost certainly underreported as many accounts note the presence of Ca-rich clinopyroxene or Ca-rich pyroxene. In most instances such Ca-rich pyroxene is probably augite, especially in Eucrites. With Diogenites the possibility of diopside is somewhat more likely.
The augite in HED meteorites is often variable, but the presence of ferroan augite — sometimes labelled 'ferroaugite' is specifically noted in a number of instances. Hedenbergite (FeCaSi2O6) is also occasionally found. In a number of instances detailed studies show significant scatter in both Ca-poor and Ca-rich Clinopyroxenes (pigeonite, diopside, augite, hedenbergite) which usually range from somewhat magnesian to mildly ferroan or even strongly ferroan in composition.
Very small amounts of Enstatite and/or Clinoenstatite are also reported for 3 Howardites. These small and chemically anomalous 'exotics' — absent detailed evidence to the contrary — suggest that the regolithic surface of Vesta and/or any other HED parent body has been enriched by an Enstatite-rich component during its the asteroidal bombardment. Ferrosilite, unusually Fe-rich pyroxene, is also reported for 2 Howardites, but would be a more likely import from Carbonaceous Chondrites.
PYROXENE TALLIES (Actual instances):
'Orthopyroxene' is reported in 48 HED falls (22 Eucrites, 15 Howardites, (all) 11 Diogenites)
Pigeonite is reported in 43 (23 Eucrites, 9 Howardites, 4 Diogenites)
Ca-rich Pyroxenes are reported in 39 HED falls: (23 Eucrites, 10 Howardites, 6 Diogenites)
Augite is reported in 36 HED falls (35 times as 'augite')
'Hypersthene, a variety of Orthopyroxene, is specifically noted 16 times
Diopside is reported in 7 HED falls. including 4 Diogenites
'Ferroaugite,' a variety of Augite, is specifically noted 7 times (5 times in eucrites)
'Bronzite', a variety of Orthopyroxene, is specifically noted 5 times
Hedenbergite is reported in 4 HED falls
Enstatite is reported in 2 Howardite falls (Bholghati, Jodzie)
Clinoenstatite is reported in 2 Howardite falls (Bholghati, Washougal)
Ferrosilite is reported in 2 Howardite falls (Jodzie, Pavlovka)
B. The HED Feldspar Group and Plagioclase Series. And, Albite, Anorthite, Bytownite, Labradorite, Maskelynite
The Feldspar Group is an extremely important Mineral Group for both terrestrial and HED mineralogy, but there is one striking geochemical difference crucial to our discussion. While Potassium-rich members of the Feldspar Group (e.g., microcline, orthoclase) are dominant components of many terrestrial lithologies, meteoritic feldspars are almost always K-poor. Consequently in the meteoritic literature, discussion of Alkali-Aluminum-rich tektosilicates is often restricted to consideration of the plagioclase series. This is particularly true for HED meteorites where both feldpathoids and K-rich phases are in short supply. Thus our discussion begins with the Plagioclase Series [Note: In the mindat.org syllabus 'Plagioclase' is treated as an exact synonym for the 'Albite-Anorthite Series'].
By definition Eucrites are dominated by pyroxene-plagioclase combinations and so, unsurprising, plagioclase is listed for all 35 Eucrite falls and all 16 Howardite falls. While plagioclase is only a minor component in Diogenites, it is still recorded for 9 of the 11 Diogenite falls. In the meteoritical literature one is more likely than not to read that a specific Eucrites or Howardite contains 'anorthitic plagioclase' — this phrase conveniently elides any explicit attention to the precise use of either anorthite (An70-An90) or 'bytownite (An70-An90) as a leading entry. The average Anorthite content is usually near the conventional dividing line between the two members of the series and these averages often fluctuate when reported by different researchers. Indeed, Plagioclase (as a stand alone label) is explicitly listed as a constituent of 57 of the HED meteorites. In three HED meteorites 'anorthite' or 'bytownite' are listed instead. However, both reporting tastes and sampling issues are involved as well. Anorthite and Bytownite are explicitly mentioned in 33 and 30 instances, respectively. Presumably, in these instances the reporter or report interpreter wished to emphasize that some regions of the meteorite were predominantly anorthite or bytownite, respectively. Both Anorthite and Bytownite are explicitly mentioned for the same meteorite in 16 instances. In these cases, we are almost certainly dealing with the sometimes quite variable plagioclase composition found in many HED meteorites. While the numbers are too small for definitive conclusions, the fact that 4 of the 8 Eucrites reporting both anorthite and bytownite are polymict eucrites (presumably sampling larger regions of the homeworld) is suggestive. The single instances of Labradorite (An50-70) in the Bilanga Diogenite and Andesine (An30-50) in the Washougal Howardite also suggest that there may have been more albite-rich regions in the homeworld(s) than our usual samples suggest. This is, however, only a suggestion as small pockets of impact melt can produce atypical compositions almost anywhere on a regolith dominated world.
Feldspar is specifically noted in 7 instances. There is a tendency for some authors to use the term Feldspar for any form of albite-rich plagioclase. The author himself avoids the term 'Feldspar' unless the Orthoclase component is greater than the Anorthite component or seems particularly significant. The K-Feldspar recorded in the Bilanga Diogenite is, compositionally at least, 98.5 % orthoclase and is described as 'exotic'. It is 'exotic' — but the unanswered question is still "Where did it come from from?" Were there any places on the Bilanga Parent Body where such K-rich phases were significant? For that matter, what sort of impacting small asteroid would contain such mineralogical assemblages?
PLAGIOCLASE AND FELDSPAR TALLIES (Actual instances):
Members of the 'Plagioclase Series' are recorded in 60 of the 62 HED Falls. (35 Eucrites, 16 Howardites, 9 Diogenites). [No Plagioclase is reported for Tatahouine and Ellemeet (Diogenites)]
End Members and the Series variants are often explicitly mentioned — Anorthite and Bytownite in 33 and 30 instances, respectively. The presence of both Anorthite and Bytownite in the same meteorite is explicitly noted in 16 instances. Eucrites (8 instances with 4 polymict Eucrites), Howardites (5 instances) and Diogenites (3 instances)
'Andesine' and 'Labradorite' varieties are mentioned in single instances (Washougal; Bilanga).
Maskelynite, glass of Plagioclase composition, is present in at least in 5 instances.
The Feldspar Group is noted in 7 HED Falls - Either as the Feldspar group (6) or as K-Feldspar (1).
Major and minor differences between the three meteorite classes for Feldspar, Plagioclase, end members, varieties, and derivative phases.
Overall, the differences in chemical composition of plagioclase in the three groups is somewhat blurred. However, Anorthite is specifically noted in 75% of Howardites Falls [12 of 16], but the percentages are significantly smaller for Eucrites (51%) [18 of 35] and for Diogenites 25% [3 of 12].
For Eucrites, Plagioclase (etc.) occurs as Anorthite (18), Bytownite (15) Maskelynite (4). Feldspar is noted on 3 occasions.
For Howardites, Plagioclase (etc.) occurs as Anorthite (12), Bytownite (8), Andesine (1), Maskelynite (1) instances. Feldspar is noted on 3 occasions.
For Diogenites, Plagioclase (etc.) occurs as Anorthite (3 instances), Bytownite (7) and Labradorite (1). Feldspar is noted on 1 occasion as K-Feldspar.
Both Anorthite and Bytownite are explicitly noted in the same meteorite in Eucrites (8 instances), including 4 polymict Eucrites, Howardites (5 instances), and Diogenites (3 instances)
C. Silica and the Silica Polymorphs Tridymite, Quartz, Cristobalite in HED falls
Polymorphs of silica are ubiquitous in HED meteorites — usually as very small grains with at least one silica phase recorded for 54 of the 62 HED falls. The recognition of a spike in silicon abundance using the microprobe makes chemical detection of silica phases relatively routine. However, determining the mineralogical identity of such phases is normally time consuming and often difficult or impossible. We note that silica polymorphs are present in 36 HED meteorites (20 Eucrites, 11 Howardites, and 3 Diogenites). Tridymite, recorded in 24 HED meteorites, is the most common identified polymorph. Tridymite is found in 10 of 16 Howardite Falls (62.5%), but only in 12 of 35 Eucrite Falls (34%). On the other hand Quartz is recorded in 9 HED meteorites - all but one of them Eucrites. Cristobalite has been recorded only in Tatahouine. In 5 instances both quartz and tridymite are found in the same meteorite. The only obvious trends here are that (1) silica phases are apparently less abundant in Diogenites than in the other two HED classes and (2) conditions for quartz formation appear to have been a little more favorable in Eucrites than in the other HED classes. Silica often appears in small aggregates of glass-containing impact melt. In this connection we note that some form of silica is found in all 7 polymict eucrites.
SILICA TALLIES (Actual instances):
Either 'Silica' or an identified polymorph of silica has been found in 54 HED meteorites
'Silica' is specifically recorded as Tridymite in 24 instances, as Quartz in 9 instances & as Cristobalite in 1 instance.
Subtallies for Individual Meteorite Groups:
'Silica' is found in 32 Eucrites; Quartz is noted in 8 instances, Tridymite in 13.
'Silica' is found in 13 Howardites; Quartz is noted in 1 instance, Tridymite in 10.
'Silica' is found in 9 Diogenites; Tridymite is noted in 2 instances, Cristobalite in 1.
Notes: Almost all HED quartz is found in Eucrites; Both Quartz and Tridymite are recorded in 5 meteorites.
D. Olivine in HED falls: Olivine = 'The Olivine series' ; (rarely, End members Forsterite and Fayalite).
In silicate melts derived from 'normal' Solar System materials with chondritic or near chondritic composition, forsteritic or Mg-rich olivine is a major component in the earliest phases which separate from the melt. In the mineralogies of meteorites derived from small asteroids (Ordinary Chondrites, Carbonaceous Chondrites) olivine is usually an important and sometimes the major phase. In the achondritic Brachinites and stony-iron Pallasites olivine is the dominant silicate. On the larger terrestrial worlds olivine-dominated rocks are usually plutonic in origin and are termed dunites when olivine is >90% of the rock. The analogous martian Chassignites are usually defined simply as 'martian dunites.' The absence of olivine-dominated rocks among HED falls has been something of a puzzle as the pyroxene-rich Diogenites are clearly of plutonic origin and where there are pyroxenites one might expect even more dunites. Results from the DAWN Mission suggest that some impacts apparently reached deep enough into Vesta's mantle to excavate large amounts of pyroxene, but perhaps not quite deep enough to excavate an equally volumetrically significant olivine component. The problem could be simply a matter of collisional circumstance. 382 Diogenite meteorites are currently listed at the Meteoritic Society's 'Meteoritical Bulletin Database' [Oct 2014] and 11 are designated as 'Olivine Diogenites.' In addition there are a small number of polymict Diogenites which may also contain significant amounts of olivine. The problem of designating the proper boundaries for HED classification is currently in a state of flux. Thus determining what is 'anomalous' and what is 'normal' about HED meteorites (in this case 'Diogenites') may remain a problem for some time. In the meantime, we return to the reported presence of minor amounts of olivine in precisely half of our 62 HED falls.
In HED meteorites olivine is recorded in 31 of the 62 Falls. Olivine is most likely to be found in Howardites (93%) [15 of 16], fairly likely to be found in Diogenites 63% [7 of 11], and least likely to be found in Eucrites (93%) [9 of 35]. End member variants, Forsterite [Mg-rich Olivine] and Fayalite [Fe-rich Olivine], are noted only in a few Howardites. One suspects that much of the olivine has been kicked around the homebody/homebodies by impacts. Otherwise, one would expect to find more olivine in the plutonic Diogenites than the partially plutonic/partially crustal Howardites. In this connection we note that more than half of the polymict Eucrites (4 of 7) contain olivine. Indeed, the Forsterite and Fayalite found in 4 Howardites may be truly 'exotic' olivine brought in by material ultimately derived from other impacting asteroids.
OLIVINE TALLIES (Actual instances):
Olivine has been reported in 31 HED falls.
End Members Forsterite (3 instances) and Fayalite (1) were reported only in Howardites.
Subtallies for Individual Meteorite Groups:
Olivine has been reported in 9 Eucrite falls,15 Howardite falls, and 7 Diogenite falls.
E. Fe-Ni Metal and other Forms of Unoxidized or Metallic Iron in HED falls
In Meteorites unoxidized or metallic iron is almost invariably accompanied by nickel (Fe-Ni metal). This most common form of unoxidized, meteoritic iron is usually labelled as Fe-Ni Metal (or some equivalent term) in the meteoritical, geochemical, and planetary science literature. Its origin is well understood — most of the universe's iron and nickel are produced in tandem during - quite literally - the last few moments before a supernova explosion and during the first hours or so of the supernova's hyper-energetic blast from the core of the exploding star up and out into the surrounding vacuum of space. During the formation of the Solar System a significant amount of this iron and accompanying nickel was incorporated into different solar system bodies — sometimes as Fe-Ni metal, iron oxides, sulfides and other compounds. Most of the earth's iron and nickel are locked up in the earth's core, but Fe-Ni metal is readily obtained from 'iron meteorites' and as tiny grains in most chondritic meteorites. Almost all meteoritic Fe-Ni metal exists as either kamacite or as a kamacite-taenite intergrowth. Kamacite has a body-centered cubical crystal structure consisting almost entirely of iron plus 4-8% Ni and .5-1.0% Co. Taenite has a face-centered cubical crystal structure and contains considerably more Nickel. There are some complications, of course, meteoritic iron may be, mineralogically speaking, somewhat disordered due to preterrestrial shocks and/or earth impact. And, recent high resolution studies show that taenite often — perhaps usually — contains small micro-domains of tetrataenite and other forms of Fe-and-Ni rich metal. Indeed, except for obviously artificial forms of iron, nickel and cobalt, almost all unoxidized metallic iron encountered on or near the earth's surface consists primarily of kamacite or kamacite-taenite intergrowths.
However, while a significant of the unoxidized iron in HED meteorites is kamacite or kamacite with taenite and/or tetrataenite, this is not true for much — perhaps most — of their unoxidized iron. As the largest single exception to the general rule that most meteoritic iron is Fe-Ni iron, a significant number of HED meteorites contain iron which is nickel poor. The problem is complicated by some terminological issues. Some meteoriticists describe almost any form of unoxidized iron as Fe-Ni metal or, when not Ni-rich, as kamacite. And, on the other hand, the International Mineralogical Society has decided that rare forms of Ni-poor cubical iron discovered in the past 2 centuries is the mineral 'Iron' while the more abundant kamacite studied by wizards, alchemists, metallurgists, and scientists for the past three millennia is a mere 'variety' of said 'Iron.' Let us try to sort out what this means for the unoxidized iron which may or may not be properly described as 'Fe-Ni metal.'
The scientific problem which seems most important to this observer is whether all, some, or none of either the Ni-poor iron or the meteoritically more 'normal' Fe-Ni metal was formed as part of the overall process which created Vesta and other HED Parent Bodies or whether it is 'exotic' material introduced by subsequent impacts with other asteroids. If Vesta formed in analogous fashion to the earth [Klaus Keil has described Vesta as 'the smallest terrestrial planet'], then most of Vesta's iron and nickel would have been incorporated into Vesta's core. [The DAWN Mission has established that Vesta has a core.] Similar considerations could possibly apply to the other large asteroidal homeworlds. According to the usual simplified models, almost all of the nickel in the proto-Vesta would have been incorporated into the core, but a significant fraction of the iron would have joined the silicate rich mantle and crust. If this were true, any Fe-Ni metal assemblages would be due to subsequent meteoritic bombardment. That is all reasonable enough, but it is not at all clear just how 'clean' such an original differentiation would have been. On the earth today, gas bubbling up in volcanic events contain odd xenon-isotopic ratios which did not rise to the top of any original magma ocean over 4 billion years ago. Instead, differentially marked by decay of radioactive isotopes which occurred over 4.5 billion years ago, this gas is only now separating itself from its original reservoirs.
As for the nearly pure iron now found in HED meteorites, one might imagine that residual reducing H-rich and/or CO-rich gas from the original solar nebula might have been at work during formation of Vesta and/or other parent bodies outer layers - but that seems purely conjectural at present. As somewhat analogous processes may have been at work on both the moon and Mars where Ni-poor iron has also been observed, it is clear that there are many unanswered questions. We turn, then, to what the record tells us.
Metallic iron as Fe-Ni metal or as Ni-poor iron is listed for 35 HED meteorites. The actual number is somewhat higher than that because in several instances authors report the presence of either iron or Fe-Ni metal without providing the actual percentage of nickel in the iron. Without that information the scientific value of any purely mineralogical record is compromised (vide infra). Still, the presently available record makes it possible to make one potentially very important observation. Iron — either as Ni-poor Iron or as Ni-containing Kamacite — is reported for all 11 of the plutonic diogenites, but for only 25% of the crustal Eucrites (14 of 35) Eucrites. This provides prima facie evidence that a significant quantity of the metallic iron is indeed indigenous iron and not impact- introduced exotica.
'Iron' as the stand alone IMA defined 'mineral' for body-centered Fe-rich metal is listed for 24 HED falls (8 Howardites, 10 Eucrites, 6 Diogenites). Most of these instances appear to be cases of Ni-poor iron, but the actual number of Ni-poor iron locations can only be tentative at this point. [The articles by Duke (1965) and Gooley and Moore (1976) give detailed accounts of metal composition of HED meteorites, but these are exceptions.] This author normally posts Ni-poor iron under the label 'Iron' with the Ni% included under the 'Mineralogical Details'. My own standard for reporting Ni-poor iron is Ni ≤ 2% (several instance of Ni < 0.5% are noted at individual meteorite Location Sites), but I know of no common standard.
Fe-Ni Metal is reported for 21 HED falls (9 Howardites, 6 Eucrites, 6 Diogenites). In 18 cases the presence of Kamacite is explicitly noticed. 'Meteoritic iron' is also reported in 3 additional meteorites in which 'kamacite' is not explicitly mentioned (Jodzie, Kirbyville, Macibini). 'Meteoritic iron' is often used when definitive mineralogical classification is not available. In this case, it should be noted that two of the meteorites mentioned above (Jodzie, Kirbyville) are very small. In 13 cases Taenite and/or Tetrataenite are associated with and sometimes adjoining the Kamacite. Thus, a first read suggests that instances of unusually Ni-poor iron and meteorite 'normal' Fe-Ni metal are roughly equal.
