Help mindat.org|Log In|Register|
Home PageMindat NewsThe Mindat ManualHistory of MindatCopyright StatusManagement TeamContact UsAdvertise on Mindat
Donate to MindatSponsor a PageSponsored PagesTop Available PagesMindat AdvertisersAdvertise on Mindat
Minerals by PropertiesMinerals by ChemistryAdvanced Locality SearchRandom MineralSearch by minIDLocalities Near MeSearch ArticlesSearch GlossaryMore Search Options
Search For:
Mineral Name:
Locality Name:
Keyword(s):
 
The Mindat ManualAdd a New PhotoRate PhotosLocality Edit ReportCoordinate Completion ReportAdd Glossary Item
StatisticsThe ElementsMember ListBooks & MagazinesMineral Shows & EventsThe Mindat DirectoryHow to Link to MindatDevice Settings
Photo SearchPhoto GalleriesNew Photos TodayNew Photos YesterdayMembers' Photo GalleriesPast Photo of the Day Gallery

Meteorite Mineralogy: An Explicitly Planetological-Planetochemical Perspective

Last Updated: 6th Sep 2017

By Lon Clay Hill

Full Title:
Minerals and Mineralites from the Earth and Sky: Constructing A Scientifically Transparent Mineralogical Terminology from a Planetochemical Perspective. A Theoretical Analysis Accompanied by An Examination of Terminological Problematics Arising from Disparate Disciplinary-Specific Objectives.

"Every mineral records an episode in the history of a rock."
[Paraphrasing Brian Harold Mason (1917-2009)]

Abstract

An epistemological-pedagogical examination of disciplinary-specific uses of mineralogical language as employed by (1) scientific disciplines primarily focused on terrestrial rocks and their lithologies [loosely, the geological sciences] and (2) other disciplines primarily focused on meteorites, other extraterrestrial stones, and other planetological issues. [Examples of confusing or contradictory use of mineralogical/planetological terms utilized within different disciplines will follow.] Our discussion begins with an examination of two questions about meteorites that arise when any human being examines a meteorite — a skyrock, a natural stone that has indeed 'fallen' from the sky. The questions are simply put: (1) Where did it come from and (2) What is it made of? Both questions receive constant attention within the scientific community as a whole. And, we will discuss both questions here. However, while the question of meteorite origins is of paramount interest to meteoriticists, it is also a more difficult question with seemingly intractable twists. So, more or less in tandem with scientific developments of the past two centuries, we will begin our main inquiry with the issue of meteorite composition and content. This second question of meteoritical constituents also has some initial irreducible complexities — within our store of recovered meteorites there are more chemical and mineralogical complexities than are found within the entire host of closely examined terrestrial stones and lithologies. Our own inquiry will be largely restricted to two of the most important subsets of the compositional question — (1) the mineralogical question [What minerals do we find in meteorites?] and (2) the chemical question [What elements, including isotopes, do we find in meteorites?].

Historically speaking, the growth of chemistry and mineralogy were more obviously intertwined during the 19th Century than they are today with our multiple disciplinary specific subdisciplines. Still, central to our theme and with the benefit of hindsight we can state: First, there continue to be a host of interesting live mineralogical issues within contemporary meteoritics. Secondly, however, detailed chemical studies — especially studies of correlated elemental and isotopic ratios — have been and continue to be more fruitful in determining the original formational worlds (original parent bodies) and/or environments of meteorites. And that leads us to the epistemological and scientific point: Our answers to either the question of meteoritic origins or the question of meteorite composition influence our framing of the other question. And, contrary to assertions that mineralogists, geologists, meteoriticist, and planetary scientists use a 'universal' mineralogical language, it appears to this author that in contemporary formal and informal meteoritical literature there are a number of circumlocutions and odd disciplinary specific uses of language that border on jargon. To meet that need the author does two things: One, the author introduces neologisms and alternative usages that be believes may add transparency to chemically-laden implications of mineralogical terms. Two, he provides several specific instances — (e.g., kamacite, enstatite…) — where traditional terrestrial-focused definitions of mineralogical terms interfere with scientific priorities in meteoritical.

Preliminary Note: This collection of sections, paragraphs, and note expresses basic thoughts which I believe are important to both mineralogy and meteoritics. I believe, however, that with more time and energy the effort could be tightened a little and improved. However, with hurricane Irma approaching South Florida and the possibility now looms that further efforts will be delayed by as much as several weeks or more. Hopefully, within a couple of months I may be able to make some improvements and any needed small or large corrections. I believe, however, that the central thesis is so important that I should put the topic up for discussion — and, in good time, attend to any improvements. Suggestions are encouraged.

Lon Clay Hill; Miramar, Florida, USA; 6 September 2017.

TABLE OF CONTENTS (Actualized)

3 Preliminary Definitions
Mineral (generic)— A natural object with (1) a defined crystallographical structure [usually a single 3D space group] and (2) a range in elemental composition as prescribed by an interested scientific group such as, for example, the International Mineralogical Society (IMA).

Mineralite — A mineral or mineralogically significant phase-or-aggregate. We specifically include minerals, mineral groups, solid solution series, mineral varieties, glasses, bi-minerals, generic multi-mineral aggregates, chemically-defined phases, and mixed chemical-mineralogical phases. Our definition includes — but is not restricted to — all IMA-minerals and all Mindat-hosted 'mineralogical items'

Planetochemical — Relating to chemical signatures that characterize planetological objects within the solar system. In this work these considerations are operationally confined mostly to the terrestrial worlds, asteroids, and meteoroids of the inner solar system along with the putative original planet bodies (OPBs) of recovered meteorites.

Part I — Epistemological and Pedagogical Framework for novices and, occasionally, for experts

Introduction: The Two Questions (Whence and What)


Several times each year a brite bolide, moving at several kilometers per second is created when a stony meteoroid enters the earth's stratosphere at cosmic velocity and immediately begins to slow down and — after shedding some material or even breaking into two or more fragments — reaches the earth and is recovered by human beings where it is called a meteorite. The meteorite's brief passage thru the earth's atmosphere may be accompanied by loud sonic booms, staccato bursts, and other sounds which may or may not be observed by those who witness the actual impact or landfall. While only a relatively few human beings witness the actual impact of a falling meteorite, these phenomena have been recognized and remembered over the past few millennia as the rare events which they are. And, in almost all instances, two important questions arise in the people who see these falls — whether they are meteorite experts or persons who have never seen a meteorite:

(1) Where did it come from?? [The question of its origin (within the Solar System)](2) What is it made of ?? [The question of elemental or mineralogical composition]

The first question is actually the more difficult question. Indeed, since ~1800 when European scientists began to fully recognize that stones do indeed fall from the sky we have learned a few things. As far as we can tell, all meteorites in our collections are small fragments of solar system worlds — usually asteroids — and a very small percentage of these skyrocks (> 1%) are known to be fragments of the moon and Mars. There are other things that we know or think we know about their origins, but there is also much that we do not know. So, like many earlier scientists, we will now turn to the second question — the question of composition. We note, however, that we will find that the question of composition will eventually force us to return to the first question — and when we do return we will see the question with different eyes and, therefore, be actually asking a somewhat different question (or set of questions).

As a scientific matter the second question — the question of composition can be most easily considered considered from two more focused perspectives. We could ask:

One, what chemical elements are found in these strange rocks?
Two, what minerals are found in these strange objects?


Now, for starters, we can say — element-wise — that these skyrock are composed of the very same chemical elements that are found in earthrocks (H,He,Li…U) [although often in different proportions]. But, mineral-wise, we can declare quite definitely that some minerals that are common in earthrocks are also common in meteorites while other minerals common in at least some meteorites are either utterly unknown or exceedingly rare in earth rocks. And then, there are a few minerals (esp. oxides, sulfides, and sulfates) that are occasionally found in both skyrocks and earthrocks. So we will begin our first general inquiry with a consideration of meteoritic minerals — those that are common in earthrocks (well known to the average geology student or amateur rockhound), those that are well known at least to geologists, and those that are unknown or virtually unknown to those who only study earthrocks and their minerals.

But even here we must be a little more technical — all skyrocks are not the same either compositionally or mineralogically. And indeed the mineralogical differences are so prominent that initial efforts to classify meteorites usually began with explicit attention to those mineralogical differences [which were, it should be stipulated, usually accompanied by important differences in chemical composition]. Indeed, at some level the few tens of thousands of meteorites we now have in our collections are a much more diverse mineralogical lot than all the millions of terrestrial rocks that have been studied in some detail. This is especially true if we consider both witnessed falls and the much larger number of meteorite 'finds' — meteorites found or recognized after their earlier unreported falls to earth. So we begin by taking immediate note of the four classical classes of meteorites — the chondrites, achondrites, stony-irons, and irons.

Meteorites: The Classical Classification Categories (and some mineralogical considerations)

Meteorites: Classical Mineralogical Categories — Chondritic stones, Achondritic stones, Stony-irons, Irons
We describe here the classification categories that have traditionally been used in all initial descriptions of recovered meteorites. I say 'traditionally' because there have been some recent moves to lump some achondrites and stony-irons into a category of differentiated meteorites. Our emphasis here is upon their mineralogical constituents. We will return to such issues as formational origins, metamorphism, collisional disruption — and differentiation — as these issues arise in what are, for the author, more natural contexts. We will begin with the chondritic stones which account for nearly 90% of all well-classified chondritic stones. Truth to tell, a few meteorites — especially the martian meteorites — resemble terrestrial rocks much more than the chondrites. However, while we return to this topic below, we wish to emphasize that we are talking here about meteorites. The fundamental epistemological premise of this essay is that even the most familiar meteoritic minerals and associated geochemical relationships cannot obscure the fact that these extraterrestrial stones are almost always fragments of an unknown solar system provenance.

I. Chondritic stones (Carbonaceous Chondrites, Enstatite Chondrites, Ordinary Chondrites)

Approximately 90% of all witnessed meteorites are chondritic stones or chondrites. Chondritic stones — like almost all terrestrial stones of the earth's crust — are compositionally and volumetrically dominated by silicate-rich minerals [e.g., olivine, pyroxene, plagioclase feldspar], minerals which are utterly familiar to terrestrial geologists. Chondrites are so named because many of them contain chondrules [chondrules] or textural-chemical relics of such chondrules. The intact chondrules appear to be frozen rock-droplets (e.g., as hailstones are frozen water droplets) and are almost never found in terrestrial rocks. The most common chondrites (H-, L-, LL-group ordinary chondrites) were for much of the 20th Century classified as Olivine-bronzite, Olivine-hypersthene, and Olivine-amphoterite chondrites, respectively. In this instance (and others) chemically defined varieties of a mineral [orthopyroxene] were used as labels for the now 'chemically' defined groups. In addition to the dominant silicates, most chondrites contain Fe-Ni metal and a few sulfides. Fe-Ni metal — as we will define it below — contains phases which are rarely if ever found in terrestrial rocks.