TALLY OF IRON-RICH PHASES (Actual instances):
Iron-rich metal is found in 35 HED falls (10 Howardites, 14 Eucrites, 11 Diogenites)
Fe-Ni Metal found in 21 HED falls (9 Howardites, 6 Eucrites, 6 Diogenites)
Formal Inventories:
Iron, possibly Ni-poor, is reported in 24 HED falls (8 Howardites, 10 Eucrites, 6 Diogenites)
'Kamacite' is reported in 18 HED falls (8 Howardites, 4 Eucrites, 6 Diogenites)
Taenite is reported in 12 HED falls (6 Howardites, 2 Eucrites, 4 Diogenites)
'Meteoritic-Iron' is reported in 6 HED falls (2 Howardites, 2 Eucrites, 2 Diogenites)
'Nickel-iron' is reported in 2 HED falls (2 Diogenites) [Possible redundant references to Taenite?]
Tetrataenite is reported in 2 HED falls (2 Diogenites)
F. Serpentine in HED falls (Howardites)
Phyllosilicates are only occasionally mentioned with respect to HED meteorites and, particularly in finds, may be products of terrestrial weathering products. However, Serpentine and minerals within the Serpentine Group are reported from 4 Howardites (Bholgati, Kapoeta, Jodzie, and Erevan). Specific instances include Cronstedtite (Erevan) and Antigorite (Bholgati). Saponite, another phyllosilicate and tochilinite, a hydrated sulfide, may also be found in associated assemblages. Work by Michael Zolensky and others (summarized in Zolensky et al., 1996) has established that there are pervasive instances of carbonaceous chondrite material in some Howardites (especially CM2 clasts and, to a lesser extent, CR2 clasts).
COMPLETE TALLY OF SERPENTINE GROUP:
Serpentine Group or minerals reported in 4 Howardites (Bholgati, Erevan, Jodzie, Kapoeta)
Cronstedtite reported from the Erevan Howardite.
Antigorite reported from the Bholgati Howardite.
G. The Apatite Association
Phosphates, particularly Ca-rich phosphates, are frequently reported in the meteoritical literature. When mineralogically identified they are almost always either apatite or merrillite. They are often — usually, I would hazard — too small to mineralogically characterize on a routine basis. Also, there have been some unsuccessful attempts to impose new standard definitions for mineral phosphates that were later rescinded. Recently, prompted especially by the usefulness of even small amounts of various radiometrically useful elements which tend to concentrate in phosphates some targeted efforts have resulted in the identification of apatite — sometimes specifically as fluorapatite — and occasionally associated merrillite. A close read of the actual data reveals that the apatite is indeed often properly characterized as fluorapatite, but varying amounts of a minor chlorapatite component are usually present as well.
We add only a little minor commentary. There seems to be little indication of hydroxyapatite, a common terrestrial component of apatite. On the other hand, phosphides are very rare in HED meteorites [Both barringerite and schreibersite are reported in two instances, but these could easily be of exotic origin]. Thus it is seems that the original HED formation epoch was largely anhydrous and characterized by an intermediate oxygen fugacity (compared to say, most iron meteorites on the one hand, or to Martian meteorites on the other.) How similar the HED formation epoch was to the formation epoch of the lunar highlands and the lunar basalts is probably both a more interesting and a more difficult question.
COMPLETE TALLY OF THE APATITE ASSOCIATION:
'Apatite', listed either as 'Apatite' or as Fluorapatite, is recorded in 13 HED meteorites (10 Eucrites & 3 Diogenites).
Fluorapatite is specificly listed in 8 HED meteorites (5 Eucrites & 3 Diogenites).
Section IV: 44 IMA defined Minerals found in HED Falls (abc order); a TABLE of Occurrence Frequency; Commentary on each individual mineral.
This table contains only those 44 mineralogical phases which (1) have been deemed by the International Mineralogical Association (IMA) as properly crystallographically and chemically defined 'minerals' and (2) are currently listed at 'mindat.org' location sites for one or more of the 62 witnessed HED meteorite falls. Each IMA defined mineral is accompanied (in parenthesis) by the number of HED falls where the mineral has been reported. References for these reports are found at the individual meteorite 'Location Sites.' The author makes no claim of completeness, but this should usually provide an introductory sample of those minerals which we would expect to see in HED meteorites, those which we might see, and those which we would be surprised to see in a moderately large HED meteorite.
[Caveat: In Section III we indicated a few mineral phases which are most likely to be underreported. One could say, of course, that as every meteorite fall is itself a fortuitous event, we have been forewarned that sampling problems are inherent in the meteoritic enterprise ab initio. E.]
In the commentary which follows we attempt to put provide additional context for the individual minerals. We, of course, give some attention to some of the expected differences between the shallower-sourced Eucrites and the plutonic Diogenites. However, some readers may be surprised to see that the Howardites appear to contain a broader source of constituents than either the Eucrites or Diogenites. The Howardites appear not only to sample a wider range in composition among, presumably, lithic constituents from various regions of the original homeworld(s), but they also seem to sample a significantly larger share of exotics — minerals derived from other meteorite types. A few high temperature minerals (e.g., perovskite) and very reduced phases (e.g., Forsterite) are likely imports from CAI-bearing carbonaceous chondrites and Enstatite-rich chondrites-and-achondrites, respectively. Carbonates and some of the hydrated silicates of likely CM chondrite origin also appear to warrant special attention. On the other hand, we do not pursue here in any real detail the interesting issue of mineralogical variations between HED subtypes (e.g., polymict versus monomict Eucrites). For that the reader needs to consult the original literature.
The issue of what constitutes an 'exotic' is replete with informative problematics. A true 'exotic' would be an imported phase or assemblage brought to the meteorite's original homeworld by an impacting meteoroid. In some cases the carbonates and phyllosilicates are components of carbonaceous clasts which can even be identified by meteorite type. For example, CM2 Carbonaceous clasts have been found in a number of HED and other meteorite types. On the other hand, HED sulfides are a bit more complicated. The presence of Fe-rich sulfides other than troilite may reflect unusual oxygen fugacities signaling the presence of a true exotic. Thus, for example, one is more likely to find pyrrhotite and pentlandite in somewhat hydrated carbonaceous chondrites than in irons, most chondrites, and most achondrites. However, their occasional detection in HED meteorites may reflect fugacity variations within the HED parent worlds as well as exotic addenda. In these cases, it is best to consult the original observations referenced in the host meteorite's Mindat location site. In addition, several of the articles in the Bibliography represent the best in contemporary amalgams of excellent science coupled to mineralogical art. [The underlying constraint, of course, is that it is a lot easier to recognize a meteorite which has fallen to earth as a geochemical stranger than it is to determine which mineralogical oddities within an HED meteorite were themselves transported from another meteoroid to the HED homeworld(s).]
TABLE: 44 IMA defined Minerals and Number of HED Listings (= # of HED hosts)
Anorthite (32) - Anorthite is formally reported for 32 HED meteorites. Anorthite is explicitly recorded in all 6 cumulate Eucrite finds.
Antigorite (1) - Antigorite, a hydrated phyllosilicate in the Serpentine Group, is formally reported only for the Bholghati Howardite.
Augite (35) - Augite is formally reported for 35 HED meteorites.
Baddeleyite (2) - Baddeleyite has been formally recorded here only for Millbillillie, a main group Eucrite, and Pasamonte, an anomalous Eucrite.
Barringerite (2) - Barringerite, a rare meteoritic phosphide, is formally reported here only for 2 Howardites, Erevan and Jodzie.
Calcite (3) - Calcite is formally reported for 3 HED meteorites, Erevan, Kapoeta, Tatahouine — all Howardites. In Tatahouine it appears to be produced by terrestrial weathering. Otherwise, it appears to be a carbonaceous exotic.
Chalcopyrite (5) - Chalcopyrite is formally reported here for 5 HED meteorites and only as a very minor accessory in 2 diogenites and 3 Howardites.
Cristobalite (1) - Cristobalite has been formally reported only for the Tatahouine Diogenite.
Chromite (54) - Chromite is formally reported for 54 HED meteorites. The only HED Meteorites not listed are 6 eucrites (Bialystok, Brient, Emmaville, Jonzac, Lakangaon, Piplia Kalan) and 2 Howardites (Chaves, Lohawat).
Cronstedtite (1) - Cronstedtite, a hydrated phyllosilicate in the Serpentine Group, has been formally reported only for the Erevan Howardite.
Diopside (7) - Diopside is reported for Bilanga, Garland, and Johnstown (diogenites); Jodzie, Kapoeta, Roda (Howardites) and Piplia Kalan (eucrite)
Dolomite (1) - Dolomite has been reported only from the Erevan Howardite.
Fayalite (1) - Fayalite, Fe-rich olivine, is reported only for the Jodzie Howardite.
Ferrosilite (2) - Ferrosilite , Fe-rich pyroxene, is reported for the Jodzie and Pavlovka Howardites
Fluorapatite (8) - Fluorapatite has been found in the Chervony Kut, Juvinas, Pasamonte, Serra de Magé and Stannern Eucrites as well as the Manegaon, Roda and Shalka Diogenites.
Forsterite (3) - Forsterite, Fe-poor olivine, is found in the Bholghati, Erevan and Kapoeta Howardites
Hedenbergite (4) - Hedenbergite is found in the Macibini and Nobleborough Eucrites and in the Kapoeta and Pavlovka Howardites
Ilmenite (46) - Ilmenite in small amounts is ubiquitous in HED meteorites. It is often associated with chromite.
Iron (26) - Iron and, specifically, Ni-poor Iron (Ni<2%) is found in small quantities in many HED meteorites. This author uses the term 'Iron' as a mineral name only for Fe-rich metal that is Ni-poor.
Mackinawite (2) - Mackinawite, a tetragonal Fe-Ni sulfide, is reported only for the Roda and Johnstown Diogenites.
Magnetite (5) - Magnetite is reported for 4 Howardites (Bholghati, Chaves Jodzie, Kapoeta) and 1 Eucrite, Peramiho. It is very easy to imagine both indigenous and exotic sources for this mildly oxidized phase which contains both ferric (Fe+++) and ferrous Fe++) iron.
Merrillite (6) - Merrillite reported only for Roda and Ibitira. On occasion merrillite has been simply defined both formally and operationally as 'extraterrestrial whitlockite'. However, detailed crystallographical studies suggest that there are minor structural differences — presumably this is at least partially because truly anhydrous minerals are less likely in almost any terrestrial setting than on worlds without atmospheres or liquid water.
Oldhamite (1) - Oldhamite is reported only for the Johnstown Diogenite. An unexpected and extremely reduced phase. However, the observation was made by none other than Paul Ramdohr.
Pentlandite (8) - Pentlandite is reported for the Bholghati, IbbenbĂĽren, Jodzie and Kapoeta Howardites as well as Bilanga, Peckelsheim, Roda and Tatahouine Diogenites. The presence of pentlandite often suggests a higher oxygen fugacity than the many meteoritical environments in which troilite is the only reported Fe-rich sulfide.
Perovskite (1) - Perovskite is reported only for the Kapoeta Howardite. Perovskite is a high temperature product.
Pigeonite (43) - Pigeonite is common in HED meteorites. Whille most common in quickly cooling lavas, it is found in most eucrite cumulates. On the other hand it is found in only 3 of the 11 Diogenite falls.
Pyrrhotite (4) - Pyrrhotite is reported for the Bholghati, Jodzie, & Kapoeta Howardites as well as the Chervony Kut Eucrite
Quartz (10) - Quartz is reported for 10 HED meteorites (including 9 eucrites): Chaves (How); Béréba, Jonzac, Juvinas, Peramiho, Stannern, Vetluga (Eu-mm); Pasamonte (Eu-pm); Serra de Magé, Vissannapeta (Eu-cum)
Saponite (3) - Saponite, a phyllosilicate, has been reported only in 3 Howardites.
Schreibersite (2) Schreibersite is reported in two diogenites, Peckelsheim and IbbenbĂĽren.
Spinel (7) - Spinel is listed for 4 Eucrite falls and 3 Howardite Falls
Taenite (12) - Taenite is listed for 2 Eucrite falls, 6 Howardites (Bholghati, Bununu, Kapoeta, Luotolax, Yurtuk, Zmenj), 4 Diogenites (IbbenbĂĽren, Johnstown, Peckelsheim, Roda)
Tetrataenite (2) - Tetrataenite is reported only for the two diogenites, Bilanga and Peckelsheim.
Titanite (1) - Titanite has only been reported for the polymict Diogenite, Garland
Tochilinite (3) - Tochilinite, a hydroxysulfide, has only been found in 3 Howardites.
Tridymite (25) - Tridymite is listed for 12 Eucrite falls, 10 Howardites, and 3 Diogenites. As the most common form of silica in HED meteorites, it is likely that many small and tiny grains identified as only as 'silica' by the microprobe are instances of unreported tridymite.
Troilite (47) - Troilite is listed for 25 Eucrite falls, 10 Diogenites & 12 Howardites
Ulvöspinel (3) - Ulvöspinel is listed only for 3 Eucrite falls
Whitlockite (2) - Whitlockite is reported only for one Eucrite (Ibitira) and one Diogenite (Roda).
Zircon (9) - is listed for 9 Eucrite falls. A targeted phase which is unusually useful for chronological studies.
Zirconolite (1) is reported only for the anomalous Eucrite, Pasamonte
Concluding Reflections: Lithological and Personal
Earlier this year I began a project to post via Mindat mineralogical inventories of the 61 witnessed meteorite falls from the Howardites, Eucrites, and Diogenite meteorite classes — those meteorites commonly referred to as the HED meteorites. My purpose was to put into a relatively accessible public format a useful introduction to what is known about the mineralogy of these particular meteorites. I restricted myself to witnessed falls in order to reduce the work into a personally manageable task. It is, of course, quite true that much of the material I have used is in the 'public record' as the term is often used. However, a considerable amount of such 'publicly available' information is contained in proprietary journals which are often difficult or expensive for many people to access. But, in any case, I managed to place this material on line within about 3 months. Many of the meteorites already had 'Location Sites' at Mindat and, when sites were not available, I was able to create them for whatever they are worth.
It then seemed reasonable to me that I should organize the implications of what I had done into what became the present essay. This effort presented more challenges than I expected and took nearly twice as long to complete as the first task. I suppose that means that it is often more difficult to know why one has done something than it is to actually do the thing itself. In any case, I summarize the meaning of these efforts with the following four paragraphs.
The meteorites which are today known as the HED achondrites were once considered a subset of a larger class of meteorites often known simply as the 'basaltic meteorites' or as 'basaltic achondrites.' They were called basaltic because their dominant minerals — pyroxene, plagioclase, and lesser olivine — are the dominant minerals of terrestrial basalts. Gradually, it has become clear that some of these meteorites are fragments of identifiable worlds. At the same time our meteorite collections grew enormously as meteorites from Antarctica and from hot deserts were identified in ever increasing numbers. We have gradually established that a few of these meteorites definitely came from Mars and a few others definitely came from the moon. At the same time it also gradually became clear that several meteorite groups also came from specific worlds — but, unfortunately, it is not clear what world is there original one. That is the situation we face today. Most meteoriticists believe — or will adapt as a working hypothesis — the notion that all or almost all of the members of such groups as the Angrites, the Aubrites, the Main Group Pallasites, the Mesosiderites, and the HED meteorites are in each case most likely derived from a single Original Parent Body (OPB). These beliefs or working hypothesis are not held by all and they are not formal credos, but they do define the current goals of current geochemical meteoritic and some asteroidal research. The Holy Grail of much contemporary works is to find and establish the OPB for as many meteorite groups as possible.
There are, however, two very important considerations which are acknowledged to stand between us and these goals. First, except for the HED meteorites, in most case there is no definite asteroidal, planetary or satellite body which stands out as a likely parent body for any specific meteorite class. Secondly, there is always a problem in defining proper membership for a meteorite group or class — the problem of 'look alikes.' Some historical perspective is helpful. The first Martian falls were only slowly separated from other meteorite groups as having peculiarities which mandated that we consider them as an ensemble meriting special attention. The Martian pyroxenite, ALH84001, was first classified as a Diogenite before it was recognized as a Martian meteorite. The olivine-rich meteorite Brachina was tentatively considered to be a likely Chassignite before it was made the prototype of a new class of primitive achondrites, the Brachinites. Which brings us back to the HED meteorites.
We have not yet put landers on Vesta, but orbital observations of the Dawn Spacecraft in 2011 and 2012 have helped to make it clear that Vesta is almost certainly the OPB of most HED meteorites. At the same time detailed high resolution of oxygen-isotope studies — coupled to other chemical ratios have made it clear that Vesta is almost certainly not the OPB of Ibitira, Pasamonte, and one or two other Eucrites. In future years and decades we will need to flesh out the details of the journey from Vesta to Earth for a goodly number of HED meteorites before we can be definite about which currently classified HED meteorites are definitely fragments of Vesta, which are merely likely fragments of Vesta, and which are fragments of another as-of-now-unidentified asteroidal body. Besides working out a more comprehensive understanding of the geochemical and mineralogical markers of original classification, subsequent metamorphism and impacts, a more comprehensive picture of the passage from a rock on Vesta to a component of an asteroidal fragment to a component on an even smaller meteoroid which struck the earth's surface will emerge. Hopefully, somewhere in there we can establish that some of the meteorites now in our laboratories have shared chronological markers that will allow us to separate what happened to these rocks when they were on or inside Vesta from what happened while they spent a billion years or so as a part of a smaller asteroid and from what happened during their past few million years as part of a boulder which collided with the earth. Since for the immediate few coming decades or, perhaps, centuries, we will be severely limited in making either human or robotic missions to asteroids and natural satellites, we will do well to work out the details of passage if we plan to identify any more homeworlds in the future.
If we do this, we may also begin to sort out the problematics of such meteorite groups as the ureilites and the mesosiderites which have been altered by impact process in even more fundamental ways than the highly brecciated howardites and lunar rocks we have been studied in recent decades with such intensity. Whatever we do and whatever new techniques we devise and refine, we will still find it useful to begin our meteorite studies by looking at their minerals. Humans use words to create languages. The lithic histories of asteroids and terrestrial worlds are embodied in their minerals. Minerals are, so to speak, a form of rockspeech.
Nota bene:
The terms used here are deemed 'indispensable' to his argument by the author. Whether that is actually true or not, they include terms which are frequently bandied about in the meteoritical literature and, presumably, are not necessarily familiar to the reader. The highlited [sic] terms are, of course, immediately available within the Mindat realm of discourse. Wikipedia definitions are also usually adequate in this context. Virtually all terminology related to meteorite classification implicitly or explicitly references the usage employed by the Meteoritical Society and employed at their 'Meteoritical Bulletin Database' [http://www.lpi.usra.edu/meteor/metbull.php] Terms which are frequently employed in somewhat different linguistic registers when employed within meteoritical literature are noted with an asterisk.
Indispensable Terms:
Achondrite -- A differentiated stony meteorite, a meteorite which was formed on a world which experienced extensive melting which resulted in the complete or partial separation of its original constituents. Here we are exclusively interested only in those meteorites which are derived from worlds which experienced melting sufficient to produce both a silicate-rich mantle and a metal-rich core. These include Martian meteorites, lunar meteorites, and those meteorite types derived from large asteroidal-sized homeworlds (Angrites, Aubrites, HED meteorites, and a few odd balls). [We do not treat the very interesting 'primitive achondrites' which appear to be derived from slitely smaller asteroidal homeworlds.]
Asteroid -- Asteroids are small (sub-lunar) silicate-rich and/or metal-rich objects whose orbits are mostly or entirely 'between' the orbits of Mars and Jupiter. Very small asteroids (<100 m diameter) may sometimes be referred to as meteoroids, especially if they are in orbits which might bring them into the vicinity of the earth.
Breccia -- A rock composed of rock fragments which have been reaggregated into a 'whole rock.' Some breccia are very friable and will crumble under the slitest pressure. Other breccia have been reconstituted as moderately sturdy stones.
Clinopyroxene Pyroxenes crystallizing in the monoclinic crystal system. Of particular interest here are both Ca-rich augite and Ca-poor pigeonite.
Eucrite — A type of differentiated, basaltic meteorite rich in pyroxene and plagioclase.
Fall [or, 'witnessed fall']— A meteorite whose fall was 'observed.' In most instances, a witnessed fall is preceded by a fireball and/or detonations and loud rumbles due to the object's fiery encounter with the earth's atmosphere. The recovered object may be partially covered by a fusion crust as well. The date and, perhaps, the hour of most falls are recorded. In a few cases, accumulated circumstantial evidence has been used to deem a recovered meteorite a witnessed 'fall' even when the date of meteorite entry was not preserved.
Fe-Ni metal — The generally preferred term within meteoritical and geochemical literature used to refer to the single and/or multiple unoxidized Fe-rich phases in meteorites. The iron is almost invariably accompanied by Ni (>4-5%),minor Co and very minor Cr. Fe-Ni metal is not a mineralogical term per se, but it is normally assumed that the predominant metallic mineralogical constituent is kamacite. This is normally true whether we are discussing irons, stony irons [The few exceptions include unusually Ni-rich metal and some severely shocked meteorites]. HED achondrites create some linguistic conundrums because some HED Fe-Ni metal is predominantly kamacite or kamacite-and-taenite, but some Fe-rich metal in HED meteorites is quite poor in Nickel content.
Find — A meteorite recovered at some unknown time after it had reached the surface of the earth. Most finds are silicate-rich 'stony meteorites' (or simply 'stones').
Homeworld — The material in a meteorite has passed thru a number of stages which can be partially deciphered once it arrives on earth. However, in almost all cases, it seems that during the first million or first few hundred million years of the solar system's existence most of the material was at one time part of a single 'world' 10-10,000 kilometers in diameter. It is customary in the meteoritical literature to refer to this hypothetical world as the 'Original Parent Body' (OPB) of a meteorite. I prefer to emphasize the hypothetical nature of this idea by using the term 'Homeworld', a near synonym, for this postulated world.
In some cases, notably with Martian and lunar meteorites, it seems clear to most scientists and to this author that the hypothetical OPB has been identified as a real solar system world. However, I would insist that while there is always the possibility that some subset of meteorites, some meteorite 'class' ( e.g., HED meteorites, EH chondrites, etc.) are derived from the same parent body, there is no guarantee that any particular meteorite class is always from the same actual solar system world. In fact, the actual history of meteoritics is that while the hypothesis of a single homeworld for various meteorite classes has been fruitful, in most cases our definition of class membership has had to continually evolve — sometimes drastically. To emphasize that I think our understanding of HED meteorite origins is fraught with oversimplification I use the term 'homeworld' as an alternative to the term 'Original Parent Body'. In the text and in the bibliography, I try to underscore why it seems almost certain to me and others that the HED meteorites are mostly fragments of Vesta, we are a long way from understanding how to definitively distinguish them from the 'look-alikes' that are also fragments of large asteroids — asteroids which are presently unidentified and which, moreover, may no longer even exist as large asteroids.
Kamacite* - The predominant mineralogical constituent of most meteoritic iron whether found in iron, stony-iron, or stony meteorites. Within the meteoritical community kamacite is the preferred term for any occurrence of Fe-Ni metal within a body-centered cubic with 5-9% Ni (known to metallurgists as 'alpha-iron').
Meteorite — A natural object or set of objects which encounters the earth and is recovered by humans. A single 'meteorite' includes all objects which arrived simultaneously and so may refer to a single relatively intact stone or metallic mass or it may consist of thousands of scattered fragments. Each meteorite is given a unique 'name' which consists of a geological reference place or object and any necessary qualifiers needed to insure the uniqueness of the name. A meteorite recovered after an unobserved arrival can be identified by several methods. For natural objects, the detection of nickel [present in unoxidized Fe-Ni metal] is usually the most easily applied means to detecting non-terrestrial objects. For those relatively few meteorites which do not possess Fe-Ni metal, oxygen isotope signatures are almost always definitive.
Meteoritic Iron - 'Meteoritic Iron' as a Mindat descriptor is a close synonym for 'Fe-Ni metal' as used in the meteoritical community. In actual usage both terms carry simultaneous mineralogical and geochemical connotations which may create ambiguities. To avoid such ambiguities, the author only uses 'Meteoritic Iron' as a geochemical descriptor for nickeliferous iron whose mineralogical character is unknown. The term 'Fe-Ni metal' as used within meteoritical literature and by this author refers to nickeliferous iron as a geochemical descriptor for nickeliferous iron whose specific mineralogical characterization is not of immediate interest. It is an unstated assumption within meteoritical literature that kamacite is almost always present in 'Fe-Ni metal' and, furthermore, that if the Ni content of the nickeliferous iron exceeds 8-9%, significant quantities of taenite and/or tetrataenite are likely present as well.
Meteoroid — A (moderately small) object which has or might encounter the earth. After such encounters, the fraction of any recovered material is or will be called a meteorite.
Mineralite — A collective term used by this author to refer to any Mineral Group or Subgroup, IMA defined mineral, recognized variety, chemically defined phase or other formal or informal term helpful in describing a mineral or mineralogically important phase. In the present instance, these means mostly any term which is recognized by Mindat algorithims for constructing Mineral Lists at Mindat Locations sites. However, the term also includes a few mixed geochemical-mineralogical terms (e.g., Ca-rich Clinopyroxene) which are not recognized by Mindat algorithms.
Orthopyroxene — Mg,Fe-rich pyroxenes which crystallize in the Orthorhombic Crystal system. Orthopyroxenes are the dominant components of all Diogenite falls and are important components of many other HED meteorites.
Plagioclase [synonym, 'Albite-Anorthite Series']- Plagioclase, sensu strictu, refers to the Albite (Na-rich) — Anorthite (Ca-rich) Solid Solution Series. As K-rich Feldspars (and K-rich compounds) are so rare within most meteorite classes, it is customary within the meteoritical literature to avoid using the term Feldspar except in those rare instance when Potassium is abundant or at least significant. This is somewhat counter to best usage within terrestrial oriented geological and mineralogical discussion where the relationship between Ca-rich, Na-rich, and K-rich Feldspars and Feldspathoids is always in the background even when not of immediate interest.
Stony meteorite — Most meteorites which fall or are recovered by trained personnel from the desserts of Africa, the America, Antarctic, or Australia are silicate-rich 'stones'. These meteorites may or may not contain chondrules [earth rocks do not] and they may or may not contain significant amounts of additional Fe-Ni metal or unusual sulfides. Nevertheless, they are still 'stones' — silicate-rich rocks containing minerals such as olivine, pyroxenes, plagioclase and/or phyllosilicates — minerals utterly familiar to all terrestrial, lunar, and meteoritical geologists.
Taenite — Taenite is a mineralogical expression of Fe-Ni metal [crystallizing in a face-centered cubical lattice] found in almost all meteorites whose Fe-Ni metal contains more than ~9% Nickel. Both taenite and unusually Ni-poor iron are found in HED meteorites, almost always in very small amounts — and sometimes in the same meteorite.
Vesta (or, '4Vesta')-- Discovered on 29 March 1807 by Heinrich Wilhelm Olbers, 4Vesta was the fourth asteroid discovered, but it is the britest asteroid. Its average distance from the sun is 2.362 a.u. (353 billion km) which places it in the inner asteroid belt. Every 16 months or so during opposition when Vesta is closest to the earth, for a few weeks it is the only asteroid which becomes visible to the naked eye in a dark sky. Vesta (diameter 525 km) is the third largest asteroid in surface area and orbits the sun, but is britest because (1) it is the closest to earth of the larger asteroids and (2) its plagioclase-rich surface is more reflective than most asteroids [with dark or extremely dark carbonaceous-rich surface materials]. A number of moderately large asteroidal fragments of past collisions of comets and other asteroids with Vesta — the Vestoids — can be distinguished by both their spectral signatures and orbital characteristics. Collisions of Vestoids with comets and other small asteroids plus gravitational perturbations combine to produce a steady stream of meteoroids in earth-crossing orbits often strike the earth. The Dawn mission orbited Vesta for 16 months in 2012 and 2013. Images, spectroscopy, and telemetry have established that the surface of Vesta is rich in the pyroxenes and plagioclase one might expect if Vesta were indeed the Original Parent Body (OPB) of many HED meteorites. The data also reveal that Vesta's surface contains a significant carbonaceous-like component due in large measure to impacting meteoroids. This dark material includes a large component of — especially and specifically — CM2 or CM2-like material.
A TALLY OF 61 MINERAL PHASES FOUND IN 62 HED FALLS (35 eucrites, 16 Howardites, & 11 Diogenites)
61 IMA MINERALS AND OTHER MINERALOGICALLY IMPORTANT PHASES FOUND IN 62 HED FALLS
FORMAL TALLIES FOR ALL HED FALLS (35 EUCRITES, 11 DIOGENITES, 16 HOWARDITES)
HED BIBLIOGRAPHY
This bibliography provides articles of general interest for understanding HED meteorites and, in addition, gives particular emphasis upon the problematics of the determining the relationships between the different HED subtypes and Vesta or other possible asteroidal parent bodies. For information about individual HED meteorites, the author has made it his practice to provide references at individual Mindat Location sites which provide historical, geochemical, and mineralogical context as well as specific references to each mineralite listed at the site. These sources can be found immediately from the 62 links provided at List A: The 62 Witnessed HED Falls. Of course, the links provided for individual meteorites at the Meteoritical Society's "Meteoritical Bulletin Data Base" are even more complete.
Whenever possible, this author prefers to cite full names. The observation of meteorite minerals involves artisan skill and know how as well as scientific knowledge. This is particularly important when one is interested in issues at the boundaries of knowledge where insight is as important as information.
Jean-Alix Barrat, Akira Yamaguchi, Richard C. Greenwood, Marcel Bohn, J. Cotton, Mathieu L. Benoit & Ian A. Franchi (2007) The Stannern trend eucrites: Contamination of main group eucritic magmas by crustal partial melts. Geochimica et Cosmochimica Acta 71, 4108—4124.
Jean-Alix Barrat, Akira Yamaguchi, Brigitte Zanda, Claire Bollinger & Marcel Bohn (2010) Relative chronology of crust formation on asteroid Vesta: Insights from the geochemistry of diogenites. Geochim. Cosmochim. Acta,74 (21):6218-6231.
Karen Susan Bartels & Timothy L. Grove (1991) High-pressure experiments on magnesian eucrite compositions -Constraints on magmatic processes in the eucrite parent body. Lunar and Planetary Science Conference, 21st Proceedings, 351-365.
Andrew W. Beck & Harry Y. McSween, Jr. (2010) Diogenites as polymict breccias composed of orthopyroxenite and harzburgite: Meteoritics and Planetary Science 45 (5): 850-872. (May 2010)
Richard P. Binzel & Shui Xu (1993) Chips off of asteroid 4 Vesta – Evidence for the parent body of basaltic achondrite meteorites. Science 260, 186-191.
Phillip A. Bland et al., (2009) An Anomalous Basaltic Meteorite from the Innermost Main Belt. Science 325, 1525-28.
Janne Blichert-Toft, Maud Boyet, Philippe Télouk & Francis Albaréde (2002) Sm-Nd and Lu¬176Hf in eucrites and the differentiation of the HED parent body. Earth and Planetary Science Letters 204, 167-181.
Joseph S. Boesenberg & Jeremy S. Delaney (1997) A model composition of the basaltic achondrite planetoid. Geochimica et Cosmochimica Acta 61(15): 3205-3225. (Aug 1997)
Donald Dale Bogard (1995) Impact ages of meteorites: A synthesis. Meteoritics, 30 (3): 244-268. (May 1995)
Donald Dale Bogard & Daniel H. Garrison (2003) 39Ar-40Ar ages of eucrites and thermal history of asteroid 4 Vesta. Meteoritics & Planetary Science 38 (5): 669-710.
Donald Dale Bogard, Larry E. Nyquist, Hiroshi Takeda, Hiroshi Mori, Tohru Aoyama, B. Bansal, Henry Wiesmann & Chi-Yu Shih (1993) Antarctic polymict eucrite Yamato 792769 and the cratering record on the HED parent body. Geochimica et Cosmochimica Acta 57 (9): 2111—2121. (May 1993)
Phillip A. Bland et al. (2009) An Anomalous Basaltic Meteorite from the Innermost Main Belt. Science 325, 1525-28.
Laurie E. Bowman, Michael N. Spilde & James J. Papike (1997) Automated dispersive spectrometer modal analysis applied to diogenites. Meteoritics & Planetary Science 32 (6): 869-875. (Nov 1997)
Paul C. Buchanan & Arch M. Reid (1991) Eucrite and Diogenite Clasts in Three Antarctic Achondrites. Abstracts of the Lunar and Planetary Science Conference 22, 149-150. (Dec 1991)
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Members of the Broward County Library Staff (and, in particular, Amy Wilson) have been especially helpful in my efforts to obtain sources which are not directly available OnLine. My two friends and colleagues, David Judd and Ray Durand, have not only listened patiently when I repeated myself, but have also encouraged me to continue my efforts.
Mineralogical Inventories of the Mineral Groups and Series, Minerals, mineral 'varieties', and other indispensable terms for all 62 Witnessed Falls of the Howardites, Eucrites and Diogenites [HED meteorites] as currently posted with MinDat ('mindat.org).
ABSTRACT
A combined essay and mineralogical inventory of the 62 witnessed HED meteorites falls. Currently 61 mineralogical important items ('Mineralites') are listed at 'mindat.org' Location Sites for one or more HED meteorite falls. The HED meteorites (Howardites, Eucrites, Diogenites) are achondritic or differentiated meteorites whose original homeworlds were large asteroidal bodies. The number of hosted mineralogical items for a single HED meteorite ranges from 5 to 29 (median 13); the number of hosted IMA-defined minerals ranges from 1 to 21 (median 6.5). In addition to presenting linked and annotated lists of the 62 HED witnessed falls and the 61 listed mineralites, the essay contains two pedagogical sections (Sections I & II, Introduction and Overview) and two more technical sections (Sections III & IV, critical evaluations of both the listed mineral Groups and the 45 IMA-defined minerals, respectively. Critical discussion of expected and unexpected differences in the mineralogical assemblages of Eucrites, Howardites, and Diogenites is included. A Bibliography with notes is also included. Present research on HED meteorites is governed by the dawning realization that most — but not all — HED meteorites are indeed fragments of the asteroid 4 Vesta. This essay-and-inventory does not attempt to resolve the issues involved in assigning individual HED meteorites to an emerging 'Main Group' of HED meteorites whose Original Parent Body (OPB) was Vesta or in assigning 'anomalous' HED meteorites to other (presently unknown) homeworlds. The essay is also limited in that only brief attention here is given to important HED 'Finds' — HED meteorites which were recovered at some unknown time after their unobserved fall. We have chosen to concentrate on the falls because they provide 'fresh' extraterrestrial material relatively free of the weathering effects which beset and complicate our ability to understand those exotic meteoritic minerals (e. g., sulfides, sulfates, phosphides,unoxidized metal…) which are most easily altered by crustal oxygen and water. Within these stated limitations, this essay attempts to provide publicly available information indicating where one can investigate the mineralogical evidence which constitutes a critical component of this continuing inquiry.
INDISPENSABLE TERMS:
Achondrite, Breccia, Clinopyroxene, Diogenite, Fe-Ni metal, Eucrite, Falls, Finds, HED meteorite, Homeworld, Howardite, Kamacite, Meteorite, Meteoritic Iron, Meteoroid, Mineral, Mineral Group, Mineralite, Mineralite Number, Orthopyroxene, Plagioclase, Stony meteorite, Taenite, Vesta. Neologisms are occasionally employed.
Mostly for reference purposes. Most terms which are not directly linked to Mindat mineralogical entries are defined in the text, as needed, and again in the Glossary.
Nota bene: The author often uses idiosyncratic spellings in words with unspoken Germanic gutturals(e.g., 'gh', 'ght') when no ambiguity is involved
Section I: Introduction
Section II: The HED Achondritic Meteorites (Howardites-Eucrites-Diogenites)
--List A: The 62 Witnessed HED Falls
Section III: Mineralogical Groups found in HED falls
Section IV: 45 IMA Minerals Found in HED Falls
Section V: Concluding Remarks and Reflections
Glossary and other Terminological issues
--List B: Formal Listing of 61 HED Minerals and 'Mineralites'
Section VI: Bibliography
Acknowlegments
- Section I: Introduction
- Section II: The HED Achondritic Meteorites (Howardites-Eucrites-Diogenites)
- The 35 Eucrite falls.