As we will also explain below, recent changes in IMA definitions of the compositional boundaries of the most common and well-known silicate minerals in meteorites have created confusion in scientific transparency for students of meteorites. The fundamental problem is that there has been a tendency to redefine and reduce minerals (esp. solid solutions series) such as Olivine, Orthopyroxene, and Plagioclase to pairs of complementary end members of such solid series [Olivine becomes either Forsterite or Fayalite, Orthopyroxene is either Enstatite or Ferrosilite, Plagioclase is either Albite or Anorthite. Other older more specific intermediate terms such as Bronzite, Hypersthene, Bytownite, and Oligoclase are either discarded or devalued as 'varieties' while end members are assigned an extended compositional range. These redefinitions often obscure and even destroy much of their utility when the object of the inquiry is to ascertain the history of the mineral and its host rock. (Vide infra: Enstatite, Forsterite).

II. Achondritic stones, differentiated (Angrites, Aubrites, Eucrites, HED meteorites, Lunar meteorites, Martian Meteorites, Ureilites…)

The Achondritic stones or achondrites represent ~5% of all witnessed meteorite falls. Truth to tell, the achondritic stones — mineralogically and texturally speaking — are the most earthlike of meteorite stones. Before the gradual adoption of explicitly chemical criteria in classification terminology, achondrites were frequently referred to as basaltic meteorites. The Martian Shergottites, some lunar meteorites, and the Eucrites are extraterrestrial basalts. They are especially similar to certain oceanic basalts such as those found in Iceland, the Hawaiian Islands and elsewhere. True, the Shergottites have been deeply shocked by the impact(s) that catapulted them from the planet Mars while the Eucrites and Lunar basalts were formed in somewhat more oxygen-deprived environments than terrestrial basalts (lower O-Fugacity). However, they are still fundamentally basalts — volcanic rocks rich in pyroxene and plagioclase that formed near the surface of their original parent bodies. Other meteorite groups formed in deeper, more plutonic regions which are also quite familiar to geologists. Thus, the Diogenites are pyroxenites (pyroxene-rich rocks); the Chassignites are Marian dunites (olivine-rich rocks).

However, among the small number of scientists who primarily study meteorite the most distinguishing feature of the achondrites is that — unlike the more common chondrites — as stones they do not and apparently never have possessed chondrules [whether chondritic material was assembled, melted down, and then reassembled is a separate issue]. In a word, they are a-chondrites — non-chondritic stones. As with the chondrites, we shall see that recent changes in definitions of primary silicate minerals in the achondrites have also created terminological barriers to scientific transparency for students of meteorites (Vide infra: Enstatite, Fassaite).

III. Stony irons (Mesosiderites, Pallasites)

The Mesosiderites and Pallasites are the most prominent of several metal-rich meteorite groups which contain also significant amounts of silicates. Both groups appear to be — in part at least — the products of catastrophically disrupted moderately large and differentiated asteroids. For our purposes it is sufficient to state that the Pallasites are distinguished by volumetrically subequal amounts of (sometimes disrupted) frozen olivine crystals floating, as it were, in a matrix of Fe-Ni metal. It has been suggested that they represent the remnants of the core-mantle interfaces from (at least 3) ancient asteroids. The Mesosiderites contain some quite complicated mixtures of fractured silicates (mostly pyroxenes and plagioclase) and seemingly injected Fe-Ni metal. They could be most simply explained as the products of a moderately large impacting iron asteroid which has disrupted a larger differentiated asteroid not unlike the asteroid 4Vesta. Vesta itself is known to be the parent body of most — but not all — of the HED achondrites. The mineralogical point of interest here is that large pyroxene clasts found in mesosiderites are surprisingly similar to the unusually orthopyroxene-rich diogenite achondrites [some diogenites are essentially monominerallic pyroxenites].

Contemporary Developments: The Meteoritical Society lists a number of additional meteorites under the heading of 'metal-rich meteorites'. They include a few carbonaceous chondrites in the CB and CH groups and several other unusual 'Stony-Irons' which have been reclassified because of their clear chemical/isotopic affinities with other meteorite groups. For a few decades two of these meteorites (Lodran and Steinbach) were listed as unique stony-irons [e.g., Lodran is now classified as a 'Lodranite' (a primitive achondrite subgroup) and Steinbach is now classified as an anomalous IVA-an Iron].

IV. Iron meteorites.


Iron meteorites represent only 4% of witnessed falls and a similarly small fraction of all recovered meteorites. However, the total mass of recovered iron meteorites far exceeds the combined mass of all other recovered meteorites. The reasons for this odd disparity include two major factors. For one, iron meteorites are so obviously different from most crustal rocks that they are more often brought to a geologist for examination and identification. Secondly, they appear to be more durable than most other types of meteorites. They may or may not fragment during their passage thru the earth's atmosphere, but they do not normally break up into hundreds or even thousands of fragments as do some stony meteorite. And, once on the surface, they do not — especially in dry climates — disappear so readily into the surrounding soil. This is actually somewhat paradoxical. Some of the common silicate minerals do are not so readily altered by the water and air that cause all iron meteorites to eventually rust away. However, the initial structural weaknesses in any stony meteorite —combined with the atmospheric weathering of any iron and sulfide grains and veins — destroy the structural integrity of stony meteorites so rapidly that surviving silicates are usually not recognized as meteorite fragments.

A general overview of iron meteorites is quite straightforward. Iron meteorites consist primary of Fe-Ni metal (a mineralogical assemblage usually dominated by two iron-rich minerals, Kamacite [metallurgical 'α-iron'] and Taenite [metallurgical 'γ-iron'] along with plessite, a microscopic Kamacite-Taenite intergrowth. Indeed, one set of iron meteorites ('hexahedrites' representing a conspicuous fraction of the IIAB irons) are essentially monominerallic objects dominated by kamacite. Furthermore, a few hexahedrites consist almost entirely of a single kamacite crystal. Most iron meteorites are also accompanied by a small assortment of Fe-rich minerals such as troilite (FeS), schreibersite (an Fe,Ni-rich phosphide), and cohenite (an iron phosphide). If the meteorite has been highly shocked, martensite, a disordered form of kamacite is sometimes produced as well. Iron meteorites appear to be the products of long, slow cooling within a set of moderately large asteroids. These asteroids apparently had initial cores which had begun to cool and solidify to the point that the original metal was beginning to separate into two predominant phases: relatively Ni-poor kamacite (Ni ~4.5-7.5 wt%) and relatively Ni-rich taenite (Ni ~10-30 wt%). At some time between 4 and 4.5 billion years ago, their parent bodies were disrupted by catastrophic collisions which dispersed the fragments — with a very small fraction (presumably fragments of fragments) eventually reaching the earth. The iron meteorites we recover today are not a homogeneous lot — the original parent bodies were of different sizes and composition and the collisions which fragmented the original parent bodies (OPBs) and their subsequent intermediate parent bodies (IPBs) left a number of disparate features upon the meteoroids which became the iron meteorites in our collections today. Most obvious petrographic features are martensitic regions (apparently heated or even melted by shock and then, almost as quickly cooled) and numerous Neumann bands. Still, as a general feature, they appear to be fragments of moderately large asteroids with metallic cores. [I do not take the numbers too literally, but I suspect that most of the OPBs had diameters in the 30-1000 km range where I add a generous excess to the continually varying estimates which show up in calculations.]

However, as mentioned above, smaller amounts of (unoxidized) Fe-Ni metal are found in most meteorites and, indeed, often provide the quickest way to determine if an unusual stone is an actual meteorite. A few meteorites lack such characteristic Fe-Ni metal (most notably the martian meteorites, HED meteorites, and many carbonaceous chondrites), but Fe-Ni metal abundances of 5-20 wt% are quite common in most freshly recovered falls. There are a number of obvious mineralogical and chemical similarities between the dominant Fe-Ni metal of iron meteorites and the accompanying Fe-Ni metal in most stony meteorites. To begin with kamacite and taenite are normally present, along with small plessitic intergrowths. But there are important differences which appear to be produced by quite different cooling histories during the formation of the meteoritic parent bodies. The most important difference appears to be the fact that most stony meteorites appear to be fragments of smaller parent bodies than the parent bodies of most iron meteorites. This has two immediate consequences — one, the Fe-Ni metal in stony meteorites, especially chondritic meteorites, was normally in close proximity to silicates when disrupting collisions occurred [Silicate-metal-sulfide fractionation occurred much slowly in small asteroids which lack the insulating cover of larger asteroids]. Secondly, collisions of unfractionated small bodies may preferentially disperse metal and sulfide which will be prevented or retarded from post-collisional re-aggregations as the nearby vacuum of space will quickly return temperatures to the ambient 100-200 K temperatures of the asteroid belt. In other words, the Fe-Ni metal of stony meteorites — while quite similar in composition and superficially similar in mineralogy to the Fe-Ni metal of iron meteorites — is found in smaller grains of quite variable composition and a larger range in post-collisional cooling histories. And this result in the following additional features of note:

Widmanstätten patterns are only occasionally found in stony meteorites — Widmanstätten patterns require long periods of time within a region sufficient distant from the cold vacuum of space found near the surface of an asteroid. Martensitic regions — produced by quick cooling of shock melted kamacite or austenite — appear to be more prominent in stony meteorites. Small dispersed spheroidal Fe-Ni metal grains — much too small for mineralogical characterization — may be a significant fraction of Fe-Ni metal in stony meteorites. And, in small Ni-rich regions, the Fe-Ni metal was not able to reequilibrate into the kamacite-taenite realm and small 'compositional islands' of tetrataenite (FeNi), awaruite (Ni3Fe) and wairauite (CoFe) were formed. Tetrataenite (found only in 1979) is surprisingly ubiquitous — and is indeed likely to be present in most chondritic meteorites (Cf. Gattacceca et al.,2014).

Fe-Ni metal in meteorites is the product of universal or cosmic processes that were already at work for nearly 9 billion years before the formation of the Solar System [circa 4.56-4.57 billion years ago]. Specifically, Fe-Ni metal in meteorites retains the clear cosmochemical signature of the supernovae that have produced 95-99% of the iron and nickel that are now present in the universe as a whole and in our Solar System. The full panoply of these chemical signatures are not found in any other forms of iron and nickel in the earth's crust today. We will address the implications of these astrophysical/cosmochemical considerations below.

An additional general mineralogical feature of note. Most recovered meteorites are not witnessed falls. Most of our iron meteorites were found years, decades, centuries, or even millennia after their landfalls. Consequently, most of them have experienced significant weathering and, at times, severe weathering. In these instances, two mineralogical features seem particularly worthy of note. One, the observed kamacite/taenite ratios may be quite variable and the original kamacite/taenite ratio may be irretrievable. Secondly, there are a number of sulfides that can be preterrestrial phases in some meteorites and terrestrial weatherates in others (isocubanite, mackinawite, pentlandite come to mind). Indeed, in some meteorites these 'earth & sky' minerals can be products of both pre-terrestrial and post-terrestrial environments. There is such a large trove of meteoritic material in these weathered meteorites that are becoming an increasingly large component of our meteorite collections, both public and private

We have, somewhat artificially, provided an introduction which is somewhat lean on chemistry. It is now time to make explicit some important chemical considerations which are particularly important in studying meteorite mineralogy.

The Complex Chemistry of Meteorites: Geochemical, Planetochemical, and Cosmochemical Considerations.


[Note to the Reader: This subsection is intended to outline the underlying chemical considerations (geochemical, planetochemical, cosmochemical) that underly contemporary meteoritics. In the Case Studies of Part II these same considerations are incorporated into the descriptions and arguments about which suggest terminological changes for specific minerals, mineral groups and series, and mineral varieties. This will, unfortunately, result in some repetition (at least in the first release).

The chemical signatures found in the minerals and mineralites of meteorites can be conveniently distinguished as cosmochemical, planetochemical, or geochemical. Cosmochemical signatures are those that we can discern from the study of the universe beyond the solar system. They could as easily be labelled as astrochemical or astrophysical signatures. Planetochemical signatures are defined here as those that we can discern from the study of other objects within the solar system. Because meteorites appear in the main to be fragments of solid bodies within the inner solar system (objects that orbit the sun or once orbited the sun between the sun and Jupiter) we could as easily speak of planetological signatures. And, of course, because stony meteorites are compositionally dominated by silicate minerals absolutely familiar to terrestrial geologists and geochemists it goes almost without saying that many chemical features found in these minerals have been produced by analogous processes. So we see geochemical signatures in meteorites. We note here that there are a number of interesting and important processes that have left isotopic signatures due to radioactive decay which we will largely omit from our discussion. Also, there are small but significant amounts of gases found in meteorites — both residual gases from the Solar System's formational epoch and products of radioactive decay. These gases will also be largely ignored because of our mineralogical focus.

We summarize here a few important chemical processes which the author believes require alternative perspectives for creating a scientifically transparent terminology for meteorites:

I: Cosmochemistry. Universal Considerations from Astronomy and Astrophysics

The ubiquitous presence of Fe-Ni metal in stony, stony-iron, and iron meteorites and its apparent chemical dominance of the earth's core reflect the universal presence of supernovae ejecta in all regions of the visible universe less than 13 billion light years away. This Fe-Ni metal — as well as the more complex odd-numbered versus even-numbered nuclei abundances in both meteorites and earth rocks — are signatures of cosmochemical processes. Particularly characteristic of meteoritic Fe-Ni metal are the relatively large accessory amounts of cobalt and chromium which were also produced in supernovae.

Stardust. Pre-solar Mineralites ('Microminerals')

Some very small minerals (Corundum, Diamond, Graphite, Hibonite, Nierite, Spinel, SiC, TiC, TiN…) have been found in incompletely metamorphosed 'primitive meteorites' — esp., carbonaceous chondrites, enstatite chondrites, and unequilibrated ordinary chondrites. We note here that our first six terms are IMA-recognized minerals. The last 3 terms (SiC, TiC, TiN) are formally chemical terms, but often refer to minerals which are best described as minerals which have not been formally recognized as minerals, among other things, because they are inconveniently small for the protocols normally used to formalize their status. In lieu of their formal recognition, the author will employ the term 'micromineral' to refer to any such inconveniently small mineral. A review highliting the astrophysics of stardust was provided by Clayton & Nittler (2004). We provide a very brief discussion of the mineralogical-meteoritcal issues in our section on Supplemental Categories near the end of the essay.

Less central to our discussion, but noted here for future reference. Rare earth elements (REE) and Platinum-group elements and their isotopes also carry supernovae and/or red giant signatures, although we will not address these issues in much detail here.

A personal Note: Elements found in meteorites which are, cosmochemically speaking, depleted in the earth's crust may also have been involved in complex planetochemical processes which are beyond this author's competence level. I mention them, however, because studies of potentially identifiable chemical cohorts is a constant subtext of meteorite research

II: Planetochemistry. Planetological Signatures of Terrestrial Worlds and Asteroidal Homes within cis-Jupiter worlds

Planetochemical parameters (e.g., Fe/Mn ratios in basalts and oxygen isotopes) are characteristically different in terrestrial, achondritic, and Martian stones. Likewise, relative populations of sulfides (pyrite, troilite, daubréelite) vary greatly within the inner solar system planets and within meteorites from the asteroid belt. It should be mentioned, however, that discovery of rare sulfides and other minerals within meteorites — especially from the aubrites and enstatite chondrites — does not create tensions between sanctioned or favored mineralogical and meteoritical dialects. Terrestrial geologists and mineralogists appreciate a new mineral discovered in a meteorite, a geode, a lava flow or a mining dump. [It is the implications of seemingly non-controversial shared terminology which often create the problems!]

III. Geochemistry, sensu stricto
Several silicate phases found frequently in both crustal rocks and meteorites can be fruitfully described by similar geochemical parameters: Fe:Mg ratios in Olivine; Ca:Fe:Mg constituents in Pyroxenes; Ca:Na:K constituents in Feldspars. However, many of our most important comments below will be with respect to some of these common minerals. Nomenclature for these minerals can create havoc for those who are pursuing questions of meteorite origins.

Sample questions of origin.
ALWAYS: On what ancient world or nebular region did this mineral first appear? [the one in my hand, the one on the microscope stage, the one in the furnace…]
Question: Was the early proto-earth assembled from a population of planetismals that included a large number of by enstatite-rich bodies? [Enstatite, sensu stricto, En≥ 99.8 mol% — not En — 51-99 mol%].
Question: Why do K-bearing feldspars appear so sparsely and so late in the accretion of the plagioclase-rich ordinary chondrites?
Question: Can the volatiles in small meteorite phosphates give us an immediate lead into the original homeworld of a meteorite!

Linguistic Interlude: Scientific Complexities Buried in Disciplinary Specific Terminology.


When speaking informally with colleagues and/or when writing for an audience which is familiar with many details of the issues important to the hearers and listeners it is both normal and, sometimes, extremely productive to use various linguistic shortcuts — abbreviations, acronyms, odd turns of speech which make it easier for the audience to confront the immediate issue of interest. This is true of lawyers, preachers, scientists, and millions of cohorts of human beings who — within the confines of a likeminded audience — think that they are only using common sense. However, from time to time, events unfold and the accumulated drift of conscious and unconscious seemingly minor or even trivial alterations in usage create new linguistic tensions and contradictions, new dialects and even new languages. What I wish to highlite here are some contrary implications that have most particularly accompanied the development of 21st Century Mineralogy and Meteoritics. The terminological proposals I make here are simply proposals — and they may or may not be adapted by others. The scientific issues addressed, however, are important and so my thoughts and terminological proposals are offered for what they are worth. A few important definitions or alternative redefinitions are proposed in the text. In the Appendix a number of abbreviations, acronyms, neologisms, and technical terms are provided for both neophytes and/or experts as appropriate. [Once I reach the 5th page of an article I can almost never remember all the pertinent acronyms which have been properly defined somewhere along the way.] In this article I will also interject, as it were, certain well-known and often ignored linguistic principles which I believe are relevant to the discussion at hand. We proffer two of them here.

I. Live languages. Why dialectical differences are natural features of spoken and written languages in all formal and informal registers.

When the English-speaking British colonists came to the new world in the 16-19th Centuries, they did not set out to subvert the proper use of British English (The King's English or The Queen's English). Rather, they did what all users of a live language do — they adapted their store of words from their mother tongue to the exigencies of their new home. They also borrowed new words from native peoples and from fellow immigrant, but the primary changes were due to innovations that grow whenever any people confront new realities. It is sometimes said that the United States and the United Kingdom are two peoples divided by the same language. [Many readers will have better examples of this fundamental linguistic principle from their own experiences.]

In similar fashion, meteoriticist and other planetary scientists — while studying stones arriving from unknown worlds and further constrained by very, very limited amounts of disposable material — have developed formal and informal terminological conventions to describe the minerals and mineralites which they find in meteorites. Scientists, however, have a specific predilection for one linguistic malady which obscures this natural process. Scientists want to speak a common or 'universal' language. It is, of course, necessary in most instances for different parties to use the same words to refer to the same realities as best they can. However, what often happens in fact is that unspoken verbal twists and circumlocutions are used to avoid the appearance of these linguistic tensions. A simple example, 'geochemical' in the meteoritical community frequently means either (1) "Pertaining to the chemistry of terrestrial worlds and smaller silicate-rich solar system worlds" or (2) "Pertaining to the chemistry of solar system worlds." Traditional geologists who focus on earth rocks, of course, will occasionally use the word in a similar (mildly) extended sense, but with much fewer unexpressed implications.

II. Transparency and Reformulation. Episodes in mathematical and Scientific History

Over 22 centuries ago Archimedes of Syracuse (circa 287-212 BCE) was able to derive the fundamental mathematical relationships to calculate the area of a circle or a parabola and both the area and volume of a sphere. To derive these relationships he used geometric language supplemented by some very helpful formulaic expressions to complete a process that we would today call mathematical integration. Over 18 centuries later, Johannes Kepler (1571-1630) used somewhat similar methods to determine the volume of semi-regular solids and the second law of planetary motion ('Areal velocity'). Proofs such as these required roughly 10-30 paragraphs of texts. Today, a competent college mathematics major or a practicing engineer — with the help of the 'Calculus' derived by Newton and Leibniz — can derive these and similar relationships ('formulae') on a single page.