- The 11 Diogenite Falls
- The 16 Howardite Falls
- The Vesta-plus Connections
- List A: The 62 Witnessed HED Falls
- Sec III: Mineralogical Groups found in HED falls
- __Tally for Pyroxenes
- __Tally for Plagioclase and Feldspar
- __Tally for Silica polymorphs
- __Tally for Olivine
- __Tally for Fe-Ni Metal and other Fe-rich Phases
- Section IV: 44 IMA Minerals Found in HED Falls
- Section V: Concluding Remarks and Reflections
- Glossary and other Terminological Issues
- List B: Formal Listing of 61 HED Minerals and 'Mineralites'
- Section VI: Bibliography
- Acknowledgments
Section I: Introduction
The Howardites, Eucrites, and Diogenites are three closely related classes of achondrite meteorites — collectively known as the HED meteorites. They are normally loosely conglomerated stony meteorites formed on moderately large asteroidal worlds, mixed together by (usually) multiple impact events, and eventually ejected into space by sufficiently violent impacts. Since their fundamental components — their primary minerals — were formed on ancient asteroids over 4.5 billion years ago, collisions, chance, circumstance, and gravitational perturbations have brought a small fraction of the ejecta into the inner solar system where they presently constitute approximately 5% of all witnessed and recovered meteorite falls. Their falls are often dramatic events — accompanied by spectacular fireballs, sonic booms, and/or whizzing sounds — but the recovered meteorites are often unspectacular, usually consisting of a few fragile stones and, perhaps, numerous small fragments. Their loud, brite arrivals and their thin black fusion crusts are often the only obvious sign to the uninitiated that these stones are actually skyrocks — visitors from distant worlds somewhere beyond the earth. Truth to tell, once we can identify a fallen stone as an HED meteorite, we now can usually say something more about them. First, like all well studied meteorites, they are bona fide members of the solar system. Secondly, most or all of them are derived from the 'Asteroid belt' which lies between the orbits of Mars and Jupiter. Third, most — but not all — of these rocks were once part of the asteroid, 4 Vesta, the britest asteroid and the closest of the larger asteroids. But, these, are conclusions — let us turn to the rocks themselves and see how these conclusions have emerged from considerations of their constituents and, most especially, their mineralogy.
Stipulations:
We add some fundamental stipulations to place these skyrocks within the framework of meteoritical research.
One, HED meteorites are called 'achondrites' because — unlike most meteorites — they do not contain small and minute silicate grains called 'chondrules' which are almost never found in earth rocks. However, that very label means they are in many ways much more like earth rocks than most meteorites, especially mineralogically.
Two. Of course, Eucrites, Howardites, and Diogenites are not simply earth rocks — they are extra-terrestrial meteorites. Our missions to the moon have helped us to understand that lunar rocks and HED meteorites are usually 'breccia' (or, at least, 'brecciated'). They are fragmental aggregates altered by countless small and large meteorite impacts. In addition, both HED meteorites and lunar rocks originated on airless and water-free worlds where ferric iron (Fe+++) and hydrated phyllosilicates are virtually non-existent.
Three. In untangling the mix of familiar and exotic minerals present in extraterrestrial materials, it has been gradually discovered that after the formation of a world and subsequent core-formation a number of isotopic and chemical ratios are exceedingly resistant to any further substantial changes. These ratios — especially, O-isotope ratios, Fe/Mn, and Rare Earth Element (REE) ratios — have helped to establish that certain meteorites are from the Moon and others are from Mars. In similar fashion, with additional aid from astronomical observations, it is now very clear that most — but not all — HED meteorites are fragments of the asteroid 4 Vesta. These last developments have also made geochemical considerations unusually important in almost all types of meteoritic research, including mineralogical reports. As we consider our meteorite mineralogical inventories we will find that the character of a 'mineral inventory' is influenced by the underlying queries which have motivated the research.
Section II: The HED Achondritic Meteorites (Howardites-Eucrites-Diogenites)
PRESENTATION ORDER: HED CLASSES; EUCRITES; DIOGENITES; HOWARDITES; VESTA
Prelude: The Skyrocks labelled 'Eucrites,' 'Howardites' and 'Diogenites' — What's in and not in a classificatory name.
Before we even begin our inquiry into the three classes of meteorites labelled 'Eucrites', 'Diogenites', and 'Howardites' and 'Diogenites' it would be best to step back briefly and speak about these particular meteorites simply as stones. For several decades it was customary to speak of the Eucrites and Howardites as basaltic meteorites because, like terrestrial basalts, they were compositionally and mineralogically dominated by various aggregates and intergrowth of pyroxene and plagioclase. The Diogenites, especially the witnessed falls, are compositionally dominated by pyroxene alone and are thus properly considered 'pyroxenites.' After the Apollo missions, it was discovered that the Eucrites and Howardites, in particular, were even closer in many ways to lunar rocks brought back from the moon. The constituent minerals of the Eucrites and Howardites, like those in the moon rocks, were formed in airless, anhydrous lavas where both water and oxygen were — by terrestrial standards — extremely scarce. And after crystallization on or near the surface of airless homeworlds, most moonrocks, Eucrites, and Howardites have been subject to meteorite bombardment and fracturing. All Eucrites, Howardites, and Diogenites are thus, to some extent, breccias. To some extent, observers simply take this into account and do not highlight this brecciation unless there are unusual lithic fragments (clasts), pervasive veining, glass and/or others noteworthy features due to impact shock. Brecciation features — like those sometimes beautiful fusion crusts formed during a meteorite's brief passage thru the atmosphere — are there, but they are (almost always) not the main mineralogical feature. Thus, when we speak about the HED meteorites — meteorites which are either Howardites, Eucrites, or Diogenites — we are speaking about meteorites, normally partially or highly fractured, whose primary minerals and common accessory minerals are, almost always, utterly familiar to terrestrial geologists.
During the 1970's and continuing into following decades, however, several developments occurred. One, both chemistry and mineralogy revealed that:
(1) Eucrites were rocks formed near or moderately near the surface [a few kilometers depth at most] of an original homeworld(s).
(2) the pyroxene-rich Diogenites were plutonic rocks formed well below the surface (~ 10-25 [?] kilometers) surface of an original homeworld(s).
(3) Howardites were mixtures of Eucritic and Diogenitic material seemingly accumulated on the surface of, perhaps, a single homeworld.
Simultaneously, it was also slowly beginning to dawn upon meteoriticists, that there were a number of other meteorites — also rich in basalts and/or other familiar igneous silicates — which were almost certainly formed on other worlds than those one or more worlds which produced the HED meteorites. Most notable, a few basaltic meteorites turned out to be lunar meteorites while a few others turned out to be Martian rocks. Crucial in these developments were the recognition that certain isotopic and elemental ratios could aid in recognizing the determination of a specific homeworld or, 'Original Parent body' (OPB). Not quite as crucial to our discussion but lurking in the background, it is slowly becoming a near certainty that the OPB for most — but not all — HED meteorites is the nearest large asteroid 4 Vesta. (Vide infra!)
With these considerations in mind, then, we look at the mineralogies of the three HED meteorite groups thru the lens of the Meteoritical Community.
The 35 Eucrite falls.
We start with the Eucrites because mineralogically they resemble familiar terrestrial basalts and, both mineralogically and texturally, they resemble the moderately familiar lunar basalts even more. Eucrites are compositionally dominated by pyroxenes and plagioclase [normally > 90 vol%]. Eucrites are named because their constituents are 'well discerned, readily distinguished' [Greek - 'eu', 'kritos']. The predominant moderate-sized pyroxene and plagioclase crystals are readily recognized with a hand lens and, except for their fusion crust, darker minerals which might obscure one's view are quite sparse. The Meteoritical Bulletin Database defines them as follows:
"Eucrites are the most abundant type of basaltic achondrite, linked by geochemical traits such as oxygen isotopic ratios and certain elemental ratios, of which Fe/Mn is the most widely cited. The main minerals in eucrites are Fe-rich pyroxene and Na-poor plagioclase. The eucrites are strongly linked with the diogenites and howardites; the three groups are collectively known as HED meteorites and may come from asteroid 4 Vesta."
We note here that the Fe-rich pyroxene is Fe-rich relative to most meteoritic pyroxenes. The Fe/Mg ratios are usually roughly between 2/1 and 1/2. The predominant pyroxene is usually either pigeonite or orthopyroxene. We also note that the plagioclase is often simply characterized as 'anorthitic plagioclase.' When referring to concentrations or averages it is common to find references to either 'anorthite' [An90—An100] or to 'bytownite' [An70—An90] or to both. The predominant pyroxene is usually either pigeonite or orthopyroxene. The pyroxene textures can be quite complex. The most obvious source of differences are rooted in contrasts in mineralogy and textures of quickly cooling rocks which formed from surface or near-surface lava and slowly cooling rocks which solidified hundreds or thousands of meters below the surface. A second consideration is that rocks can be remelted — at least partially — by powerful impacts which can heat as well as shock any nearby rocks and rock layers. In addition to the simple physics of combined heating, cooling, and shock events, there is an additional complication in regolith composition. Some of the brecciated meteorites which reach us had formed on a surface whose constituents were mostly quite local and compositionally similar. These meteorites are classified as 'monomict eucrites'. Other brecciated eucrites have significant compositional heterogeneities. These meteorites apparently were part of a surface which sampled ejecta from more distant regions with their concomitant geochemical and physical diversity. Eucrites with significant constituent diversity are classified as 'polymict eucrites'. The 'cumulate eucrites' apparently grew most slowly in moderately deep magma chambers and, when ejected, often escaped some of the drastic fracturing that befell most eucrites. It may even be the case that the cumulate eucrites were formed only a few kilometers or so above the pyroxene-rich region(s) that produced the diogenites.
Thirty one of the 35 Eucrite falls are thus further characterized or subdivided as follows:
18 monomict Eucrites (eu-mm), 9 polymict Eucrites (eu-pm); 4 Cumulate Eucrites (eu-cm).
Of the remaining four Eucrites, one is simply described as 'brecciated' and another as 'unbrecciated.' Two are simply described as 'Eucrites' without further characterization. There are a number of interesting Eucrite finds which we note only en passant. A few of them have MinDat location sites. One Eucrite (Camel Donga, a 1984 find from Australia) contains about 2% Fe-rich metal.
The 11 Diogenite Falls
We now consider the Diogenites. As extraterrestrial pyroxene-rich stones they represent a rock type which is not quite as common as the basaltic Eucrites, but is still quite familiar to terrestrial geologists. All Diogenites falls and almost all Diogenite finds are compositionally dominated by orthopyroxene (85-99%). Being essentially monominerallic stones they are well-characterized as extraterrestrial plutonic pyroxenites.
Diogenites are named for Diogenes of Apollonia, a Greek philosopher (circa 424 BCE) who suggested that meteorites were from the heavens:
"With the visible stars revolve stones which are invisible, and for that reason nameless. They often fall on the ground and are extinguished, like the stone star that came down on fire at Aegospotami." [Following Wikipedia here.]
The Meteoritical Bulletin Database defines Diogenites as follows:
"Diogenites are an abundant type of achondrite, linked by geochemical traits such as oxygen isotopic ratios and certain elemental ratios, of which Fe/Mn is the most widely cited. The dominant mineral in diogenites is orthopyroxene. The diogenites are strongly linked with two other achondrite groups: eucrites and howardites; the three groups are collectively known as HED meteorites and may come from asteroid 4 Vesta."
Most Diogenite falls (8 of 11) are simply characterized as 'Diogenites'. 3 are further characterized as polymict Diogenites (diog-pm).
The pyroxene is predominantly orthopyroxene as the original rocks (or rock formations) solidified slowly while ~10 or more kilometers below the surface. Other features are consistent with original formation in a deep magma chamber as we would expect in a plutonic rock. On the basis of the record here, the Ca-rich clinopyroxenes were perhaps a little more likely to equilibrate as diopside than the augite which is found more frequently in Eucrites. (Augite lattices are a little more tolerant of 'impurities' than diopside lattices.) Chromite and ilmenite are minor, but ubiquitous components of both terrestrial plutonic rocks and the Diogenites. However, the nature of the small amounts of iron-rich metal are puzzling (kamacite, some taenite, and some very Ni-poor Iron are found). Whether these instances of unoxidized metal are residues of incomplete core-mantle separation of the original homeworld(s), exotics due to meteorite impacts, or by-products of metamorphic reduction processes with the original homeworld(s) is not at all clear at this time.
While the impacts that lifted the diogenites from their original plutonic sites deep in the original homeworld onto the surface or directly into space would have been relatively powerful, components that are sufficiently distant are sometimes hoisted into space with only moderated fracturing and disruption. Furthermore, being blasted into space with after as a single impact or two may give a solitary plutonic rock a better chance to hide from impact damage than a surface rock which is near the epicenter of multiple large and small impacts. In any case, only a relatively small number of Diogenite falls are classified as 'polymict Diogenites'. This is true of Diogenite finds as well. While it is outside the scope of this study, it should be mentioned that olivine - a common terrestrial plutonic mineral — is a relatively minor component of the Diogenites falls. However, about 6% (10 of 159) of all diogenites - all finds are classified as olivine-rich diogenites. [At least one olivine-rich diogenite, Miller Range 03443, has a Mindat location site.] It may be a merely stochastic feature of the present epoch that all 11 diogenite falls collected in the past two centuries were pyroxenites. A few millennia or hundred megayears from now the fraction of olivine-rich meteorites from the Diogenite homeworld(s) may be significantly different.
The 16 Howardite Falls
While Howardites are the initial component of the HED acronym, there are two good reasons for treating them after discussing the Eucrites and Diogenites. First, Howardites have been created by the commingling of eucritic & diogenitic material by impact — a process which in itself creates additional complexities. Secondly, there are additional components in Howardites which are very rare in the other two types. Compositionally odd eucritic and diogenitic clasts appear to be from different lithologies within the HED homeworld(s) than those represented by the Eucrites and Diogenites in our collection. Furthermore, Howardites are more likely to harbor exotic clasts which are residues of meteoroidal impacts from other non-HED meteorites (e.g., carbonaceous and ordinary chondrites).
Howardites are named after Edward Charles Howard (1774-1816), a British Chemist who helped establish the extra-terrestrial nature of several Fe-Ni metal containing meteorites :
The Meteoritical Bulletin Database defines Howardites as follows:
"Howardites are an abundant group of polymict-breccia achondrites that appear to represent mixtures of eucrites + diogenites (these three linked groups are collectively known as HED meteorites and may come from asteroid 4 Vesta). The main minerals in howardites are pyroxene (largely orthopyroxene) and Na-poor plagioclase. A minority of howardites are rich in solar-wind noble gases and thus inferred to be regolith breccias."
All 16 Howardite falls are simply characterized as 'Howardites'. In actual fact, the working definition for a howardite is that it contains at least 10% Eucritic material and at least 10% Diogenitic material. Whether the mix is 10-90 or 90-10 Eucrite: Diogenite does not affect the definition, but obviously it would affect the mineralogical and geochemical ratios. More difficult are borderline cases with 5-10% exotic components (CM2 material, in particular, may be present at the several % level). As all Diogenites are by definition 2-or-more component breccias, sampling problems can also result in different classifications by different observers.
To a first order one can understand Howardites as mechanical mixtures of Eucrites rich in pigeonite and plagioclase and Diogenites rich in orthopyroxene. An indeed, one finds individual clasts in Howardites that still retain their Eucritic or Diogenitic identity. However, there are several features that require some additional explanations. For one, the range of pyroxene compositions in individual Howardites exceeds the total range observed in both Eucrites and Diogenites. Thus the Howardite Kapoeta has some pyroxene components that are more Mg-rich than those observed in Eucrites and Howardites. (The Howardite find, Monticello, also has some unusually Mg-rich pyroxene). It appears that the Howardites have been enriched by fragments from a wider range of surface area and, perhaps, from other deep regions than we observe in the Eucrites and Diogenites that are in our collections. Secondly, after material from diverse places was brought together on or near the surface of any original homeworld(s) significant fractions have been partially mobilized and equilibrated by additional heating. The heating may have been due of the original radioactive sources which melted significant fractions of many early solar system worlds. Or, the heating may have been due to the material being trapped under hot debris from various impacts. Finally, a significant fraction of exotic material from Carbonaceous Chondrites and other meteorite types has been found in a number of Howardites. In several meteorites it appears that at least some of the material has been involved in more than one scattering impact and in more than one impact heating event. In some cases these events have created inhomogeneities and, in other instances, equilibrating metamorphosis. These processes have created some especially complex textures in those pyroxenes that have alternately cooled and slowly equilibrated and then been shocked-heated and subsequently reequilibrated at varying rates. For example, some orthopyroxenes have two or more generations of thin augite lamellae which has exsolved out of cooling orthopyroxene. Another issue that this author will not attempt to explain is that more or less ordinary meteoritic Fe-Ni metal (kamacite) and some very meteoritically unusual Ni-poor Fe metal is found in very small amounts here and there in some Howardites.
The Vesta-plus Connections
The asteroid 4 Vesta was discovered on 29 March 1807 by Heinrich Wilhelm Olbers. Although it is the britest asteroid, it was the 4th to be discovered - hence its official designation as 4 Vesta. Occasionally barely visible to the naked eye it is the third largest astroid (diameter ~550 km). It is the britest asteroid both because it is the closest of the larger asteroids and its surface is considerably more reflective than most asteroids. And, it is also very unusual because its spectra reveals the prominent presence of ferrous and calcic silicates which are rarely observed in the vast majority of asteroidal spectra. Over four decades ago it was discovered that there were especially strong similarities between the spectra of Vesta and the reflection spectra of those meteorites which we today call Eucrites and Howardites. Approximately two decades ago it was discovered that there is a large set of much smaller asteroids (called 'Vestoids') whose spectral properties are remarkably similar to the those of Vesta and HED meteorites. The orbital properties of the Vestoids suggest that they are a family of Vesta impact ejectites and that a significant number of them are or are well placed to be perturbed into earth-crossing orbits (i.e., potential meteorite sources). During the past three decades parallel studies with oxygen isotope ratios in meteorites (and moon rocks) have proved to be powerful discriminants in narrowing possible parent bodies for a number of meteorite groups. The essential underpinning for such methods is the realization that once a planetary body is formed and roughly equilibrated thru worldwide magmatic differentiation into a core and mantle, oxygen isotope ratios are tightly constrained as changes are mostly mass dependent. Changes in O-16/O-17 ratios are roughly half any changes in O-16/O-18 ratios and vice versa. These studies have revealed — in particular — that lunar rocks and meteorites, Martian meteorites, and HED meteorites have oxygen isotope ratios that separate them from each other and from other meteorites. A number of textural, geochemical, and radiometric measures accompany these oxygen isotope groupings. Finally, the recent DAWN spacecraft mission to Vesta spent several months mapping the surface of Vesta and revealed a surface covered with properties closely matching those of Howardite meteorites, especially, with other regions more closely resembling Eucrites and Howardites. The surface even contains a small admixture of Carbonaceous Chondrite material — material presumably derived from the asteroids responsible for many of the impact features of the current Vesta surface and the brecciated nature of most HED meteorites. There is also a vary large impact crater, Rheasilvia, created somewhat over a billion years ago which may well be related to studies of Ar-40 and other radiometric isotope species which suggest that some Howardites experienced a significant energetic event at approximately the same time. One might even be tempted to consider the case closed.
Actually, the case is somewhat more complicated than suggested above. New very high resolution studies of Oxygen-isotope ratios have revealed that several HED meteorites — including three Eucrite falls — are almost certainly not derived from the same parent body as most HED meteorites. Thus, while most HED meteorites are indeed likely fragments of Vesta — establishing a definitive homeworld identification for specific HED meteorites will in most cases required a more detailed concordance of O-isotopes, Fe/Mn and REE ratios, and chronological information than that presently available. One emerging topic which may aid in establishing definitive homeworld connections is the possibility of determining 'orbital cohorts.' Different isotopic ratios of radioactive elements record different events within a rock or even within different minerals in that rock. Slowly, dating methods are confirming that some HED meteorites — or components of these meteorites — either (1) formed at approximately the same time and/or (2) experienced unusually strong impacts at approximately the same time and/or (3) were exposed to cosmic rays after subsequent impact(s) created a very small meteoroid in an earth-crossing orbit at approximately the same time. At the present time these groupings of shared history are too small in membership and are too imprecise for definitive conclusions — but that may change.
But this is overview, the main theme of our essay is to provide some perspective on the actual mineralogical entities which comprise the HED meteorites. In our conclusions we will offer some additional remarks on the HED-Vesta connection. The continuing attempts to determine which of the HED meteorites are fragments of Vesta and other asteroids is indeed one of the hot topics of the day - with quite interesting complexities. In our bibliography we also provide a number of references to the relevant astronomical and geochemical inquiries which form the backdrop for our more specifically meteoritic and mineralogical inquiry.
List A: The 62 Witnessed HED Falls
The 62 HED meteorites classified as witnessed falls by the Meteoritical Society before November 2014.
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Sec III: Mineralogical Groups found in HED falls
SECTION III (Expanded Title): PRIMARY AND ACCESSORY MINERALOGICAL GROUPS, SUBGROUPS, SERIES, etc. FOUND IN HED FALLS
Featuring: Pyroxenes, Feldspars, Silica Polymorphs, Olivine, Fe-rich Metal, Serpentine, Apatite
The Petrography and Mineralogy of HED meteorites are dominated by minerals within the Pyroxene and Feldspar Mineral Groups. The Pyroxenes include both Orthopyroxenes [Orthorhombic crystal system] and Clinopyroxenes [Monoclinic crystal system]. HED Feldspars are mostly Ca-rich Plagioclase. Both because meteoritic research must often be limited to small samples and because geochemical information is often given greater weight in meteoritical studies than in terrestrial mineralogically focused studies, reports by different research groups on the ingredients of the very same meteorite have different degrees of mineralogical specificity and are sometimes indeterminate or even seriously incomplete from a strictly mineralogical perspective. In particular, listed mineralogically important phases and assemblages of a given meteorite are often reported in terms of overlapping categories of Mineral Groups, Mineral Subgroups, Mineral Series, IMA defined Minerals, or Mineral Varieties. These categories constitute the primary labels utilized at 'mindat.org', but chemically defined terms and other more informal labels are also used. So we will begin our description by looking at the actual frequency of occurrence of some important mineralogical groups (and subgroups, etc.) found in HED meteorites. [All such groups are found in our List B which has definitional links to precise definitions of all such terms.]
In this section — unless otherwise indicated — we combine instances from overlapping categories to give actual sums based on all operative labels that apply. Later, in Section IV we examine the 45 IMA defined minerals found in the HED falls. In that section it is more sensible to adhere to the formal listings without combining items listed from overlapping categories.
A. HED pyroxenes. The Pyroxene Group; Orthopyroxene and Clinopyroxene subgroups; Pigeonite, Augite, Hypersthene, Ferroaugite, etc.
As most Diogenites are pyroxenites (~90% vol% pyroxene) and Howardites are mixtures of Diogenitic material intermingled with Eucritic material dominated by their pyroxene intergrowths, it is obvious that pyroxenes are the dominant mineral group of most HED meteorites. The only exception are those Eucrites and a few rare Howardites which contain slitely more plagioclase than pyroxene. Within the framework of this rather straightforward statement, however, a host of complexities abound. The dominant pyroxene, reported in all Diogenite falls and almost all Diogenite Falls, is mildly ferroan orthopyroxene (Fs ~26 mol%) — which is sometimes referred to as either 'hypersthene' or 'bronzite'. In Eucrites — often cooling quickly after their intrusion onto or near the surface — pigeonite is more common. Naturally enough, in Howardites the relative proportions of orthopyroxene and pigeonite vary considerably. The problem is actually much more complicated than this. In a number of HED meteorites slow re-equilibration of initially formed pyroxenes occurred — often leading to exsolution of augite lamellae and/or to partial or complete inversion of the pigeonite into orthopyroxene. Furthermore, foreign clasts of significantly different pyroxene composition are always a possibility when dealing with these brecciated rocks which have, usually, experienced multiple impacts since the initial crystallization of these rocks. We provide here, then, only a rough indication — a sample — of the embedded mineralogical narratives waiting to be further decoded.
Augite is reported in a majority of Eucrites and Howardites and in some Diogenites. Augite is a primary mineral in some instances, but is usually found as one or more generations of thin or very thin augite exsolution lamellae. In addition, to different cooling rates at different depths during the formation of parent body crust, heating episodes triggered by impact events have also been followed by subsequent exsolution events. The actual number of HED meteorites with augite lamellae is almost certainly underreported as many accounts note the presence of Ca-rich clinopyroxene or Ca-rich pyroxene. In most instances such Ca-rich pyroxene is probably augite, especially in Eucrites. With Diogenites the possibility of diopside is somewhat more likely.
The augite in HED meteorites is often variable, but the presence of ferroan augite — sometimes labelled 'ferroaugite' is specifically noted in a number of instances. Hedenbergite (FeCaSi2O6) is also occasionally found. In a number of instances detailed studies show significant scatter in both Ca-poor and Ca-rich Clinopyroxenes (pigeonite, diopside, augite, hedenbergite) which usually range from somewhat magnesian to mildly ferroan or even strongly ferroan in composition.
Very small amounts of Enstatite and/or Clinoenstatite are also reported for 3 Howardites. These small and chemically anomalous 'exotics' — absent detailed evidence to the contrary — suggest that the regolithic surface of Vesta and/or any other HED parent body has been enriched by an Enstatite-rich component during its the asteroidal bombardment. Ferrosilite, unusually Fe-rich pyroxene, is also reported for 2 Howardites, but would be a more likely import from Carbonaceous Chondrites.
__Tally for Pyroxenes
PYROXENE TALLIES (Actual instances):
'Orthopyroxene' is reported in 48 HED falls (22 Eucrites, 15 Howardites, (all) 11 Diogenites)
Pigeonite is reported in 43 (23 Eucrites, 9 Howardites, 4 Diogenites)
Ca-rich Pyroxenes are reported in 39 HED falls: (23 Eucrites, 10 Howardites, 6 Diogenites)
Augite is reported in 36 HED falls (35 times as 'augite')
'Hypersthene, a variety of Orthopyroxene, is specifically noted 16 times
Diopside is reported in 7 HED falls. including 4 Diogenites
'Ferroaugite,' a variety of Augite, is specifically noted 7 times (5 times in eucrites)
'Bronzite', a variety of Orthopyroxene, is specifically noted 5 times
Hedenbergite is reported in 4 HED falls
Enstatite is reported in 2 Howardite falls (Bholghati, Jodzie)
Clinoenstatite is reported in 2 Howardite falls (Bholghati, Washougal)
Ferrosilite is reported in 2 Howardite falls (Jodzie, Pavlovka)
B. The HED Feldspar Group and Plagioclase Series. And, Albite, Anorthite, Bytownite, Labradorite, Maskelynite
The Feldspar Group is an extremely important Mineral Group for both terrestrial and HED mineralogy, but there is one striking geochemical difference crucial to our discussion. While Potassium-rich members of the Feldspar Group (e.g., microcline, orthoclase) are dominant components of many terrestrial lithologies, meteoritic feldspars are almost always K-poor. Consequently in the meteoritic literature, discussion of Alkali-Aluminum-rich tektosilicates is often restricted to consideration of the plagioclase series. This is particularly true for HED meteorites where both feldpathoids and K-rich phases are in short supply. Thus our discussion begins with the Plagioclase Series [Note: In the mindat.org syllabus 'Plagioclase' is treated as an exact synonym for the 'Albite-Anorthite Series'].
By definition Eucrites are dominated by pyroxene-plagioclase combinations and so, unsurprising, plagioclase is listed for all 35 Eucrite falls and all 16 Howardite falls. While plagioclase is only a minor component in Diogenites, it is still recorded for 9 of the 11 Diogenite falls. In the meteoritical literature one is more likely than not to read that a specific Eucrites or Howardite contains 'anorthitic plagioclase' — this phrase conveniently elides any explicit attention to the precise use of either anorthite (An70-An90) or 'bytownite (An70-An90) as a leading entry. The average Anorthite content is usually near the conventional dividing line between the two members of the series and these averages often fluctuate when reported by different researchers. Indeed, Plagioclase (as a stand alone label) is explicitly listed as a constituent of 57 of the HED meteorites. In three HED meteorites 'anorthite' or 'bytownite' are listed instead. However, both reporting tastes and sampling issues are involved as well. Anorthite and Bytownite are explicitly mentioned in 33 and 30 instances, respectively. Presumably, in these instances the reporter or report interpreter wished to emphasize that some regions of the meteorite were predominantly anorthite or bytownite, respectively. Both Anorthite and Bytownite are explicitly mentioned for the same meteorite in 16 instances. In these cases, we are almost certainly dealing with the sometimes quite variable plagioclase composition found in many HED meteorites. While the numbers are too small for definitive conclusions, the fact that 4 of the 8 Eucrites reporting both anorthite and bytownite are polymict eucrites (presumably sampling larger regions of the homeworld) is suggestive. The single instances of Labradorite (An50-70) in the Bilanga Diogenite and Andesine (An30-50) in the Washougal Howardite also suggest that there may have been more albite-rich regions in the homeworld(s) than our usual samples suggest. This is, however, only a suggestion as small pockets of impact melt can produce atypical compositions almost anywhere on a regolith dominated world.
Feldspar is specifically noted in 7 instances. There is a tendency for some authors to use the term Feldspar for any form of albite-rich plagioclase. The author himself avoids the term 'Feldspar' unless the Orthoclase component is greater than the Anorthite component or seems particularly significant. The K-Feldspar recorded in the Bilanga Diogenite is, compositionally at least, 98.5 % orthoclase and is described as 'exotic'. It is 'exotic' — but the unanswered question is still "Where did it come from from?" Were there any places on the Bilanga Parent Body where such K-rich phases were significant? For that matter, what sort of impacting small asteroid would contain such mineralogical assemblages?
__Tally for Plagioclase and Feldspar
PLAGIOCLASE AND FELDSPAR TALLIES (Actual instances):
Members of the 'Plagioclase Series' are recorded in 60 of the 62 HED Falls. (35 Eucrites, 16 Howardites, 9 Diogenites). [No Plagioclase is reported for Tatahouine and Ellemeet (Diogenites)]
End Members and the Series variants are often explicitly mentioned — Anorthite and Bytownite in 33 and 30 instances, respectively. The presence of both Anorthite and Bytownite in the same meteorite is explicitly noted in 16 instances. Eucrites (8 instances with 4 polymict Eucrites), Howardites (5 instances) and Diogenites (3 instances)
'Andesine' and 'Labradorite' varieties are mentioned in single instances (Washougal; Bilanga).
Maskelynite, glass of Plagioclase composition, is present in at least in 5 instances.
The Feldspar Group is noted in 7 HED Falls - Either as the Feldspar group (6) or as K-Feldspar (1).
Major and minor differences between the three meteorite classes for Feldspar, Plagioclase, end members, varieties, and derivative phases.
Overall, the differences in chemical composition of plagioclase in the three groups is somewhat blurred. However, Anorthite is specifically noted in 75% of Howardites Falls [12 of 16], but the percentages are significantly smaller for Eucrites (51%) [18 of 35] and for Diogenites 25% [3 of 12].
For Eucrites, Plagioclase (etc.) occurs as Anorthite (18), Bytownite (15) Maskelynite (4). Feldspar is noted on 3 occasions.
For Howardites, Plagioclase (etc.) occurs as Anorthite (12), Bytownite (8), Andesine (1), Maskelynite (1) instances. Feldspar is noted on 3 occasions.
For Diogenites, Plagioclase (etc.) occurs as Anorthite (3 instances), Bytownite (7) and Labradorite (1). Feldspar is noted on 1 occasion as K-Feldspar.
Both Anorthite and Bytownite are explicitly noted in the same meteorite in Eucrites (8 instances), including 4 polymict Eucrites, Howardites (5 instances), and Diogenites (3 instances)
C. Silica and the Silica Polymorphs Tridymite, Quartz, Cristobalite in HED falls
Polymorphs of silica are ubiquitous in HED meteorites — usually as very small grains with at least one silica phase recorded for 54 of the 62 HED falls. The recognition of a spike in silicon abundance using the microprobe makes chemical detection of silica phases relatively routine. However, determining the mineralogical identity of such phases is normally time consuming and often difficult or impossible. We note that silica polymorphs are present in 36 HED meteorites (20 Eucrites, 11 Howardites, and 3 Diogenites). Tridymite, recorded in 24 HED meteorites, is the most common identified polymorph. Tridymite is found in 10 of 16 Howardite Falls (62.5%), but only in 12 of 35 Eucrite Falls (34%). On the other hand Quartz is recorded in 9 HED meteorites - all but one of them Eucrites. Cristobalite has been recorded only in Tatahouine. In 5 instances both quartz and tridymite are found in the same meteorite. The only obvious trends here are that (1) silica phases are apparently less abundant in Diogenites than in the other two HED classes and (2) conditions for quartz formation appear to have been a little more favorable in Eucrites than in the other HED classes. Silica often appears in small aggregates of glass-containing impact melt. In this connection we note that some form of silica is found in all 7 polymict eucrites.
__Tally for Silica polymorphs
SILICA TALLIES (Actual instances):
Either 'Silica' or an identified polymorph of silica has been found in 54 HED meteorites
'Silica' is specifically recorded as Tridymite in 24 instances, as Quartz in 9 instances & as Cristobalite in 1 instance.
Subtallies for Individual Meteorite Groups:
'Silica' is found in 32 Eucrites; Quartz is noted in 8 instances, Tridymite in 13.
'Silica' is found in 13 Howardites; Quartz is noted in 1 instance, Tridymite in 10.
'Silica' is found in 9 Diogenites; Tridymite is noted in 2 instances, Cristobalite in 1.
Notes: Almost all HED quartz is found in Eucrites; Both Quartz and Tridymite are recorded in 5 meteorites.
D. Olivine in HED falls: Olivine = 'The Olivine series' ; (rarely, End members Forsterite and Fayalite).
In silicate melts derived from 'normal' Solar System materials with chondritic or near chondritic composition, forsteritic or Mg-rich olivine is a major component in the earliest phases which separate from the melt. In the mineralogies of meteorites derived from small asteroids (Ordinary Chondrites, Carbonaceous Chondrites) olivine is usually an important and sometimes the major phase. In the achondritic Brachinites and stony-iron Pallasites olivine is the dominant silicate. On the larger terrestrial worlds olivine-dominated rocks are usually plutonic in origin and are termed dunites when olivine is >90% of the rock. The analogous martian Chassignites are usually defined simply as 'martian dunites.' The absence of olivine-dominated rocks among HED falls has been something of a puzzle as the pyroxene-rich Diogenites are clearly of plutonic origin and where there are pyroxenites one might expect even more dunites. Results from the DAWN Mission suggest that some impacts apparently reached deep enough into Vesta's mantle to excavate large amounts of pyroxene, but perhaps not quite deep enough to excavate an equally volumetrically significant olivine component. The problem could be simply a matter of collisional circumstance. 382 Diogenite meteorites are currently listed at the Meteoritic Society's 'Meteoritical Bulletin Database' [Oct 2014] and 11 are designated as 'Olivine Diogenites.' In addition there are a small number of polymict Diogenites which may also contain significant amounts of olivine. The problem of designating the proper boundaries for HED classification is currently in a state of flux. Thus determining what is 'anomalous' and what is 'normal' about HED meteorites (in this case 'Diogenites') may remain a problem for some time. In the meantime, we return to the reported presence of minor amounts of olivine in precisely half of our 62 HED falls.
In HED meteorites olivine is recorded in 31 of the 62 Falls. Olivine is most likely to be found in Howardites (93%) [15 of 16], fairly likely to be found in Diogenites 63% [7 of 11], and least likely to be found in Eucrites (93%) [9 of 35]. End member variants, Forsterite [Mg-rich Olivine] and Fayalite [Fe-rich Olivine], are noted only in a few Howardites. One suspects that much of the olivine has been kicked around the homebody/homebodies by impacts. Otherwise, one would expect to find more olivine in the plutonic Diogenites than the partially plutonic/partially crustal Howardites. In this connection we note that more than half of the polymict Eucrites (4 of 7) contain olivine. Indeed, the Forsterite and Fayalite found in 4 Howardites may be truly 'exotic' olivine brought in by material ultimately derived from other impacting asteroids.
__Tally for Olivine
OLIVINE TALLIES (Actual instances):
Olivine has been reported in 31 HED falls.
End Members Forsterite (3 instances) and Fayalite (1) were reported only in Howardites.
Subtallies for Individual Meteorite Groups:
Olivine has been reported in 9 Eucrite falls,15 Howardite falls, and 7 Diogenite falls.