Blaise Pascal (1623-1662), while commenting on his famous 'Pascal Triangle', stated that one should not think there was nothing 'new' in his rearrangement in tabular form of previously recognized relationships. The rearrangement was in fact a new, better, deeper, and more pregnant way to look at the relationships which form the framework of the binomial theorem.

PART II — Case Studies in Planetochemical Mineralogy: Fe-Ni metal, 8 mineralites, and 3 Supplemental Categories

Case Studies & Supplements (Complete List): Fe-Ni Metal; 8 mineralites — Kamacite, Enstatite, Forsterite, Feldspar, Fassaite, Plagioclase, Apatite, Merrillite; 3 supplemental categories — Ca-poor pyroxenes; Stardust; Ni-rich metal.

[Note to the Reader: Part II utilizes the explicit recognition of 'chemical' distinctions found in the above "The Complex Chemistry of Meteorites" subsection of Part I. This has resulted in the necessary repetition of these chemical distinctions in the individual 'Case Studies' of Part II. This is particularly evident in the "Primordial Metal — Fe-Ni metal" subsection below.

One of the casualties of scientific progress is the development of disciplinary specific terminology that can be either opaque or inconsistent with common usage in other scientific disciplines. Both the meteoritic and mineralogical communities are not immune from these developments. There have been and continue to be a number of meteoriticists who are both mineralogical experts and respected meteoriticists. However, the task is quite difficult. Terrestrial mineralogist generally work within a framework where sufficient samples of important rocks and mineral are either on hand or are readily available because their places of origin are known. Meteoriticists, however, are almost always restricted to small samples and, with some very interesting exceptions, do not know in any specific manner where in the solar system a particular meteorite came from. Normally, we are pretty sure that most of them have originated in the solar system — and that is precisely where we, the meteoriticists, begin.

There is, however, a deeper issue embedded in the practical exigencies of how one utilizes the predominantly terrestrially-focused language of mineralogy to describe the mineralogy of meteorites. Meteorites are samples of a far greater range of lithological environments than those which inform most geologists, geochemists, and mineralogists. Consequently, meteoriticists — both in their formal publications and, even more so, in their informal communications, must try to blend the minerals which, so to speak, appear before their 'eyes' [in their hand, in the microscope,…] and their quest to determine the sources, the original homeworlds of the skyrocks within which the minerals are embedded. And, to drive the point home, I provide eight examples of mineralogical terms often used within the meteoritical literature in a manner inconsistent with IMA-preferred or IMA-sanctioned usage. I will also add less extensive remarks about three related examples of primary putatively 'chemical' terms which are entangled in mineralogical realities. These candidates for future discussions include (A) some seemingly outdated terms used to describe Ca-poor Pyroxenes, (B) Stardust [pre-solar minerals], and (C) the curiously named Ni-rich minerals found in the Fe-Ni metal of meteorites and, incidentally, also found as exceedingly rare terrestrial phases.

A Planetochemical Definition:
Mineral (planetochemical)— A natural solid phase or compound with (1) a defined crystallographical structure [usually a single 3D space group] and (2) an elemental compositional range which transparently references those geochemical, planetological, and cosmochemical considerations deemed best suited to determine the original homeworld (OPB) or formational environment.

Mineralogical Neologism, Definition:
Mineralite — A mineral or mineralogically significant phase or aggregate. We specifically include minerals, mineral groups, solid solution series, mineral varieties, glasses, bi-minerals, generic multi-mineral aggregates, chemically-defined phases, and mixed chemical-mineralogical phases. Our definition includes — but is not restricted to — all IMA-minerals and all Mindat-hosted 'mineralogical items'. In addition, we include chemically-defined phases such as silicon carbide and mixed chemical-mineralogical terms [Ca-rich Pyroxene, Ca-phosphate]. These additional terms are frequently utilized in meteoritic literature, but are very sparsely utilized in the generic geological and geochemical literature.

Primordial Metal — Fe-Ni metal: The Cosmochemical Messenger

Fe-Ni metal — Ni-bearing metallic (non-oxidized) iron which is found only in meteorites and never in terrestrial metallic iron (aka, native iron).

Within the meteorite community Fe-Ni metal is the preferred chemically-defined term for meteoritic Ni-bearing iron. Nota bene: The 'chemistry' referenced here are those cosmochemical/planetological signatures which indicate that all meteoritic iron is of non-terrestrial origin.
Synonym: Nickeliferous iron, an early 20th Century scientifically preferred term for (today's) Fe-Ni metal.

Until human beings were technologically able to separate oxygen from the natural iron oxides which surround us, the only metallic iron which human beings encountered was almost always found in meteorites both as the dominant constituent in iron meteorites ('irons') and as a significant constituent in most stony meteorites. Detecting nickel in such 'Meteoritic Iron' is usually more than sufficient to establish that a suspected meteorite is indeed an actual extra-terrestrially derived meteorite. Furthermore, a number of additional chemical and mineralogical signatures are associated with this extraterrestrial Fe-Ni metal. Bulk Fe-Ni metal in meteorites is characterized the (1) dominance of metallic iron in all but a few instances, (2) the invariable presence of, usually, 5%-30% Ni [with a few Ni-rich outliers], (3) smaller amounts of equally ubiquitous Cobalt [usually, 0.25%—3%], as well as (4) even smaller, but detectable amounts of accessory chromium, phosphorus, and various other 'siderophilic' elements. Chromium and phosphorus are present in iron meteorites and in ordinary chondritic Fe-Ni metal at levels well above those normally found in terrestrial crustal rocks. However, the actual Cr and P abundances within such Fe-Ni metal phases are critically dependent upon the O-Fugacity levels during the formation and cooling of these metal phases. Furthermore, during the actual formation of the meteoritic parent bodies, 'competing' P- and Cr-bearing oxides, sulfides, phosphides and phosphates would have diminished the Cr and P content of the metal. Nevertheless, before the end of the 19th century, the only natural and non-meteoritic unoxidized metallic iron known to human being were some massive iron deposits used by Greenland Eskimos. Much of this (crystalized) native iron was accompanied by 'alloyed' nickel (roughly 2-3%) and various Pt-group elements with non-meteoritic signatures.

Meteoritic Fe-Ni metal is cosmochemically important because it carries clear the clear signatures of its formation within supernovae — most clearly seen in the abundance and ratios of the 'Iron Group' elements and their isotopes within the iron meteorites, stony-irons, and in chondrites. These supernova-derived constituents have been somewhat modified within these meteorites — presumably due to variations in primordial silicate-iron ratios and different physical conditions of mantle-core formation, and other factors. Nevertheless, the supernova signatures are still clearly in evidence in these 'secondary' products of solar system's formational epoch.

Occasionally, metallic iron in meteorites is Ni-poor — apparently due to secondary and tertiary processes during the formational epoch of the meteoritic OPBs within the ancient Solar Nebula. In a similar manner, the original Fe/Ni, Fe/Co and Ni/Co ratios in native terrestrial oxides, sulfides, and extremely rare metallic iron and other metallic alloys, have been severely altered and effectively erased by tertiary and quaternary processes associated with the earth's continuous history of orogeny, plate tectonics, erosional alterations, and even minor chemical alterations due to incoming meteorites [occasionally important at the 'local' level]. The most important exception to this generalization are the isotopic ratios of Iron Group elements and other siderophiles.

It is the author's predilection to refer to all metallic iron with Ni≥ 4% as Fe-Ni metal. Instances of terrestrial or meteoritic metallic iron with less than 4% Ni is — in the author's opinion — best labelled simply as iron. We will discuss the mineralogical issues associated with these primary cosmochemical realities as they arise (Vide infra: kamacite, Ni-bearing metals).

Synonyms, details:
Meteoritic Iron (Mindat) — Strictly speaking, Mindat's 'Meteoritic Iron' signifies mineralogically uncharacterized Fe-Ni metal.

'Fe-Ni metal' (meteoritic literature) — Normally a chemical/cosmochemical descriptor which is mineralogically neutral i.e., there is no explicit mineralogical content. The following usages (often implicit) are among the most common:
I. 'Fe-Ni metal' may be used when the authors have made no explicit or extensive mineralogical investigation of the meteorite metal.
II. 'Fe-Ni metal' may be used when any Fe- and Ni-bearing minerals known to be present (e.g., kamacite, taenite, tetrataenite…) are immaterial to the author's thesis.
III. 'Fe-Ni metal' may be used as a mineralogically deferential statement. [If phases other than kamacite are suspected, the term 'Fe-Ni metal' may be used in lieu of more definitive mineralogical information that may arise later.]

To be sure, it goes 'without saying' that kamacite is usually 'expected.' It is understood that the 'Fe-Ni metal' is usually predominantly kamacite and that in almost all meteorites the 'Fe-Ni metal' contains at least a modicum of kamacite. Exceptions might include compositionally anomalous meteorites, highly shocked meteorites, and — increasingly important in our meteorite collections — heavily weathered meteorites.

Eight Featured Case Studies of Meteoritic Minerals — Formational Signatures in Scientific Definitions

8 minerals and mineralites — Kamacite, Enstatite, Forsterite, Plagioclase, Fassaite, Feldspar, Apatite, Merrillite

The 8 cases considered here illustrate instances of tensions between required and/or preferred usages of IMA-defined mineral groups and subgroups and the need for scientific transparency in utilizing these same terms in the meteoritical and planetological sciences. The terms are presented roughly in order of their importance for understanding meteorite origins. In most instances here, the author is advocating the use well-understood mineralogical terms (labels) in a manner which is reasonably consistent with the problematics of contemporary meteoritics. These labels may or may not be the preferred terms within most IMA-sanctioned discourse. The more important question is whether these or similar suggestions can promote the development of a mineralogical idiom which does not muddy the geo-, planeto-, cosmo-chemical questions which drive meteoritics today.

Mineral Case Study #1: Kamacite ('α-iron') — The Primordial Mineral

If we look back thru the mists of human history, weapons, tools, jewelry, tombs, ancient scripts, hieroglyphs and other artifacts reveal that human beings have occasionally discovered masses rich in Fe-Ni metal (using today's preferred term). Etymology and anthropology make it clear that some societies also knew that this iron had 'fallen' from the sky. While nickel-free iron was produced by the Hittites and has gradually become a staple of modern industrial societies, almost all non-artificial forms of unoxidized or 'free' iron found upon the planet earth are instances of Fe-Ni metal, iron-invariably-accompanied by nickel. As witchcraft, wizardry, and alchemy gradually evolved into chemistry, metallurgy and mineralogy it was gradually recognized that most Fe-Ni metal found in meteorites came in two distinguishable mineralogical phases. The most abundant phase is kamacite, a face-centered cubical mineral that ordinarily contains from 5-7.5 % Ni and 0.25-3.0 wt% CO. In most iron meteorites ['Octahedrites'], kamacite is the primary constituent and is accompanied by subequal amounts of taenite and lesser amounts of troilite, schreibersite and other minor phases. Among the IIab iron meteorites we even find a small number of essential monominerallic meteorites ['hexahedrites'] dominated by kamacite. In a few instances, the kamacite is found as a single crystal [masses may exceed 1,000 kg!]. Kamacite has been studied by metallurgists for a few centuries and eventually became designated in metallurgical circles as alpha-iron ('α-iron'). [Even now it often seems unnatural to call a metallic mineral a 'mineral' — so that even in scientific literature kamacite is often called an alloy. More importantly, it appears that communication lines between metallurgists and mineralogists studying silicate-rich phases have not always been very keen.] Kamacite is also found in 'plessitic' iron meteorites where it forms microscopic kamacite-taenite intergrowths. Kamacite features can also be drastically disordered in highly shocked and in shock-melted meteorites. Finally, we must also note the occasional exception. In a few unusually Ni-rich meteorites kamacite appears to be missing or, at least, exceedingly sparse.

Today, kamacite is still the predominant Fe-rich mineral which any terrestrial geologist will encounter on the planet earth whether he or she is looking for iron ores or meteorites anywhere on or below the surface of the earth. To the meteoriticist, kamacite is not — historically or practically — a 'variety' of iron. It is the most common form of natural metallic iron which we will find anywhere near the surface of any planets or asteroids in the next few centuries. It is, as noted earlier, a cosmochemically significant mineral.

Terminological Niceties and Planetochemical Imperatives
Kamacite (meteoritic literature) — Normally a chemical/cosmochemical descriptor which is mineralogically neutral, i.e. there is no explicit mineralogical content. The term may be used
(1) when the authors have made no explicit mineralogical study of the meteorite metal;
(2) as a mineralogically deferent statement. [If phases other than kamacite are suspected, the term 'Fe-Ni metal' may be used in lieu of more definitive mineralogical information that may arise later.];
(3) when Fe- and Ni-bearing minerals known to be present (e.g., kamacite, taenite, tetrataenite…) are immaterial to the author's thesis.
(4) Any meteoritic metallic iron with isometric body-centered crystallographic structure including Ni-poor varieties (Ni ≤ 4 wt%). Application of the term kamacite such 'tertiary' iron in which the characteristic meteoritic Ni-signature has been largely suppressed is, in the author's view, an improper use of the term.

Kamacite v. Iron — Linguistic Consistency and Planetochemical Transparency.

It is has been known for over a century that meteoritically ubiquitous troilite (FeS) is an end member of the terrestrially encountered pyrrhotite (Fe1-xS8), a magnetic mineral. However, stoichiometric troilite is not only geochemically anomalous, it is also non-magnetic. This recognition is illustrative of a general principle within meteoritic and planetochemical mineralogy. Endmember compositions of mineralogical solid solutions series are frequently diagnostic of critical parameters in both terrestrial and meteoritic environments.

Thus, on the earth troilite is extremely rare, but pyrrhotite is a well-known members of various sulfide facies. In meteorites, troilite is ubiquitous while primary pyrrhotite is found within a quite circumscribed set of meteorites (normally those which have been exposed to preterrestrial hydration or oxidation).

In similar fashion, on earth, kamacite is an unusual but obvious extra-terrestrial exotic while native iron is extremely rare. Contra-wise, kamacite is an expected component of most meteorites but Ni-poor metallic iron is a product of tertiary processes (usually iron removal from planetologically normal Fe-Ni). From a consistent planetochemical perspective, then, terrestrial Ni-poor iron is an endmember of the kamacite solid-solution mineral series. Cardinal principle: End members of almost any solid-solution mineral series may require linguistically explicit recognition.

Mineral Case Study #2: Enstatite [sensu stricto] — Extremely 'oxygen starved' environments plus the earth's formation
Enstatite (Mg-rich, Fe-,Ca-poor Orthopyroxene)

Almost all stony meteorites, many stony-irons (e.g., mesosiderites) and even a few iron meteorites (iron meteorites w. silicate inclusions) contain significant amounts of one or more pyroxenes. A small number of meteorites (primarily the enstatite chondrites and aubrites) are unusually rich in an extremely iron-poor form of the common Ca-poor orthopyroxenes found in many terrestrial rocks, lunar basalts, and the majority of meteorites. Both the common Ca-poor orthopyroxenes and the unusually Fe-poor orthopyroxenes are examples of the solid-solution ferrosilite-enstatite series. Among terrestrial geologist, the convention has arisen of labeling members of the series as 'Enstatite (En)' if they contain more than 50 mol% enstatite (En>50) and, conversely, 'Ferrosilite (Fs)', if they contain more than 50 mol% ferrosilite (Fs>50). However, within the meteoritical literature there has been a tendency to use restrict the term 'enstatite' to exceedingly Fe-poor orthopyroxene (En>95 or, even, En>98 or En>99). The scientific grounds for this linguistic hesitation are quite simple. In the solar system and in the cosmos, oxygen fugacities found on or near the earth's surface are, statistically speaking, almost always planetologically and cosmically anomalous. In more cosmically normal environments, we would expect that ambient gases such as hydrogen, methane, or carbon monoxide in most rock-forming or planet-forming environments would prevent the formation of Fe-bearing silicates. As a general rule, we would expect — in laymen's terms — that both planetary atmospheres and star-forming nebulae have been and will continue to be more starved for oxygen than those that shaped the formation of our moon. More technically, but — perhaps — even more to the point, evidence is continuing to mount that available oxygen during the earliest epoch's of our own planet's formation would have been exceedingly restricted compared to the subsequent rock and mineral formation of the past 4 billion years. The earth may be — mostly — a pile of Fe-poor silicates, Fe-Ni metal, and a few sulfides which were altered only superficially by the late addition of a host of comets and carbonaceous asteroids.

The author's solution.
The scientific stakes are sufficiently weighty that I have adopted the practice of restricting the term 'enstatite' to those orthopyroxenes which are more than 90 mol% enstatite (En>90).

Specifically, I adopt the following conventions:
'Enstatite' = 'Enstatite, sensu stricto' = Ca-poor Orthopyroxene with En≥ 90 mol%.
Orthopyroxene = Ca-poor Orthopyroxene with En 10-90 mol%.
'Clinoenstatite = 'Clinoenstatite, sensu stricto' = Ca-poor Clinopyroxene with En≥ 90 mol%.
Meteoritical purists might also be interested in the following convention:
'Enstatite' = 'Enstatite, sensu puro' = Ca-poor Orthopyroxene with En≥ 98 mol%.

When citing (usually older) sources which refer to the bronzite and hypersthene varieties of orthopyroxene, the author may be more explicit:
Orthopyroxene ('bronzite') = Ca-poor Orthopyroxene [Enstatite 80-90 mol%]
Orthopyroxene ('hypersthene') = Ca-poor Orthopyroxene [Enstatite 70-80 mol%]

The occasional contemporary references to 'bronzite' or 'hypersthene' are compositionally more precise terms than my recommended 'orthopyroxene.' However, these terms have often been defined differently in the meteoritical literature and in the earth-focused mineralogical literature. Thus, it seems unclear whether these terms can be salvaged for general use. Still, the need for the restricted definition of 'enstatite' appears to be critical to discussions about the original homeworlds of the enstatite chondrites, aubrites, and some UOCs as well as to determining the original population of earth-forming planetismals! The extraordinary purity of enstatite in some meteorites [En 98.0-99.9 mol%] is so striking that it simply cannot be ignored. An older review (Keil, 1989) is as good a starting point as any to begin exploring these issues.

Mineral Case Study #3: Forsterite — Following the shadows of extremely low oxygen fugacities.

Olivine is the dominant 'mineral' (or, 'mineral series') in most ordinary chondrites, many carbonaceous chondrites, the rare martian chassignites, and is usually at least an accessory component in most other stony meteorites. It is also the dominant silicate in the stony-iron pallasites. In most instances, meteoritic olivine is 'forsteritic' — the Mg component is usually dominant, but not excessively so. Indeed, terrestrial olivine often exhibits similar compositional trends. Olivine in dunites and other ultramafic rocks is also usually forsteritic. However, when terrestrial olivine is found in various reducing environments — particularly those involving metamorphosed limestone — the iron may be lost with the remaining olivine becoming unusually iron-poor. During the 20th Century, unusually Mg-rich Olivine [Fa≤10 mol%] was often designated as 'Forsterite.'

Olivine in the unequilibrated ordinary chondrites (and in other stony meteorites with unequilibrated silicates) can be quite variable and usually includes a conspicuous minority population of Fe-poor 'Forsterites.' Furthermore, within the more unequilibrated chondrites this population often includes a small population of extremely Fe-poor olivine (Fo 98-99.9 mol%) grains. Such Forsterite is additional evidence that the present population of asteroids, planets, and meteoroid relicts were created from feeding zones within the original planetismal populations whose elemental and mineralogical variations were well outside the normative compositions of the majority of the surviving bodies. Like Enstatite and Clinoenstatite, preservation of compositional transparency for Forsterite is an utterly critical issue in disentangling the history of the Solar System — especially within the relatively hotter and more crowded regions near and interior to the proto-earth.

Planetochemically Preferred Terminology for Olivine (Forsterite—Fayalite Solid Solution Series):
Forsterite (Forsterite, sensu stricto) = Olivine (Fo ≥ 90 mol%; Fa < 10 mol%)
Olivine (generic) — Olivine (Fa 10-90 mol%) or, equivalently, Olivine (Fo 10-90 mol%)
Fayalite — Olivine (Fa >50 mol%)

Informal Usage.
Olivine is now defined in normal geochemical discourse as the Forsterite-Fayalite 'Solid Solution Series' rather than as a per se IMA 'mineral.' However, olivine in an hand lens or in a microscope is, phenomenologically speaking, simply a 'mineral' (the issue of precise mineralogical identity is not normally an immediate issue as it might be when viewing, say, spinels or sulfides).

Mineral Case Study #4: Plagioclase (Plagioclase Feldspar): Meteoritically Normal Feldspar

Mindat Convention: Plagioclase is an exact synonym for the 'Albite-Anorthite solid solution series' in Mindat Algorithms.