E. Fe-Ni Metal and other Forms of Unoxidized or Metallic Iron in HED falls
In Meteorites unoxidized or metallic iron is almost invariably accompanied by nickel (Fe-Ni metal). This most common form of unoxidized, meteoritic iron is usually labelled as Fe-Ni Metal (or some equivalent term) in the meteoritical, geochemical, and planetary science literature. Its origin is well understood — most of the universe's iron and nickel are produced in tandem during - quite literally - the last few moments before a supernova explosion and during the first hours or so of the supernova's hyper-energetic blast from the core of the exploding star up and out into the surrounding vacuum of space. During the formation of the Solar System a significant amount of this iron and accompanying nickel was incorporated into different solar system bodies — sometimes as Fe-Ni metal, iron oxides, sulfides and other compounds. Most of the earth's iron and nickel are locked up in the earth's core, but Fe-Ni metal is readily obtained from 'iron meteorites' and as tiny grains in most chondritic meteorites. Almost all meteoritic Fe-Ni metal exists as either kamacite or as a kamacite-taenite intergrowth. Kamacite has a body-centered cubical crystal structure consisting almost entirely of iron plus 4-8% Ni and .5-1.0% Co. Taenite has a face-centered cubical crystal structure and contains considerably more Nickel. There are some complications, of course, meteoritic iron may be, mineralogically speaking, somewhat disordered due to preterrestrial shocks and/or earth impact. And, recent high resolution studies show that taenite often — perhaps usually — contains small micro-domains of tetrataenite and other forms of Fe-and-Ni rich metal. Indeed, except for obviously artificial forms of iron, nickel and cobalt, almost all unoxidized metallic iron encountered on or near the earth's surface consists primarily of kamacite or kamacite-taenite intergrowths.
However, while a significant of the unoxidized iron in HED meteorites is kamacite or kamacite with taenite and/or tetrataenite, this is not true for much — perhaps most — of their unoxidized iron. As the largest single exception to the general rule that most meteoritic iron is Fe-Ni iron, a significant number of HED meteorites contain iron which is nickel poor. The problem is complicated by some terminological issues. Some meteoriticists describe almost any form of unoxidized iron as Fe-Ni metal or, when not Ni-rich, as kamacite. And, on the other hand, the International Mineralogical Society has decided that rare forms of Ni-poor cubical iron discovered in the past 2 centuries is the mineral 'Iron' while the more abundant kamacite studied by wizards, alchemists, metallurgists, and scientists for the past three millennia is a mere 'variety' of said 'Iron.' Let us try to sort out what this means for the unoxidized iron which may or may not be properly described as 'Fe-Ni metal.'
The scientific problem which seems most important to this observer is whether all, some, or none of either the Ni-poor iron or the meteoritically more 'normal' Fe-Ni metal was formed as part of the overall process which created Vesta and other HED Parent Bodies or whether it is 'exotic' material introduced by subsequent impacts with other asteroids. If Vesta formed in analogous fashion to the earth [Klaus Keil has described Vesta as 'the smallest terrestrial planet'], then most of Vesta's iron and nickel would have been incorporated into Vesta's core. [The DAWN Mission has established that Vesta has a core.] Similar considerations could possibly apply to the other large asteroidal homeworlds. According to the usual simplified models, almost all of the nickel in the proto-Vesta would have been incorporated into the core, but a significant fraction of the iron would have joined the silicate rich mantle and crust. If this were true, any Fe-Ni metal assemblages would be due to subsequent meteoritic bombardment. That is all reasonable enough, but it is not at all clear just how 'clean' such an original differentiation would have been. On the earth today, gas bubbling up in volcanic events contain odd xenon-isotopic ratios which did not rise to the top of any original magma ocean over 4 billion years ago. Instead, differentially marked by decay of radioactive isotopes which occurred over 4.5 billion years ago, this gas is only now separating itself from its original reservoirs.
As for the nearly pure iron now found in HED meteorites, one might imagine that residual reducing H-rich and/or CO-rich gas from the original solar nebula might have been at work during formation of Vesta and/or other parent bodies outer layers - but that seems purely conjectural at present. As somewhat analogous processes may have been at work on both the moon and Mars where Ni-poor iron has also been observed, it is clear that there are many unanswered questions. We turn, then, to what the record tells us.
Metallic iron as Fe-Ni metal or as Ni-poor iron is listed for 35 HED meteorites. The actual number is somewhat higher than that because in several instances authors report the presence of either iron or Fe-Ni metal without providing the actual percentage of nickel in the iron. Without that information the scientific value of any purely mineralogical record is compromised (vide infra). Still, the presently available record makes it possible to make one potentially very important observation. Iron — either as Ni-poor Iron or as Ni-containing Kamacite — is reported for all 11 of the plutonic diogenites, but for only 25% of the crustal Eucrites (14 of 35) Eucrites. This provides prima facie evidence that a significant quantity of the metallic iron is indeed indigenous iron and not impact- introduced exotica.
'Iron' as the stand alone IMA defined 'mineral' for body-centered Fe-rich metal is listed for 24 HED falls (8 Howardites, 10 Eucrites, 6 Diogenites). Most of these instances appear to be cases of Ni-poor iron, but the actual number of Ni-poor iron locations can only be tentative at this point. [The articles by Duke (1965) and Gooley and Moore (1976) give detailed accounts of metal composition of HED meteorites, but these are exceptions.] This author normally posts Ni-poor iron under the label 'Iron' with the Ni% included under the 'Mineralogical Details'. My own standard for reporting Ni-poor iron is Ni ≤ 2% (several instance of Ni < 0.5% are noted at individual meteorite Location Sites), but I know of no common standard.
Fe-Ni Metal is reported for 21 HED falls (9 Howardites, 6 Eucrites, 6 Diogenites). In 18 cases the presence of Kamacite is explicitly noticed. 'Meteoritic iron' is also reported in 3 additional meteorites in which 'kamacite' is not explicitly mentioned (Jodzie, Kirbyville, Macibini). 'Meteoritic iron' is often used when definitive mineralogical classification is not available. In this case, it should be noted that two of the meteorites mentioned above (Jodzie, Kirbyville) are very small. In 13 cases Taenite and/or Tetrataenite are associated with and sometimes adjoining the Kamacite. Thus, a first read suggests that instances of unusually Ni-poor iron and meteorite 'normal' Fe-Ni metal are roughly equal.
__Tally for Fe-Ni Metal and other Fe-rich Phases
TALLY OF IRON-RICH PHASES (Actual instances):
Iron-rich metal is found in 35 HED falls (10 Howardites, 14 Eucrites, 11 Diogenites)
Fe-Ni Metal found in 21 HED falls (9 Howardites, 6 Eucrites, 6 Diogenites)
Formal Inventories:
Iron, possibly Ni-poor, is reported in 24 HED falls (8 Howardites, 10 Eucrites, 6 Diogenites)
'Kamacite' is reported in 18 HED falls (8 Howardites, 4 Eucrites, 6 Diogenites)
Taenite is reported in 12 HED falls (6 Howardites, 2 Eucrites, 4 Diogenites)
'Meteoritic-Iron' is reported in 6 HED falls (2 Howardites, 2 Eucrites, 2 Diogenites)
'Nickel-iron' is reported in 2 HED falls (2 Diogenites) [Possible redundant references to Taenite?]
Tetrataenite is reported in 2 HED falls (2 Diogenites)
F. Serpentine in HED falls (Howardites)
Phyllosilicates are only occasionally mentioned with respect to HED meteorites and, particularly in finds, may be products of terrestrial weathering products. However, Serpentine and minerals within the Serpentine Group are reported from 4 Howardites (Bholgati, Kapoeta, Jodzie, and Erevan). Specific instances include Cronstedtite (Erevan) and Antigorite (Bholgati). Saponite, another phyllosilicate and tochilinite, a hydrated sulfide, may also be found in associated assemblages. Work by Michael Zolensky and others (summarized in Zolensky et al., 1996) has established that there are pervasive instances of carbonaceous chondrite material in some Howardites (especially CM2 clasts and, to a lesser extent, CR2 clasts).
COMPLETE TALLY OF SERPENTINE GROUP:
Serpentine Group or minerals reported in 4 Howardites (Bholgati, Erevan, Jodzie, Kapoeta)
Cronstedtite reported from the Erevan Howardite.
Antigorite reported from the Bholgati Howardite.
G. The Apatite Association
Phosphates, particularly Ca-rich phosphates, are frequently reported in the meteoritical literature. When mineralogically identified they are almost always either apatite or merrillite. They are often — usually, I would hazard — too small to mineralogically characterize on a routine basis. Also, there have been some unsuccessful attempts to impose new standard definitions for mineral phosphates that were later rescinded. Recently, prompted especially by the usefulness of even small amounts of various radiometrically useful elements which tend to concentrate in phosphates some targeted efforts have resulted in the identification of apatite — sometimes specifically as fluorapatite — and occasionally associated merrillite. A close read of the actual data reveals that the apatite is indeed often properly characterized as fluorapatite, but varying amounts of a minor chlorapatite component are usually present as well.
We add only a little minor commentary. There seems to be little indication of hydroxyapatite, a common terrestrial component of apatite. On the other hand, phosphides are very rare in HED meteorites [Both barringerite and schreibersite are reported in two instances, but these could easily be of exotic origin]. Thus it is seems that the original HED formation epoch was largely anhydrous and characterized by an intermediate oxygen fugacity (compared to say, most iron meteorites on the one hand, or to Martian meteorites on the other.) How similar the HED formation epoch was to the formation epoch of the lunar highlands and the lunar basalts is probably both a more interesting and a more difficult question.
COMPLETE TALLY OF THE APATITE ASSOCIATION:
'Apatite', listed either as 'Apatite' or as Fluorapatite, is recorded in 13 HED meteorites (10 Eucrites & 3 Diogenites).
Fluorapatite is specificly listed in 8 HED meteorites (5 Eucrites & 3 Diogenites).
Section IV: 44 IMA Minerals Found in HED Falls
Section IV: 44 IMA defined Minerals found in HED Falls (abc order); a TABLE of Occurrence Frequency; Commentary on each individual mineral.
This table contains only those 44 mineralogical phases which (1) have been deemed by the International Mineralogical Association (IMA) as properly crystallographically and chemically defined 'minerals' and (2) are currently listed at 'mindat.org' location sites for one or more of the 62 witnessed HED meteorite falls. Each IMA defined mineral is accompanied (in parenthesis) by the number of HED falls where the mineral has been reported. References for these reports are found at the individual meteorite 'Location Sites.' The author makes no claim of completeness, but this should usually provide an introductory sample of those minerals which we would expect to see in HED meteorites, those which we might see, and those which we would be surprised to see in a moderately large HED meteorite.
[Caveat: In Section III we indicated a few mineral phases which are most likely to be underreported. One could say, of course, that as every meteorite fall is itself a fortuitous event, we have been forewarned that sampling problems are inherent in the meteoritic enterprise ab initio. E.]
In the commentary which follows we attempt to put provide additional context for the individual minerals. We, of course, give some attention to some of the expected differences between the shallower-sourced Eucrites and the plutonic Diogenites. However, some readers may be surprised to see that the Howardites appear to contain a broader source of constituents than either the Eucrites or Diogenites. The Howardites appear not only to sample a wider range in composition among, presumably, lithic constituents from various regions of the original homeworld(s), but they also seem to sample a significantly larger share of exotics — minerals derived from other meteorite types. A few high temperature minerals (e.g., perovskite) and very reduced phases (e.g., Forsterite) are likely imports from CAI-bearing carbonaceous chondrites and Enstatite-rich chondrites-and-achondrites, respectively. Carbonates and some of the hydrated silicates of likely CM chondrite origin also appear to warrant special attention. On the other hand, we do not pursue here in any real detail the interesting issue of mineralogical variations between HED subtypes (e.g., polymict versus monomict Eucrites). For that the reader needs to consult the original literature.
The issue of what constitutes an 'exotic' is replete with informative problematics. A true 'exotic' would be an imported phase or assemblage brought to the meteorite's original homeworld by an impacting meteoroid. In some cases the carbonates and phyllosilicates are components of carbonaceous clasts which can even be identified by meteorite type. For example, CM2 Carbonaceous clasts have been found in a number of HED and other meteorite types. On the other hand, HED sulfides are a bit more complicated. The presence of Fe-rich sulfides other than troilite may reflect unusual oxygen fugacities signaling the presence of a true exotic. Thus, for example, one is more likely to find pyrrhotite and pentlandite in somewhat hydrated carbonaceous chondrites than in irons, most chondrites, and most achondrites. However, their occasional detection in HED meteorites may reflect fugacity variations within the HED parent worlds as well as exotic addenda. In these cases, it is best to consult the original observations referenced in the host meteorite's Mindat location site. In addition, several of the articles in the Bibliography represent the best in contemporary amalgams of excellent science coupled to mineralogical art. [The underlying constraint, of course, is that it is a lot easier to recognize a meteorite which has fallen to earth as a geochemical stranger than it is to determine which mineralogical oddities within an HED meteorite were themselves transported from another meteoroid to the HED homeworld(s).]
TABLE: 44 IMA defined Minerals and Number of HED Listings (= # of HED hosts)
44 Minerals and Number of HED Listings | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Anorthite (32) - Anorthite is formally reported for 32 HED meteorites. Anorthite is explicitly recorded in all 6 cumulate Eucrite finds.
Antigorite (1) - Antigorite, a hydrated phyllosilicate in the Serpentine Group, is formally reported only for the Bholghati Howardite.
Augite (35) - Augite is formally reported for 35 HED meteorites.
Baddeleyite (2) - Baddeleyite has been formally recorded here only for Millbillillie, a main group Eucrite, and Pasamonte, an anomalous Eucrite.
Barringerite (2) - Barringerite, a rare meteoritic phosphide, is formally reported here only for 2 Howardites, Erevan and Jodzie.
Calcite (3) - Calcite is formally reported for 3 HED meteorites, Erevan, Kapoeta, Tatahouine — all Howardites. In Tatahouine it appears to be produced by terrestrial weathering. Otherwise, it appears to be a carbonaceous exotic.
Chalcopyrite (5) - Chalcopyrite is formally reported here for 5 HED meteorites and only as a very minor accessory in 2 diogenites and 3 Howardites.
Cristobalite (1) - Cristobalite has been formally reported only for the Tatahouine Diogenite.
Chromite (54) - Chromite is formally reported for 54 HED meteorites. The only HED Meteorites not listed are 6 eucrites (Bialystok, Brient, Emmaville, Jonzac, Lakangaon, Piplia Kalan) and 2 Howardites (Chaves, Lohawat).
Cronstedtite (1) - Cronstedtite, a hydrated phyllosilicate in the Serpentine Group, has been formally reported only for the Erevan Howardite.
Diopside (7) - Diopside is reported for Bilanga, Garland, and Johnstown (diogenites); Jodzie, Kapoeta, Roda (Howardites) and Piplia Kalan (eucrite)
Dolomite (1) - Dolomite has been reported only from the Erevan Howardite.
Fayalite (1) - Fayalite, Fe-rich olivine, is reported only for the Jodzie Howardite.
Ferrosilite (2) - Ferrosilite , Fe-rich pyroxene, is reported for the Jodzie and Pavlovka Howardites
Fluorapatite (8) - Fluorapatite has been found in the Chervony Kut, Juvinas, Pasamonte, Serra de Magé and Stannern Eucrites as well as the Manegaon, Roda and Shalka Diogenites.
Forsterite (3) - Forsterite, Fe-poor olivine, is found in the Bholghati, Erevan and Kapoeta Howardites
Hedenbergite (4) - Hedenbergite is found in the Macibini and Nobleborough Eucrites and in the Kapoeta and Pavlovka Howardites
Ilmenite (46) - Ilmenite in small amounts is ubiquitous in HED meteorites. It is often associated with chromite.
Iron (26) - Iron and, specifically, Ni-poor Iron (Ni<2%) is found in small quantities in many HED meteorites. This author uses the term 'Iron' as a mineral name only for Fe-rich metal that is Ni-poor.
Mackinawite (2) - Mackinawite, a tetragonal Fe-Ni sulfide, is reported only for the Roda and Johnstown Diogenites.
Magnetite (5) - Magnetite is reported for 4 Howardites (Bholghati, Chaves Jodzie, Kapoeta) and 1 Eucrite, Peramiho. It is very easy to imagine both indigenous and exotic sources for this mildly oxidized phase which contains both ferric (Fe+++) and ferrous Fe++) iron.
Merrillite (6) - Merrillite reported only for Roda and Ibitira. On occasion merrillite has been simply defined both formally and operationally as 'extraterrestrial whitlockite'. However, detailed crystallographical studies suggest that there are minor structural differences — presumably this is at least partially because truly anhydrous minerals are less likely in almost any terrestrial setting than on worlds without atmospheres or liquid water.
Oldhamite (1) - Oldhamite is reported only for the Johnstown Diogenite. An unexpected and extremely reduced phase. However, the observation was made by none other than Paul Ramdohr.
Pentlandite (8) - Pentlandite is reported for the Bholghati, IbbenbĂĽren, Jodzie and Kapoeta Howardites as well as Bilanga, Peckelsheim, Roda and Tatahouine Diogenites. The presence of pentlandite often suggests a higher oxygen fugacity than the many meteoritical environments in which troilite is the only reported Fe-rich sulfide.
Perovskite (1) - Perovskite is reported only for the Kapoeta Howardite. Perovskite is a high temperature product.
Pigeonite (43) - Pigeonite is common in HED meteorites. Whille most common in quickly cooling lavas, it is found in most eucrite cumulates. On the other hand it is found in only 3 of the 11 Diogenite falls.
Pyrrhotite (4) - Pyrrhotite is reported for the Bholghati, Jodzie, & Kapoeta Howardites as well as the Chervony Kut Eucrite
Quartz (10) - Quartz is reported for 10 HED meteorites (including 9 eucrites): Chaves (How); Béréba, Jonzac, Juvinas, Peramiho, Stannern, Vetluga (Eu-mm); Pasamonte (Eu-pm); Serra de Magé, Vissannapeta (Eu-cum)
Saponite (3) - Saponite, a phyllosilicate, has been reported only in 3 Howardites.
Schreibersite (2) Schreibersite is reported in two diogenites, Peckelsheim and IbbenbĂĽren.
Spinel (7) - Spinel is listed for 4 Eucrite falls and 3 Howardite Falls
Taenite (12) - Taenite is listed for 2 Eucrite falls, 6 Howardites (Bholghati, Bununu, Kapoeta, Luotolax, Yurtuk, Zmenj), 4 Diogenites (IbbenbĂĽren, Johnstown, Peckelsheim, Roda)
Tetrataenite (2) - Tetrataenite is reported only for the two diogenites, Bilanga and Peckelsheim.
Titanite (1) - Titanite has only been reported for the polymict Diogenite, Garland
Tochilinite (3) - Tochilinite, a hydroxysulfide, has only been found in 3 Howardites.
Tridymite (25) - Tridymite is listed for 12 Eucrite falls, 10 Howardites, and 3 Diogenites. As the most common form of silica in HED meteorites, it is likely that many small and tiny grains identified as only as 'silica' by the microprobe are instances of unreported tridymite.