Meteoritic feldspars are usually potassium-poor plagioclase feldspars with triclinic symmetry. The predominate aluminum-silicate of ordinary chondrites is usually albitic oligoclase, the predominate aluminum-silicate of the HED meteorites is usually anorthitic bytownite, and a few lunar rocks are so rich in anorthite that they are referred to simply as 'anorthosites.' Consequently, within strictly meteoritic and planetological literature, plagioclase is usually the preferred term for the K-poor varieties of the 'Feldspar Group'.

To be sure, potassium-rich feldspar is an occasional minor mineralogical component of various meteorites. When potassium-rich feldspar is present, it merits explicit attention. Otherwise, it seems appropriate to refer to plagioclase feldspar simply as plagioclase. [In other words, Alles in Ordnung!]

Secondary Issue: Albite and Anorthite.
Unusually Albitic and Anorthic varieties of Plagioclase are sometimes found in UOCs and elsewhere. In many instances unusually Ca-rich varieties appear to represent Hi-T environments. It seems prudent, here, best to utilize the following conventions:
Albite (Albite, sensu stricto) — Plagioclase (Ab≥90)
Anorthite (Anorthite, sensu stricto) — Plagioclase (An≥90)

Again, we wish to isolate those end-member constituents which contain poorly understood mineralogical and chemical varieties within the population of terrestrial forming planetismals. These suggested conventions are intermediate between current IMA definitions of Plagioclase and earlier standard 20th Century definitions of the 'Albite, Oligoclase, Labradorite, Bytownite, Anorthite' sequence.

Mineral Case Study #5: Fassaite

Fassaite, a Ti-bearing variety of Augite, was first reported from the Fassa Valley in the Monzoni Mountains of Italy. Both in terrestrial rocks and in meteorites it appears to be produced in relatively high temperature (high-T) environments. At high temperatures incipient clinopyroxenes can more readily accommodate aluminum and titanium (and in terrestrial rocks more ferric iron [Fe+++]) than can clinopyroxenes formed at the more normal 1000 K—1500 K formational ranges found in most meteorites and terrestrial rocks. Its importance in meteorites is exhibited as minor high-T inclusions in unequilibrated ordinary chondrites and carbonaceous chondrites and as a major or even dominant component of the rare Angrites, a very unusual chemical group of achondritic stones. A detailed petrographic examination of fassaite in the Allende meteorite in Simon & Grossman (2006) illustrates the importance of fassaite as an indicator of important Hi-T formational environments in the very early solar system. Fassaite is also the primary constituent of the Angra dos Reis meteorite, the prototype for the achondritic Angrites and the only witnesses fall. The presence of Fassaite [and Kirschteinite (Ca-rich member of the Olivine group)] in the angrites has been recognized as 'a matter of note' for over 4 decades [e.g., Mason (1972), Rubin (1997), Keil (2012) …].

However, in embedded in two recent essays we find a very curious descriptor for Fassaite: Angrites consist of major… 'Al-Ti-bearing diopside-hedenbergite (formerly called fassaite)' [Mittlefehldt et al. (2002);Keil (2012)]. A little research will reveal that the term 'Fassaite' has been 'discredited' by the IMA. This has all of the appearance of a genuinely earthbound perspective. There may be not be very many instances that one needs to refer to Fassaite in referring to terrestrial geological processes (this author is fundamentally ignorant about the matter). However, the issue of Fassaite for meteorites is a critical one for meteoritics. The angrites are one of 5 important differentiated achondritic meteorite groups (Angrites, Aubrites, HED meteorites, Lunar meteorites, Martian meteorites) which are fragments of relatively large asteroidal or planetary OPBS. The 'discrediting' of this robust and genetically significant term for a mineral variety appears — from the planetochemical perspective — to be lacking in scientific relevance.

Of course, one solution for this would not be to simply 'grandfather' the term. The term may not be as useful in terrestrial geochemical discourse as it is in planetological/planetochemical research. We could, instead, mark the term as a meteoritics-specific mineral variety. In other words, we should recognize that a more robust and transparent scientific language must honor the reality and utility of scientific dialects. We will never have a truly universal scientific language — the universe is not only stranger than we can imagine, it is clothed in perplexities that we cannot always beword. We can, however, make our formal language(s) more cosmically open.

Mineral Case Study #6: Feldspar (esp., K-bearing varieties)

Approximately 60% of the earth's crust consists of feldspar and several similar minerals and, of course, their weathering products. However, the potassium component of the sodium-calcium-potassium mineralogical triumvirate that dominates the mineralogy of the terrestrial crust is usually missing in action in meteoritic mineralogical inventories. However, when potassium-rich feldspar is present — for example, among the primitive 'unequilibrated ordinary chondrites' (UOCs), we need to take note. There is some evidence that relatively rich K-rich bodies were among an important late-arriving cohort of minor asteroidal bodies associated, perhaps, with the history of the LL chondrites or, maybe, even with the late lunar bombardment (~4 Ga BP). My own practice thus is normally to restrict the term 'Feldspar' with respect to meteorites to instances where the orthoclase component is significant. Normally, an orthoclase (Or~≥10 mol% or Or>An)component is sufficient to merit explicit notice.

Unfortunately, for the unequilibrated ordinary chondrites and/or highly brecciated meteorites, an additional factor comes into play. It is frequently very difficult to determine the composition of very small feldspar grains. When the actual composition of such grains becomes highly uncertain — whether they are simply too small to measure and/or too variable to characterize, the term 'feldspar' may become the term of choice.

Mineral Case Study #7: Apatite, A ubiquitous phosphate group.

Apatite and Apatites have been defined in various ways over the years. Major redefinitions involve the recent creation of an Apatite super Group as well as redefinitions of various minerals and varieties within the Apatite Group of minerals. We are here concerned primarily with 3 phases currently defined as minerals — Chlorapatite, Fluorapatite and Hydroxylapatite. Chemically speaking, these 3 hexagonal minerals are essentially calcium phosphate minerals with a single lattice site for different volatiles. Quoting MinDat: "Most [terrestrial] "apatite" is fluorapatite, whereas hydroxylapatite is much less common and chlorapatite is very rare."

The situation for meteoritic apatites is somewhat different. Chlorapatite appears to be relatively much more common in meteorites. Indeed, it was the only apatite mentioned in the Mason (1972) review of meteoritic mineralogy and appeared prominently in the later Rubin (1997) review. On the other hand, hydroxylapatite is much less common in meteorites for the obvious reason that most meteorites were formed and have remained in relatively anhydrous environments. Unsurprisingly, hydroxylapatite has been reported in Martian meteorites and Carbonaceous Chondrites.

As apatite and other phosphate grains in meteorites are usually small and difficult to distinguish mineralogically it is probably to be expected that relatively complete mineralogical descriptions of apatite minerals are few and scattered. Indeed, it appears that chronologically studies pursuing Pb206/Pb207 ratios have provided us with some of our best mineral specific studies of apatite minerals. The enhanced lead abundances in apatites (and merrillite [vide infra]) have given researchers extra incentives to be much more mineralogically precise than the frequent cavalier 'minor Ca-rich phosphates' that provide the only information about phosphate minerals in the studies of all-too-many individual meteorites. However, there is another matter that contributes to mineralogical imprecision in the study of meteoritic apatites.

As a general rule, then, the primary component of meteoritic appears to be Chlorapatite with Fluorapatite usually the second-most important component. In spite of the fact that many meteoritic Apatites have a large Chlorapatite component, determining that a small Apatite grain is 50+ mol% Chlorapatite (or, Fluorapatite) is often a difficult and/or time-consuming task. The difficulty is compounded by two additional considerations. One, other volatiles may be present and contribute additional ions into the mix. Rubin's review specifically mentions 'carbonate-fluorapatite' and 'fluor-chlorapatite' melanges to the mix. Thus, Apatite (sensu lacto) may be the preferred term to use when statistical fluctuations make overly precise language somewhat precious.

Mineralogically and chemically detailed studies of meteoritic Apatites do occasionally appear in studies of the unequilibrated chondrites and in chronometric studies. Apatites are enriched in uranium so that phase specific determination of the lead isotope ratios so important in the U/Pb dating of the most ancient meteorites sometimes provides more mineralogical detail than do straight forward petrological-mineralogical studies.

Another apparent mineralogical desideratum that does not appear to get as much attention as it should are the Apatite/Merrillite ratios within the phosphate budget of specific meteorites. Both Apatite and Merrillite are present in many meteorites and their relative proportions should be quite sensitive to volatile fugacities. [Cf. Mineral Case Study #9: Merrillite)

Mineral Case Study #9: Merrillite, A meteoritically ubiquitous anhydrous phosphate.Merrillite is an anhydrous Ca-phosphate found in meteorites and other extraterrestrial rocks. It was believed to have the chemical formula [Ca3 (PO4)2] and has often been referred to simply as ‘extraterrestrial Whitlockite.’ Merrillite has been used as to describe meteoritical phosphate for several decades, but in many instances its properties were apparently indistinguishable from the ‘Whitlockite’ studied by terrestrial geologists and mineralogists. However, it now appears that at least in some instances, Merrillite has a composition of Ca9(Na,Mg,Fe)(PO4)7 and can be assigned the crystallographic class [H-M symbol (3m), space group (R 3m). On the other hand, the New Hampshire type specimen for Whitlockite apparently has a composition of Ca9(Mg,Fe)(PO4)6(PO3OH) and crystallographic H-M symbol (3m) and space group (R 3c). Looking closely at slitely diverse formulae one notes that it may be considerably more difficult to produce truly anhydrous sulfates on earth than in some extraterrestrial environments. Stay tuned!

Case Studies, 3 Supplemental Categories: Suggestions for Future Investigations

Ca-poor pyroxenes; Stardust; Ni-rich minerals

This subsection presents other instances in which mineralogically precise terminology is often lacking for some planetological and astrophysically significant phases found in meteoritics.

Case Study #B1: Ca-poor pyroxenes
Ca-poor Pyroxenes (Enstatite, Bronzite, Hypersthene, Clinoenstatite, Orthopyroxene, Ferrosilite; Pigeonite [!?])
4 IMA-defined Minerals (Ca-poor Enstatite, Clinoenstatite, Ferrosilite) and moderately Ca-poor Pigeonite ; 2 IMA-defined Ca-poor Mineral Subgroups (Clinopyroxene, Orthopyroxene); and 2 'retired' Ca-poor Mineral Varieties (Bronzite, Hypersthene) are routinely examined as part of the Classification Process. While in many cases (e.g., ordinary chondrites), it is possible to make a provisionally meaningful classification by determining the average composition of the olivine supplemented by determining the Cobalt content of the kamacite, it is always desirable and frequently necessary to determine the average composition of the dominant Ca-poor pyroxene. We have already indicated that the chemical composition of dominant silicates is primary datum for all meteorites, most especially when enstatite or clinoenstatite (sensu stricto) are present. In addition to the compositional parameters of classification, the textural analysis which explicitly and implicitly attempt to reconstruct metamorphic and brecciation processes (OC petrologic types, etc., etc.) are normally and, again, sometimes necessarily involved with the recognition of those specific mineralogical species and varieties which encoded those processes.

The scientific point is simple — fundamental genetic parameters are encoded in the mineralogical terms that are used to describe meteorites. Meteoritic Ca-poor pyroxenes, in particular, are found in multiple physical and chemical varieties within single meteorites. A plethora of fundamental mineralogically significant distinctions which were formally recognized in the past have now been replaced by a sparse and inadequate mineralogical vocabulary which must be augmented by informal, non-standard labels and/or elaborate circumlocutions in most scientific discussions which attempt to describe and explain the complex journey of any given meteorite from an identifiable habitat within the ancient solar system (OPB or nebular region) thru a series of metamorphic and collisional events until its current status as a studied meteorite. Similar, but less complex considerations apply to the Ca-rich pyroxenes. The reasons for the impoverished vocabulary used to formally discuss meteorite origins. We note three. One, the scientific issues entertained in the study of meteoritic materials are complex. Two, currently all scientists are earthbound — and any scientist, even one who has studied extraterrestrial rocks and astronomy for most of his/her professional life will necessarily carry some measure of earthly preconceptions into his/her work no matter how creative and how intellectually open he/she might be. Three, the meteoritics community itself has sometimes been spellbound by the discovery that certain chemical signatures can provide us with robust hypotheses about the separate, multiple provenance of meteoritic parent bodies and, indeed, the occasional definite determination of such parent bodies as the moon and Mars. It has been, indeed, a scientific marvel. However, every meteorite which has reached the earth has travelled on what has been an intrinsically idiosyncratic journey. And, each mineralite within the meteorite — every mineralogically significant entity (mineral, mineral variety, bimineral, glass, chemical variation of such phases) — contains a potentially decipherable record of one or more event in the meteorite's history. These discrete mineralogical records, the author believes will be an indispensable component in our search to discover the extent to which our current hypotheses about meteoritic origins are, in fact, partially or even mostly true — and the the extent to which they must be modified or even abandoned.

Historical Note. The author would suggest that the discovery of natural groupings of iron meteorites into an initial four Ga-Ge groups and the realization that we already have several Martian meteorites in our collections were seminal influences in the gradual reorganization from the explicitly mineralogical, but also implicitly chemical science of meteoritics to the current explicitly chemical, but also implicitly mineralogical science of meteoritics. This characterization is, of course, an oversimplification. The point is, however, that science is a human activity and that it, like other human activities, may become over enthralled with the dernier cri.

Case Study #B2: Stardust (SiC, TiC, TiN)
Moissanite and Silicon Carbide (SiC)
Circa 1905, the mineral Moissanite was discovered within the Canyon Diablo meteorite. There was some apprehension that the Moissanite, an isometric polymorph of silicon carbide (SiC), might be a contamination product of carborundum, a synthetic ceramic. However, it has been gradually established that moissanite is present in both meteorites and kimberlite pipes. A large number of silicon carbide polymorph have been studied in the laboratory and consequently Moissanite, the isometric polymorph has been labelled as the 'β−SiC' polymorph.

In recent decades astrophysical studies have revealed the presence of silicon carbide [SiC] grains in the surrounding ejecta of very cool red supergiant atmospheres ['carbon stars']. First indications are that the silicon carbide found near stars is a hexagonal mineral which had been labeled the 'α−SiC' in crystallographical studies of the SiC polymorphs. This astrophysical finding was supplemented by the discovery of extremely small grains of the isometric mineral Moissanite ('β−SiC') in primitive meteorites. These (‘pre-solar’) grain are apparently remnants from the gas cloud which produced the solar nebula and the Solar System ). Initial detection of star dust frequently involves detection of rare gases with unusual isotopic ratios (e.g., cosmochemical signatures). However, sooner or later mineralogical issues come to the fore. In the case of silicon carbide we are introduced to a schizophrenic scientific terminology which treats one Si-C polymorph as true mineral and another equally significant Si-C polymorph essentially as merely a chemically interesting isomer. The result has been that in the astrophysical and in much of the cosmochemically-focused meteoritic literature critical mineralogically important issues have been shunted aside. The consequence is that mineral grains of star dust are labelled as SiC, TiC, TiN, and Alumina. The "Alumina" terminology is a particularly glaring — the very nearly pure aluminum oxide, in fact, is almost certainly the mineral known as 'corundum'. We understand that jewelers have developed a special language for the special colors of corundum. Indeed, we understand that the special colors of Ruby, Sapphire, and other varieties of Corundum have aesthetic and economic value. We even understand that the special colors are usually due to small impurities of 1 or 2 transition elements. But for an important fundamental process involving these marvelous microminerals we have no mineralogical language.

The mineralogical implications of stardust are striking. In cooling gases outside of various stars (supernovae, novae, red giants, Wolf-Rayet stars…) solid grains of various composition condense. Presumably, they condense as various mixes of stable or metastable minerals or as highly-highly disordered glassy materials. Over time some of the metastable phases will adapt more stable mineralogical structures and glasses will partially or completely devitrify. In the case of silicon carbide it appears that the initial condensates were rich in the hexagonal polymorph, but by the time they have reached us the silicon carbide had largely converted into the isometric polymorph. Observations of intermediate stages and variants are not discussed in the literature available to the author, but the science of star dust is still young. Of course, it is not true that the meteoriticists who report on stardust ignore the mineralogy. In the case of graphite, for example, a mineral with many properties already known before the detection of graphite in stardust references to petrographic and mineralogical properties are reported as a matter of course. Cf. Croat et.al. (2003) and Chigai et.al. (1999).

Case Study #B3: Ni-rich metal Phases in Meteorites — the Minerals and their Names.
Taenite; Tetrataenite (FeNi); Awaruite (Ni3Fe); Wairauite (CoFe).

Three separate Ni-rich minerals and a Co-rich mineral have been recognized as components of the Fe-Ni metal found in meteorites. Tetrataenite (found only in 1979) is surprisingly ubiquitous tetragonal mineral— and is indeed likely to be present in most chondritic meteorites. It consists of iron and nickel atoms present in nearly equal amounts. It is somewhat surprising that Awaruite and Wairauite were found as terrestrial minerals before they were recognized in meteorites but such appears to be the case. Both minerals were found on South Island, New Zealand. Their names sound so similar because their discovery locations, Awarua Bay and Wairau Valley, respectively, bear Maori names.

Taenite is a form of Fe-Ni metal with a cubic face-centered structure and can be quite Ni-rich (up to 48%).
It is, however, quite surprising that the 'type locality' for taenite is listed as the Gorge river in New Zealand (also the site of Awaruite's discovered. Perhaps, larger samples of the terrestrial taenite are available for distribution than meteorite specimens which are usually curated under quite strict conditions. Taenite has been reported from several serpentine-bearing localities and even in a few lunar samples. It was not, however, 'discovered' in terrestrially native ores. Some detailed studies and comparisons of minor and trace element distributions in taenite from both meteorites and terrestrial rocks might be quite illuminating. The name itself is derived from meteorite studies (Cf. von Reichenbach,1861)

Conclusions


Our argument has been made. In the meteoritic community both formal and informal verbal and written discourse frequently use mineralogical terms which are replete with odd nuances and non-standard meanings. When meteoriticists use IMA-preferred terminology they often must blur important planetological and cosmochemical considerations or utilize elaborate circumlocutions to convey such chemically significant information. There are a number of ways to address these issues. We will list them here in order of their implications.

One way that meteoriticists can communicate important mineralogical information within current IMA sanctioned and preferred venues is to restrict their use of 'End Member' terms for Solid Solution Series to traditional usage of these terms. Thus, the author has argued that such terms as Albite, Anorthite, Enstatite, and Forsterite should be — like Troilite — should be restricted to phases that contain at least 90 mol% (or more!) of the End Member component of the Solid Solution Series. If such usage is inconsistent with more terrestrially focused venues, the IMA should sanction the use of disciplinary specific terms (as has already happened with the use of whitlockite and merrillite).

The issue of preferred usage for 'kamacite' is more fundamental. From three different perspectives — historical, terrestrial abundance, and cosmochemical — terrestrially native, non-oxidized cubical iron is a variety of kamacite. Common sense, I believe, will eventually lead those who study terrestrial and extra-terrestrial minerals to recognize that formal scientific terminology should reflect as broad and 'universal' perspective as we can muster.

The third and final issue has to do with scientific transparency and the author's introduction of some more specific terminology. Practicing scientists, like all humans, tend to infuse their own preconceptions into their own use of common terms. In the interest of such transparency, it is time for those who study minerals, rocks, and natural metals to recognize that the terrestrially, planetologically, and astrophysically oriented disciplines need to make explicit their own geochemical, planetochemical, and cosmochemical perspectives. Jargon is not created by the utilization of new terms when they are needed. Rather, jargon is created when we use terms within our own scientific disciplines as if we were only talking to each other. The author believes that the search for the original homes of these weird stones that fall from the sky has reached a level of maturity that requires some new thinking about proper mineralogical terminology. Minerals, of course, are themselves also quite weird as well as interesting, beautiful, and economically important. But, 'to see a universe in a grain of sand' sometimes require us to describe those grains with new scientific words as well as with new microscopes or wonderful poems. Whether the author's suggestions meet such standards or, at least, raise the proper questions which will lead us towards such standards is for others to decide.