Troilite (47) - Troilite is listed for 25 Eucrite falls, 10 Diogenites & 12 Howardites
Ulvöspinel (3) - Ulvöspinel is listed only for 3 Eucrite falls
Whitlockite (2) - Whitlockite is reported only for one Eucrite (Ibitira) and one Diogenite (Roda).
Zircon (9) - is listed for 9 Eucrite falls. A targeted phase which is unusually useful for chronological studies.
Zirconolite (1) is reported only for the anomalous Eucrite, Pasamonte
Section V: Concluding Remarks and Reflections
Concluding Reflections: Lithological and Personal
Earlier this year I began a project to post via Mindat mineralogical inventories of the 61 witnessed meteorite falls from the Howardites, Eucrites, and Diogenite meteorite classes — those meteorites commonly referred to as the HED meteorites. My purpose was to put into a relatively accessible public format a useful introduction to what is known about the mineralogy of these particular meteorites. I restricted myself to witnessed falls in order to reduce the work into a personally manageable task. It is, of course, quite true that much of the material I have used is in the 'public record' as the term is often used. However, a considerable amount of such 'publicly available' information is contained in proprietary journals which are often difficult or expensive for many people to access. But, in any case, I managed to place this material on line within about 3 months. Many of the meteorites already had 'Location Sites' at Mindat and, when sites were not available, I was able to create them for whatever they are worth.
It then seemed reasonable to me that I should organize the implications of what I had done into what became the present essay. This effort presented more challenges than I expected and took nearly twice as long to complete as the first task. I suppose that means that it is often more difficult to know why one has done something than it is to actually do the thing itself. In any case, I summarize the meaning of these efforts with the following four paragraphs.
The meteorites which are today known as the HED achondrites were once considered a subset of a larger class of meteorites often known simply as the 'basaltic meteorites' or as 'basaltic achondrites.' They were called basaltic because their dominant minerals — pyroxene, plagioclase, and lesser olivine — are the dominant minerals of terrestrial basalts. Gradually, it has become clear that some of these meteorites are fragments of identifiable worlds. At the same time our meteorite collections grew enormously as meteorites from Antarctica and from hot deserts were identified in ever increasing numbers. We have gradually established that a few of these meteorites definitely came from Mars and a few others definitely came from the moon. At the same time it also gradually became clear that several meteorite groups also came from specific worlds — but, unfortunately, it is not clear what world is there original one. That is the situation we face today. Most meteoriticists believe — or will adapt as a working hypothesis — the notion that all or almost all of the members of such groups as the Angrites, the Aubrites, the Main Group Pallasites, the Mesosiderites, and the HED meteorites are in each case most likely derived from a single Original Parent Body (OPB). These beliefs or working hypothesis are not held by all and they are not formal credos, but they do define the current goals of current geochemical meteoritic and some asteroidal research. The Holy Grail of much contemporary works is to find and establish the OPB for as many meteorite groups as possible.
There are, however, two very important considerations which are acknowledged to stand between us and these goals. First, except for the HED meteorites, in most case there is no definite asteroidal, planetary or satellite body which stands out as a likely parent body for any specific meteorite class. Secondly, there is always a problem in defining proper membership for a meteorite group or class — the problem of 'look alikes.' Some historical perspective is helpful. The first Martian falls were only slowly separated from other meteorite groups as having peculiarities which mandated that we consider them as an ensemble meriting special attention. The Martian pyroxenite, ALH84001, was first classified as a Diogenite before it was recognized as a Martian meteorite. The olivine-rich meteorite Brachina was tentatively considered to be a likely Chassignite before it was made the prototype of a new class of primitive achondrites, the Brachinites. Which brings us back to the HED meteorites.
We have not yet put landers on Vesta, but orbital observations of the Dawn Spacecraft in 2011 and 2012 have helped to make it clear that Vesta is almost certainly the OPB of most HED meteorites. At the same time detailed high resolution of oxygen-isotope studies — coupled to other chemical ratios have made it clear that Vesta is almost certainly not the OPB of Ibitira, Pasamonte, and one or two other Eucrites. In future years and decades we will need to flesh out the details of the journey from Vesta to Earth for a goodly number of HED meteorites before we can be definite about which currently classified HED meteorites are definitely fragments of Vesta, which are merely likely fragments of Vesta, and which are fragments of another as-of-now-unidentified asteroidal body. Besides working out a more comprehensive understanding of the geochemical and mineralogical markers of original classification, subsequent metamorphism and impacts, a more comprehensive picture of the passage from a rock on Vesta to a component of an asteroidal fragment to a component on an even smaller meteoroid which struck the earth's surface will emerge. Hopefully, somewhere in there we can establish that some of the meteorites now in our laboratories have shared chronological markers that will allow us to separate what happened to these rocks when they were on or inside Vesta from what happened while they spent a billion years or so as a part of a smaller asteroid and from what happened during their past few million years as part of a boulder which collided with the earth. Since for the immediate few coming decades or, perhaps, centuries, we will be severely limited in making either human or robotic missions to asteroids and natural satellites, we will do well to work out the details of passage if we plan to identify any more homeworlds in the future.
If we do this, we may also begin to sort out the problematics of such meteorite groups as the ureilites and the mesosiderites which have been altered by impact process in even more fundamental ways than the highly brecciated howardites and lunar rocks we have been studied in recent decades with such intensity. Whatever we do and whatever new techniques we devise and refine, we will still find it useful to begin our meteorite studies by looking at their minerals. Humans use words to create languages. The lithic histories of asteroids and terrestrial worlds are embodied in their minerals. Minerals are, so to speak, a form of rockspeech.
Glossary and other Terminological Issues
Nota bene:
The terms used here are deemed 'indispensable' to his argument by the author. Whether that is actually true or not, they include terms which are frequently bandied about in the meteoritical literature and, presumably, are not necessarily familiar to the reader. The highlited [sic] terms are, of course, immediately available within the Mindat realm of discourse. Wikipedia definitions are also usually adequate in this context. Virtually all terminology related to meteorite classification implicitly or explicitly references the usage employed by the Meteoritical Society and employed at their 'Meteoritical Bulletin Database' [http://www.lpi.usra.edu/meteor/metbull.php] Terms which are frequently employed in somewhat different linguistic registers when employed within meteoritical literature are noted with an asterisk.
Indispensable Terms:
Achondrite -- A differentiated stony meteorite, a meteorite which was formed on a world which experienced extensive melting which resulted in the complete or partial separation of its original constituents. Here we are exclusively interested only in those meteorites which are derived from worlds which experienced melting sufficient to produce both a silicate-rich mantle and a metal-rich core. These include Martian meteorites, lunar meteorites, and those meteorite types derived from large asteroidal-sized homeworlds (Angrites, Aubrites, HED meteorites, and a few odd balls). [We do not treat the very interesting 'primitive achondrites' which appear to be derived from slitely smaller asteroidal homeworlds.]
Asteroid -- Asteroids are small (sub-lunar) silicate-rich and/or metal-rich objects whose orbits are mostly or entirely 'between' the orbits of Mars and Jupiter. Very small asteroids (<100 m diameter) may sometimes be referred to as meteoroids, especially if they are in orbits which might bring them into the vicinity of the earth.
Breccia -- A rock composed of rock fragments which have been reaggregated into a 'whole rock.' Some breccia are very friable and will crumble under the slitest pressure. Other breccia have been reconstituted as moderately sturdy stones.
Clinopyroxene Pyroxenes crystallizing in the monoclinic crystal system. Of particular interest here are both Ca-rich augite and Ca-poor pigeonite.
Eucrite — A type of differentiated, basaltic meteorite rich in pyroxene and plagioclase.
Fall [or, 'witnessed fall']— A meteorite whose fall was 'observed.' In most instances, a witnessed fall is preceded by a fireball and/or detonations and loud rumbles due to the object's fiery encounter with the earth's atmosphere. The recovered object may be partially covered by a fusion crust as well. The date and, perhaps, the hour of most falls are recorded. In a few cases, accumulated circumstantial evidence has been used to deem a recovered meteorite a witnessed 'fall' even when the date of meteorite entry was not preserved.
Fe-Ni metal — The generally preferred term within meteoritical and geochemical literature used to refer to the single and/or multiple unoxidized Fe-rich phases in meteorites. The iron is almost invariably accompanied by Ni (>4-5%),minor Co and very minor Cr. Fe-Ni metal is not a mineralogical term per se, but it is normally assumed that the predominant metallic mineralogical constituent is kamacite. This is normally true whether we are discussing irons, stony irons [The few exceptions include unusually Ni-rich metal and some severely shocked meteorites]. HED achondrites create some linguistic conundrums because some HED Fe-Ni metal is predominantly kamacite or kamacite-and-taenite, but some Fe-rich metal in HED meteorites is quite poor in Nickel content.
Find — A meteorite recovered at some unknown time after it had reached the surface of the earth. Most finds are silicate-rich 'stony meteorites' (or simply 'stones').
Homeworld — The material in a meteorite has passed thru a number of stages which can be partially deciphered once it arrives on earth. However, in almost all cases, it seems that during the first million or first few hundred million years of the solar system's existence most of the material was at one time part of a single 'world' 10-10,000 kilometers in diameter. It is customary in the meteoritical literature to refer to this hypothetical world as the 'Original Parent Body' (OPB) of a meteorite. I prefer to emphasize the hypothetical nature of this idea by using the term 'Homeworld', a near synonym, for this postulated world.
In some cases, notably with Martian and lunar meteorites, it seems clear to most scientists and to this author that the hypothetical OPB has been identified as a real solar system world. However, I would insist that while there is always the possibility that some subset of meteorites, some meteorite 'class' ( e.g., HED meteorites, EH chondrites, etc.) are derived from the same parent body, there is no guarantee that any particular meteorite class is always from the same actual solar system world. In fact, the actual history of meteoritics is that while the hypothesis of a single homeworld for various meteorite classes has been fruitful, in most cases our definition of class membership has had to continually evolve — sometimes drastically. To emphasize that I think our understanding of HED meteorite origins is fraught with oversimplification I use the term 'homeworld' as an alternative to the term 'Original Parent Body'. In the text and in the bibliography, I try to underscore why it seems almost certain to me and others that the HED meteorites are mostly fragments of Vesta, we are a long way from understanding how to definitively distinguish them from the 'look-alikes' that are also fragments of large asteroids — asteroids which are presently unidentified and which, moreover, may no longer even exist as large asteroids.
Kamacite* - The predominant mineralogical constituent of most meteoritic iron whether found in iron, stony-iron, or stony meteorites. Within the meteoritical community kamacite is the preferred term for any occurrence of Fe-Ni metal within a body-centered cubic with 5-9% Ni (known to metallurgists as 'alpha-iron').
Meteorite — A natural object or set of objects which encounters the earth and is recovered by humans. A single 'meteorite' includes all objects which arrived simultaneously and so may refer to a single relatively intact stone or metallic mass or it may consist of thousands of scattered fragments. Each meteorite is given a unique 'name' which consists of a geological reference place or object and any necessary qualifiers needed to insure the uniqueness of the name. A meteorite recovered after an unobserved arrival can be identified by several methods. For natural objects, the detection of nickel [present in unoxidized Fe-Ni metal] is usually the most easily applied means to detecting non-terrestrial objects. For those relatively few meteorites which do not possess Fe-Ni metal, oxygen isotope signatures are almost always definitive.
Meteoritic Iron - 'Meteoritic Iron' as a Mindat descriptor is a close synonym for 'Fe-Ni metal' as used in the meteoritical community. In actual usage both terms carry simultaneous mineralogical and geochemical connotations which may create ambiguities. To avoid such ambiguities, the author only uses 'Meteoritic Iron' as a geochemical descriptor for nickeliferous iron whose mineralogical character is unknown. The term 'Fe-Ni metal' as used within meteoritical literature and by this author refers to nickeliferous iron as a geochemical descriptor for nickeliferous iron whose specific mineralogical characterization is not of immediate interest. It is an unstated assumption within meteoritical literature that kamacite is almost always present in 'Fe-Ni metal' and, furthermore, that if the Ni content of the nickeliferous iron exceeds 8-9%, significant quantities of taenite and/or tetrataenite are likely present as well.
Meteoroid — A (moderately small) object which has or might encounter the earth. After such encounters, the fraction of any recovered material is or will be called a meteorite.
Mineralite — A collective term used by this author to refer to any Mineral Group or Subgroup, IMA defined mineral, recognized variety, chemically defined phase or other formal or informal term helpful in describing a mineral or mineralogically important phase. In the present instance, these means mostly any term which is recognized by Mindat algorithims for constructing Mineral Lists at Mindat Locations sites. However, the term also includes a few mixed geochemical-mineralogical terms (e.g., Ca-rich Clinopyroxene) which are not recognized by Mindat algorithms.
Orthopyroxene — Mg,Fe-rich pyroxenes which crystallize in the Orthorhombic Crystal system. Orthopyroxenes are the dominant components of all Diogenite falls and are important components of many other HED meteorites.
Plagioclase [synonym, 'Albite-Anorthite Series']- Plagioclase, sensu strictu, refers to the Albite (Na-rich) — Anorthite (Ca-rich) Solid Solution Series. As K-rich Feldspars (and K-rich compounds) are so rare within most meteorite classes, it is customary within the meteoritical literature to avoid using the term Feldspar except in those rare instance when Potassium is abundant or at least significant. This is somewhat counter to best usage within terrestrial oriented geological and mineralogical discussion where the relationship between Ca-rich, Na-rich, and K-rich Feldspars and Feldspathoids is always in the background even when not of immediate interest.
Stony meteorite — Most meteorites which fall or are recovered by trained personnel from the desserts of Africa, the America, Antarctic, or Australia are silicate-rich 'stones'. These meteorites may or may not contain chondrules [earth rocks do not] and they may or may not contain significant amounts of additional Fe-Ni metal or unusual sulfides. Nevertheless, they are still 'stones' — silicate-rich rocks containing minerals such as olivine, pyroxenes, plagioclase and/or phyllosilicates — minerals utterly familiar to all terrestrial, lunar, and meteoritical geologists.
Taenite — Taenite is a mineralogical expression of Fe-Ni metal [crystallizing in a face-centered cubical lattice] found in almost all meteorites whose Fe-Ni metal contains more than ~9% Nickel. Both taenite and unusually Ni-poor iron are found in HED meteorites, almost always in very small amounts — and sometimes in the same meteorite.
Vesta (or, '4Vesta')-- Discovered on 29 March 1807 by Heinrich Wilhelm Olbers, 4Vesta was the fourth asteroid discovered, but it is the britest asteroid. Its average distance from the sun is 2.362 a.u. (353 billion km) which places it in the inner asteroid belt. Every 16 months or so during opposition when Vesta is closest to the earth, for a few weeks it is the only asteroid which becomes visible to the naked eye in a dark sky. Vesta (diameter 525 km) is the third largest asteroid in surface area and orbits the sun, but is britest because (1) it is the closest to earth of the larger asteroids and (2) its plagioclase-rich surface is more reflective than most asteroids [with dark or extremely dark carbonaceous-rich surface materials]. A number of moderately large asteroidal fragments of past collisions of comets and other asteroids with Vesta — the Vestoids — can be distinguished by both their spectral signatures and orbital characteristics. Collisions of Vestoids with comets and other small asteroids plus gravitational perturbations combine to produce a steady stream of meteoroids in earth-crossing orbits often strike the earth. The Dawn mission orbited Vesta for 16 months in 2012 and 2013. Images, spectroscopy, and telemetry have established that the surface of Vesta is rich in the pyroxenes and plagioclase one might expect if Vesta were indeed the Original Parent Body (OPB) of many HED meteorites. The data also reveal that Vesta's surface contains a significant carbonaceous-like component due in large measure to impacting meteoroids. This dark material includes a large component of — especially and specifically — CM2 or CM2-like material.
List B: Formal Listing of 61 HED Minerals and 'Mineralites'
A TALLY OF 61 MINERAL PHASES FOUND IN 62 HED FALLS (35 eucrites, 16 Howardites, & 11 Diogenites)
61 IMA MINERALS AND OTHER MINERALOGICALLY IMPORTANT PHASES FOUND IN 62 HED FALLS
FORMAL TALLIES FOR ALL HED FALLS (35 EUCRITES, 11 DIOGENITES, 16 HOWARDITES)
FORMAL MINERALITE TALLY FOR ALL HED FALLS | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Section VI: Bibliography
HED BIBLIOGRAPHY
This bibliography provides articles of general interest for understanding HED meteorites and, in addition, gives particular emphasis upon the problematics of the determining the relationships between the different HED subtypes and Vesta or other possible asteroidal parent bodies. For information about individual HED meteorites, the author has made it his practice to provide references at individual Mindat Location sites which provide historical, geochemical, and mineralogical context as well as specific references to each mineralite listed at the site. These sources can be found immediately from the 62 links provided at List A: The 62 Witnessed HED Falls. Of course, the links provided for individual meteorites at the Meteoritical Society's "Meteoritical Bulletin Data Base" are even more complete.
Whenever possible, this author prefers to cite full names. The observation of meteorite minerals involves artisan skill and know how as well as scientific knowledge. This is particularly important when one is interested in issues at the boundaries of knowledge where insight is as important as information.
Jean-Alix Barrat, Akira Yamaguchi, Richard C. Greenwood, Marcel Bohn, J. Cotton, Mathieu L. Benoit & Ian A. Franchi (2007) The Stannern trend eucrites: Contamination of main group eucritic magmas by crustal partial melts. Geochimica et Cosmochimica Acta 71, 4108—4124.
Jean-Alix Barrat, Akira Yamaguchi, Brigitte Zanda, Claire Bollinger & Marcel Bohn (2010) Relative chronology of crust formation on asteroid Vesta: Insights from the geochemistry of diogenites. Geochim. Cosmochim. Acta,74 (21):6218-6231.
Karen Susan Bartels & Timothy L. Grove (1991) High-pressure experiments on magnesian eucrite compositions -Constraints on magmatic processes in the eucrite parent body. Lunar and Planetary Science Conference, 21st Proceedings, 351-365.
Andrew W. Beck & Harry Y. McSween, Jr. (2010) Diogenites as polymict breccias composed of orthopyroxenite and harzburgite: Meteoritics and Planetary Science 45 (5): 850-872. (May 2010)
Richard P. Binzel & Shui Xu (1993) Chips off of asteroid 4 Vesta – Evidence for the parent body of basaltic achondrite meteorites. Science 260, 186-191.
Phillip A. Bland et al., (2009) An Anomalous Basaltic Meteorite from the Innermost Main Belt. Science 325, 1525-28.
Janne Blichert-Toft, Maud Boyet, Philippe Télouk & Francis Albaréde (2002) Sm-Nd and Lu¬176Hf in eucrites and the differentiation of the HED parent body. Earth and Planetary Science Letters 204, 167-181.
Joseph S. Boesenberg & Jeremy S. Delaney (1997) A model composition of the basaltic achondrite planetoid. Geochimica et Cosmochimica Acta 61(15): 3205-3225. (Aug 1997)
Donald Dale Bogard (1995) Impact ages of meteorites: A synthesis. Meteoritics, 30 (3): 244-268. (May 1995)
Donald Dale Bogard & Daniel H. Garrison (2003) 39Ar-40Ar ages of eucrites and thermal history of asteroid 4 Vesta. Meteoritics & Planetary Science 38 (5): 669-710.
Donald Dale Bogard, Larry E. Nyquist, Hiroshi Takeda, Hiroshi Mori, Tohru Aoyama, B. Bansal, Henry Wiesmann & Chi-Yu Shih (1993) Antarctic polymict eucrite Yamato 792769 and the cratering record on the HED parent body. Geochimica et Cosmochimica Acta 57 (9): 2111—2121. (May 1993)
Phillip A. Bland et al. (2009) An Anomalous Basaltic Meteorite from the Innermost Main Belt. Science 325, 1525-28.
Laurie E. Bowman, Michael N. Spilde & James J. Papike (1997) Automated dispersive spectrometer modal analysis applied to diogenites. Meteoritics & Planetary Science 32 (6): 869-875. (Nov 1997)
Paul C. Buchanan & Arch M. Reid (1991) Eucrite and Diogenite Clasts in Three Antarctic Achondrites. Abstracts of the Lunar and Planetary Science Conference 22, 149-150. (Dec 1991)
Laurie E. Bowman, Jay J. Papike & Michael N. Spilde (1999) Diogenites as asteroidal cumulates: Insights from spinel chemistry: American Mineralogist: 84 (7-8): 1020–1026. (July-August 1999)
Paul C. Buchanan, Arch M. Reid & Carol Schwarz. (1990) Clast Populations in Three Antarctic Achondrites. Abstracts of the Lunar and Planetary Science Conference 21, 141-142.
Paul C. Buchanan, Michael E. Zolensky & Arch M. Reid, 1993b. Carbonaceous chondrite clasts in the howardites Bholghati and EET87513. Meteoritics 28, 659-682.
Paul C. Buchanan, David J. Lindstrom, David W. Mittlefehldt, Christian Koeberl & Wolf Uwe Reimold (2000) The South African polymict eucrite Macibini. Meteoritics & Planetary Science 35, 1321-1331.
Paul C. Buchanan & David W. Mittlefehldt (2003) Lithic components in the paired howardites EET 87503 and EET 87513: Characterization of the regolith of 4 Vesta. Antarctic Meteorite Research, vol 16, p.128-151
Ian S.E. Carmichael, Francis J. Turner & John Verhoogen ( 1974) Igneous Petrology. McGraw-Hill (New York), 739 pp.
Robert N. Clayton & Toshiko K. Mayeda (1983) Oxygen isotopes in eucrites, shergottites, nakhlites, and chassignites. Earth and Planetary Science Letters 62, 1-6.
Guy Consolmagno, SJ & Michael Julian Drake (1977) Composition and evolution of the eucrite parent body: Evidence from rare earth elements. Geochimica et Cosmochimica Acta 41, 1271-1282.
Jeremy S. Delaney, Hiroshi Takeda , Martin Prinz, Cherukupalli E. Nehru & George E. Harlow (1983) The nomenclature of polymict basaltic achondrites. Meteoritics 18, 103-111.
Michael Julian Drake (2001) The eucrite/Vesta story. Meteoritics & Planetary Science, vol. 36, no. 4, pp. 501-513.
Michael Julian Drake (1979) Geochemical evolution of the eucrite parent body -Possible nature and evolution of asteroid 4 Vesta. In: Asteroids. Tucson, Ariz., University of Arizona Press, 1979, pp. 765-782.
Michael B. Duke (1965) Metallic iron in basaltic achondrites: Journal of Geophysical Research 70 (6): 1523-1527. (March 1965)
Michael B. Duke & Leon T. Silver (1967) Petrology of eucrites, howardites, and mesosiderites. Geochimica et Cosmochimica Acta, 31, 1637-1665.
Paolo Farinella & David Vokrouhlicky (1999) Semimajor Axis Mobility of Asteroidal Fragments. Science 283, 1507-10.
Robert J. Floran (1978) Silicate petrography, classification, and origin of the mesosiderites ¬Review and new observations. Lunar and Planetary Science Conference, 9th, Proceedings. Volume 1, New York, Pergamon Press, 1053-1081.
Michael J. Gaffey (1997) Surface lithologic heterogeneity of asteroid 4 Vesta. Icarus 127, 130¬157.
Stephen J. G. Galer & Guenter W. Lugmair (1996) Lead Isotope Systematics of Noncumulate Eucrites. Meteoritics & Planetary Science 31, A47.
Rainer Gersonde, Frank T. Kyte, Ulrich Bleil, Bernhard Diekmann, Jose-Abel Flores, Karsten Gohl, Gregor Grahl, Rick Hagen, Gerhard Kuhn & Francisco J. Sierro (1997) Geological record and reconstruction of the late Pliocene impact of the Eltanin asteroid in the Southern Ocean. Nature 390, 357-363.
Cyrena A. Goodrich & Jeremy S. Delaney (2000) Fe/Mg–Fe/Mn relations of meteorites and primary heterogeneity of primitive achondrite parent bodies. Geochim Cosmochim Acta 64, 149-160.
Ron Gooley & Carleton B. Moore ( 1976) Native metal in diogenite meteorites: American Mineralogist 61 (5/6, Part I): 373-378. (May-June 1976)
Mathieu Gounelle, Marc Chaussidon, Alessandro Morbidelli, Jean-Alix Barrat, Ceale Engrand, Michael E. Zolensky & Kevin D. McKeegan (2009) A unique basaltic micrometeorite expands the inventory of solar system planetary crusts. Proceedings of the National Academy of Sciences 106, 6904-6909.
Richard C. Greenwood, Ian A. Franchi, Albert Jambon & Paul C. Buchanan (2005) Widespread magma oceans on asteroidal bodies in the early Solar System. Nature 435, 916¬918.
Timothy L. Grove & Karen S. Bartels (1992) The Relation Between Diogenite Cumulates and Eucrite Magmas. Proceedings of the 22nd Lunar and Planetary Science Conference, 437-445.
Roger H. Hewins & Horton E. Newsom (1988) Igneous activity in the early solar system. In: Meteorites and the early solar system. University of Arizona Press, Tucson, AZ, 73-101.
Weibao Hsu & Ghislaine Crozaz (1996) Mineral chemistry and the petrogenesis of eucrites: I. Noncumulate eucrites. Geochimica et Cosmochimica Acta, 60, no. 22, 4571¬4591.
Weibao Hsu & Ghislaine Crozaz (1997) Mineral chemistry and the petrogenesis of eucrites: II. Cumulate eucrites. Geochimica et Cosmochimica Acta, 61, 1293-1302.
Ikuko Ikeda & Hiroshi Takeda (1985) A model for the origin of basaltic achondrites based on the Yamato 7308 howardite. Journal Geophysical Research, Supplement 90, C649-C663.
Amy Jo Goldmintz Jurewicz, David W. Mittlefehldt & John H. Jones (1993) Experimental partial melting of the Allende (CV) and Murchison (CM) chondrites and the origin of asteroidal basalts . Geochimica et Cosmochimica Acta 57, 2123-2139.
Amy Jo Goldmintz Jurewicz, David W. Mittlefehldt & John H. Jones (1995) Experimental partial melting of the St. Severin (LL) and Lost City (H) chondrites. Geochimica et Cosmochimica Acta 59, 391-408.
Klaus Keil (2002) Geological History of Asteroid 4 Vesta: The “Smallest Terrestrial Planet”. In Asteroids III, William F. Bottke Jr., Alberto Cellino, Paolo Paolicchi & Richard P. Binzel (eds), University of Arizona Press, Tucson, 573-584.
Thorsten Kleine, Mathieu Touboul, Bernard Bourdon, Francis Nimmo, Knut Metzler, Herbert Palme, Stein B. Jacobsen, Qing-Zhu Yin & Alexander N. Halliday (2009) Hf–W chronology of the accretion and early evolution of asteroids and terrestrial planets. Geochimica et Cosmochimica Acta 73, 5150-5188.
Randy L. Korotev (2005) Lunar geochemistry as told by lunar meteorites. Chemie der Erde 65, 297–346.
Harold P. Larsen & Uwe Fink (1975) Infrared spectral observations of asteroid 4 Vesta. Icarus 26, 420-427.
Daniela Lazzaro, Tatiana A. Michtchenko, Jorge M. Carvano, Richard P. Binzel, Shelte J. Bus, Thomas H. Burbine, Thais Mothé-Diniz, T. Florczak, Claudia A. Angeli & Alan W. Harris (2000) Discovery of a Basaltic Asteroid in the Outer Main Belt. Science 288, 2033-2035.
Der-Chuen Lee & Alexander N. Halliday (1996) Hf-W Isotopic Evidence for Rapid Accretion and Differentiation in the Early Solar System. Science 274. 1876 – 1879.
Der-Chuen Lee & Alexander N. Halliday, Gregory A. Snyder & Lawrence A. Taylor (1997) Age and Origin of the Moon. Science 278, 1098–1103.
Guenter W. Lugmair & Shukolyukov, A. (1998) Early solar system timescales according to 53Mn-53Cr systematics. Geochimica et Cosmochimica Acta 62, 2863-2886.
Guenter W. Lugmair & Stephen J. G. Galer (1992) Age and isotopic relationships among the angrites Lewis Cliff 86010 and Angra dos Reis. Geochim. Cosmochim. Acta 56, 1673-1694.
Francesco Marzari, Alberto Cellino, Donald R. Davis, Paolo Farinella, Vincenzo ZappalĂ & V. Vanzani (1996) Origin and evolution of the Vesta asteroid family. Astronomy and Astrophysics 316, 248-262.
Brian Harold Mason, B. (1963). The Hypersthene Achondrites. American Museum Novitates, 2155, 13 pages.
R.G. Mayne, Harry Y. McSween, Jr., Timothy J. McCoy & A. Gale (2009) Petrology of the unbrecciated eucrites. Geochimica et Cosmochimica Acta 73, 794-819.
Thomas B. McCord, John B. Adams & Torrence V. Johnson (1970) Asteroid Vesta: Spectral Reflectivity and Compositional Implications. Science 168, 1445-1447.
Harry Y. McSween, Jr. & 11 others (1913). Dawn, the Vesta-HED connection; and the Geological context for eucrites, diogenites, and howardites. Meteoritics & Planetary Science 48 (13): 2090-2104. (Nov 2013)
Knut Metzler, Klaus D. Bobe, Herbert Palme, Bernhard Spettel & Dieter Stöffler (1995) Thermal and impact metamorphism on the HED parent asteroid. Planetary and Space Science 43, 499-525.
Fabbio Migliorini, Alessandro Morbidelli, Vincenzo Zappalà , Brett J. Gladman, Mark E. Bailey & Alberto Cellino (1997) Vesta fragments from v6 and 3:1 resonances: Implications for V-type near-Earth asteroids and howardite, eucrite and diogenite meteorites. Meteoritics & Planetary Science 32, 903¬915.
Keiji Misawa, Akira Yamaguchi & Hiroshi Kaiden (2005) U-Pb and Pb-Pb ages of zircons from basaltic eucrites: Implications for early basaltic volcanism on the eucrite parent body. Geochimica et Cosmochimica Acta 69, 5847-5861.
David W. Mittlefehldt (2005) Ibitira: A basaltic achondrite from a distinct parent asteroid and implications for the Dawn mission. Meteoritics & Planetary Science 40, 665-677.
David W. Mittlefehldt & Marilyn M. Lindstrom (1991) Geochemistry of 5 Antarctic Howardites and Their Clasts. Abstracts of the Lunar and Planetary Science Conference 22, 901-902.
David W. Mittlefehldt, Timothy J. McCoy, Cyrena A. Goodrich & Alfred Kracher (1998). Non-chondritic meteorites from asteroidal bodies. In: Planetary Materials (James J. Papike, Editor): Chapter 4, 195 pages. Mineralogical Society of America: Washington, DC, USA.
David W. Mittlefehldt, Andrew W. Beck, Cin-Ty A. Lee & Harry Y. McSween Jr. (2009) Chemistry of diogenites and evolution of their parent asteroid. LPSC XL, 1038.
Masamichi Miyamoto & Hiroshi Takeda (1981) Thermal and deformational histories of diogenites as inferred from their microtextures of orthopyroxene. Earth Planet. Sci. Lett. 53, 266-274.
Masamichi Miyamoto & Hiroshi Takeda (1994) Evidence for excavation of deep crustal material of a Vesta-like body from Ca compositional gradients in pyroxene. Earth and Planetary Science Letters 122, 343-349.
Nicholas A. Moskovitz, Robert Jedicke, Eric Gaidos, Mark Willman, David Nesvorný, Ronald Fevig & Zelijko Ivezić (2008) The distribution of basaltic asteroids in the Main Belt. Icarus 198, 77-90.
David Nesvorný, Fernando Roig, Brett J. Gladman, Daniela Lazzaro, Valerio Carruba & Mothé-Diniz, T. (2008) Fugitives from the Vesta family. Icarus 193, 85-95.
Dimitri A. Papanastassiou, R. Sundar Rajan, John C. Huneke & Gerald J. Wasserburg (1974) Rb-Sr Ages and Lunar analogs in a Basaltic Achondrite; Implications for Early Solar System Chronologies. Lunar and Planetary Science Conference V, 583-585.
Carlé M. Pieters, Richard P. Binzel, Donald Dale Bogard, Takahiro Hiroi, David W. Mittlefehldt, Larry E. Nyquist, Andrew S. Rivkin & Hiroshi Takeda (2005) Asteroid-meteorite links: the Vesta conundrum(s) Proceedings IAU Symposium No. 229, 273-288.
Alain Prinzhofer, Dimitri A. Papanastassiou & Gerald J. Wasserburg (1992) Samarium-neodymium evolution of meteorites. Geochimica et Cosmochimica Acta 56, 797-815.
Ghylaine Quitté, Jean-Louis Birck & Claude Jean Allègre (2000) Hf-W systematics in eucrites: the puzzle of iron segregation in the early solar system. Earth and Planetary Science Letters 184, 83-94.
Arch M. Reid & Barnard, B.M. (1979) Unequilibrated and Equilibrated Eucrites. Lunar and Planetary Science X: 1019-1022.
Kevin Righter & Michael Julian Drake (1997) A magma ocean on Vesta: Core formation and petrogenesis of eucrites and diogenites. Meteoritics & Planetary Science 32, (6) 929-944. (Nov 1997)
Minik T. Rosing & Henning Haack (2004) The First Mesosiderite-like Clast in a Howardite. 35th Lunar and Planetary Science Conference, March 15-19, 2004, League City, Texas, abstract no.1487.
Christopher T. Russell, Fabrizio Capaccioni, Angiloletta Coradini, Maria Cristina de Sanctis, William C. Feldman, Ralph Jaumann, Horst Uwe Keller, Thomas B. McCord, Lucy A. McFadden, Stefano Mottola, Carlé M. Pieters, Thomas H. Prettyman, Carol A. Raymond, Mark V. Sykes, David E. Smith & Maria T. Zuber (2007) Dawn Mission to Vesta and Ceres: Symbiosis between Terrestrial Observations and Robotic Exploration. Earth Moon Planet 101, 65–91.
Alex Ruzicka, Gregory A. Snyder & Lawrence A. Taylor (1997) Vesta as the howardite, eucrite and diogenite parent body: Implications for the size of a core and for large-scale differentiation. Meteoritics & Planetary Science 32 (6), 825-840. (Nov 1997)
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Martin Schiller, Joel A. Baker, Martin Bizzarro, John Creech & Anthony J. Irving (2010) Timing and mechanisms of the evolution of the magma ocean on the HED parent body. Meteoritics 45, A181. (July 2010)
David N. Schramm, Fouad Tera & Gerald J. Wasserburg (1970) The isotopic abundance of26Mg and limits on26Al in the early solar system. Earth Planet. Sci. Lett. 10, 44-59.
Christian Schroeder & 28 authors (2008) Meteorites on Mars observed with the Mars exploration rovers. Journal of geophysical research 113, E06S22.
Jeffrey M. Schwartz & I. Stewart McCallum (2005) Comparative study of equilibrated and unequilibrated eucrites: Subsolidus thermal histories of Haraiya and Pasamonte. American Mineralogist 90 (11-12): 1871-1886. (Nov-Dec 2005)
Edward R. D. Scott, Richard C. Greenwood, Ian A. Franchi & Ian S. Sanders (2009) Oxygen isotopic constraints on the origin and parent bodies of eucrites, diogenites, and howardites: Geochimica et Cosmochimica Acta 73 (19): 5835-5853. (Oct 2009)
Charles K. Shearer Jr., Grant W. Fowler & James J. Papike (1997) Petrogenetic models for magmatism on the eucrite parent body: Evidence from orthopyroxene in diogenites. Meteoritics & Planetary Science 32 (6) , 877 – 889. (Nov 1997)
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Acknowledgments
Members of the Broward County Library Staff (and, in particular, Amy Wilson) have been especially helpful in my efforts to obtain sources which are not directly available OnLine. My two friends and colleagues, David Judd and Ray Durand, have not only listened patiently when I repeated myself, but have also encouraged me to continue my efforts.
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Quick NavSection I: IntroductionSection II: The HED Achondritic Meteorites (Howardites-Eucrites-Diogenites)The 35 Eucrite falls.The 11 Diogenite FallsThe 16 Howardite FallsThe Vesta-plus ConnectionsList A: The 62 Witnessed HED FallsSec III: Mineralogical Groups found in HED fallsTally for PyroxenesTally for Plagioclase and FeldsparTally for Silica polymorphsTally for OlivineTally for Fe-Ni Metal and other Fe-rich PhasesSection IV: 44 IMA Minerals Found in HED FallsSection V: Concluding Remarks and ReflectionsGlossary and other Terminological IssuesList B: Formal Listing of 61 HED Minerals and 'Mineralites'Section VI: BibliographyAcknowledgments