Bibliography


Adrian J. Brearley & Rhian H. Jones (1998). Chondritic Meteorites. In: Planetary Materials (Papike, J. J., Editor): Chapter 3, 398 pages. Mineralogical Society of America: Washington, DC, USA.
Takeshi Chigai, Tetsuo Yamamoto & Takashi Kozasa (1999) Formation conditions of presolar TiC core-graphite mantle spherules in the Murchison meteorite. Astrophysical Journal 510: 999-1010. (Jan 1999).
Donald D. Clayton & Larry R. Nittler (2004). Astrophysics with Presolar Stardust. Annual Review of Astronomy and Astrophysics, Vol. 42, pp. 39-78. (Sept 2004).
T. Kevin Croat, Thomas J. Bernatowicz, Sachiko Amari, Scott R. Messenger & Frank J. Stadermann (2003) Structural, chemical, and isotopic microanalytical investigations of graphite from supernovae. Geochimica et Cosmochimica Acta 67(24): 4705-4725. (Dec 2003).
Monica Mary Grady (2000). Catalogue of Meteorites (5/e). Cambridge University Press: Cambridge; New York; Oakleigh; Madrid; Cape Town. 689 pages.
Monica Mary Grady, Giovanni Pratesi & Vanni Moggi-Cecchi (2015) Atlas of Meteorites. Cambridge University Press: Cambridge, United Kingdom. 373 pages.
Robert Hutchinson (2004) Meteorites: A Petrologic, Chemical and Isotopic Synthesis. Cambridge University Press: Cambridge, New York, Melbourne, Cape Town, Madrid, São Paolo. 506 pages.
Rhian H. Jones (1990) Petrology and Petrology of Type II, FeO-rich chondrules in Semarkona (LL3.0). Geochimica et Cosmochimica Acta 54 (6): 1785-1802. (June 1990).
Klaus Keil (1989) Enstatite meteorites and their parent bodies: Meteoritics 24(3): 195-208. (Dec 1989).
Klaus Keil (2012) Angrites, a small but diverse suite of ancient, silica-undersaturated volcanic-plutonic mafic meteorites, and the history of their parent asteroid (Invited Review): Chemie der Erde 72(3): 191-218. (Sept 2012).
Brian Harold Mason (1972) The Mineralogy of Meteorites: Meteoritics 7(3): 309—326. (Sept 1972)
Harry Y. McSween, Jr. & Allan H. Treiman (1998) Martian Meteorites. In: Planetary Materials (Papike, J. J., Editor): Chapter 6, 53 pages. Mineralogical Society of America: Washington, DC, USA.
David W. Mittlefehldt, Timothy J. McCoy, Cyrena Anne Goodrich & Alfred Kracher (1998). Non-chondritic meteorites from asteroidal bodies. In: Planetary Materials (Papike, J. J., Editor): Chapter 4, 195 pages. Mineralogical Society of America: Washington, DC, USA. (1998).
David W. Mittlefehldt, Marvin Killgore & Michael T. Lee (2002) Petrology and geochemistry of D’Orbigny, geochemistry of Sahara 99555, and the origin of angrites. Meteoritics & Planetary Science 37(3): 345–369. (March 2002).
Donald D. Clayton & Larry R. Nittler (2004). Astrophysics with Presolar Stardust. Annual Review of Astronomy and Astrophysics, Vol. 42: 39-78.
Allan Edward Rubin (1997) Mineralogy of Mineral Groups. Meteoritics Planetary Science 32(2), 231-247. (March 1997).
Steven B. Simon & Lawrence Grossman (2006) A comparative study of melilite and fassaite in Types B1 and B2 refractory inclusions: Geochimica et Cosmochimica Acta 70(4): 780–798. (Feb 2006).
Karl L. von Reichenbach (1861) Übe das innere Gefüge der näheren Bestandtheil des Meteoreisens. Annalen der Physik 114:99-132, 250- 274, ,620-636.

ebiblio
http://www.lpi.usra.edu/meteor/metbull.php - Meteoritical Bulletin Database
http://web.missouri.edu/~speckan/witch-stuff/Research/chapter6/chapter6.html - SiC grains in atmospheres of cool carbon stars (C>O!!)

Appendix: Terminological Reference — Abbreviations, Acronyms, Definitions, Neologisms.

Preview: Abbreviations, Acronyms, Definitions, and Neologisms for Neophytes and Experts
Unless otherwise indicated, our Definitions (and their acronymic abbreviations) for Minerals and Meteorites are normally pedagogically motivated brief renderings of terms defined more precisely (technically speaking) by the International Mineralogical Association and The Meteoritical Society, respectively. In the format of this essay, the author finds that mindat.org ('Mindat') — formally, a 'secondary source' — is usually more than adequate for our purposes so that immediate Mindat links accompany many mineralites discussed here. Here, almost all exceptions to 'normal' or 'preferred' usage come under 'Neologisms' and 'Solid Solutions Series.'
Acronyms — The use of acronyms is standard in most professional scientific journals. However, for neophytes, interested experts from other fields, and or dummies like myself who cannot always remember an acronym which was clearly defined 3-10 pages before, I have provided a generous selection of the acronyms used here. [Exception: Chemical symbols for the elements are normally used without explication.]
Neologisms — Definitions here may provide slight amplifications to these terms as found in the text itself. The 4 LCHj-introduced neologisms are Mineral (Planetochemically defined), Mineralite, Planetochemical, and Weatherate.

Planetochemical — Relating to chemical signatures that characterize planetological objects within the solar system. In this work these considerations are operationally confined mostly to the terrestrial worlds, asteroids, and meteoroids of the inner solar system along with the putative original planet bodies (OPBs) of recovered meteorites.

Abbreviations...
Chemical Group — The usual and preferred term in formal meteoritical literature to describe the primary classificational cohorts to which each meteorite is assigned when recognized as an actual meteorite. The term (and its near synonym) is actually a shorthand for a complex set of associated chemical, mineralogical, and isotopic characteristics which bespeak, perhaps, a common original parent body (OPB) or formational environment. The chemical characteristics are, however, are highlited because — in many cases at least — the chemical (or geochemical) ratios remain relatively unchanged during lithological processes (vulcanism, metamorphism, collisions) which may drastically change petrologic and mineralogical constituents and features.

Cosmochemical (adjective) — Of chemical characteristics which have universal significance. In the meteoritical literature, 'cosmochemical' is normally applied to those chemical signatures that implicitly or explicitly reference the intersection of astronomy and astrophysics. Prime examplar: The ubiquitous presence of Fe-Ni metal in stony, stony-iron, and iron meteorites and its apparent chemical dominance of the earth's core reflect the universal presence of supernovae ejecta in all regions of the visible universe less than 13 billion light years away.l

Daughter Asteroid — A common informal term for IPBs and other moderately large asteroids produced by the collisional disruption of a larger asteroid (esp. asteroid 'families').

Earthfound (adjective) — A non-subtle and perhaps even unnecessary reminder that — except for several meteorites discovered by the Mars Rovers and others returned by Apollo astronauts — the meteorites in our collections are a minute fraction of the solar system's population of small bodies and they have reached us by seemingly haphazard, but actually non-random gravitational and collisional processes which are not fully understood.

Equilibrated Ordinary Chondrite (EOC) — An ordinary chondrite which is 'equilibrated'. In 'equilibrated' ordinary chondrites the composition of different olivine and Ca-poor pyroxenes grains are either constant or nearly so. Almost all EOCs are classified as belonging to petrologic types 4 thru 6. (e.g., H4, L6, LL5, etc.). EOCs constitute more than 90% of the entire ordinary chondrite (OC) clan.

Fall = Witnessed Fall — A meteorite which was seen to fall or can has been deemed to have fallen at a more or less definite time by the Meteoritical Society. [A meteorite which hit the roof with a thud and is discovered in the attic three weeks later is called a (Witnessed) Fall.]

Find — A meteorite which is discovered ('found') at some indefinite interval after its unreported fall to the earth.

Geochemical (adjective, primary meaning) — Of chemical characteristics which are important in understanding terrestrial geology. In the meteoritical literature, 'geochemical' is often applied to chemical signatures that are similar to those found in terrestrial lithologies. Prime exemplars: Terrestrial and meteorite silicates can be described by common geochemical parameters — Fe: Mg ratios in Olivine; Ca:Fe:Mg constituents in Pyroxenes; Ca:Na:K constituents in Feldspars.
CAVEAT: The word 'geochemical' when applied indiscriminately to meteoritic lithological and metallic relationships may carry planetological or planetochemical implications foreign to those common within terrestrially referenced geochemical discourse.

Intermediate parent body (IPB) — A body which was collisionally separated (ejected) from a putative original parent body (OPB) and subsequently suffered one or more collisions which produced a (usually) much smaller meteorite-producing meteoroid. Indeed, it appears that between the putative OPBs and the earth-landing meteorites there were — in most cases — a sequence of smaller and smaller IPBs (fragments of fragments) which eventually produced the meteorites we now have in our collections.
SCIENCE ALERT: The chains of collisions which have produced our earthfound meteorites have altered the composition of all meteorites. All meteorites contain microscopic inclusions, most meteorites have small and somewhat foreign ['exotic'] inclusions which may be identified either with a hand lens and/or a microscope, and a few meteorites are the compacted mingling of two or more distinct lithological 'units.'

International Mineralogical Association (IMA)— An international scientific society which provides directives and guidance in all things mineralogical, including definitions of mineral names and, roughly, their compositional boundaries.

Meteoroid — The term used in the meteoritical literature for a (usually) relatively small asteroid which, upon striking the earth, has produced or will produce a meteorite. In the past two decades, attempts to determine realistic estimates of the pre-terrestrial mass of the impacting meteoroids has become an issue of intense interest. When a meteorite producing bolide produces many fragments and is observed by numerous observers and devices, very interesting results are produced.

Mindat — 'mindat.org', an organization dedicated to publicizing the physical locations of important IMA-defined minerals and a relatively large set of mineral varieties and other significant mineralogical items.

Micromineral — A mineral with a known crystallographic structure and chemical composition, which for want of a sufficiently large type sample, is not officially recognized as a "true" mineral. Used especially with reference to stardust.

Mineral — A physical aggregate which possesses (1) a known crystal structure, almost always one of the 230 space groups which belong to 32 classes, and (2) a 'defined' chemical composition. In scientific literature the 'defined' chemical structure is promulgated by a scientific organization such as the International Mineralogical Society (IMA).

Mineral List (Mindat) — A 'Mineral List' accompanies virtually all meteorites that are posted at Mindat. The list contains both IMA sanctioned terms and other older and informal terms that are utilized by significant numbers of geologically and geochemically interested parties (scientists, prospectors, rock hounds, jewelers, miners, teachers, etc., etc.). On the Mindat Mineral List all IMA-defined minerals are designated as 'valid minerals'.

Mineralite — A mineralogical significant phase or aggregate such as a mineral, mineral series, mineral variety, bimineral aggregate, or other item that is posted at 'mindat.org' or appear in the meteoritical literature. As defined here the term includes chemically defined phases [e.g., silica, silicon carbide] and mixed chemical-mineralogical terms [e.g., Ca-rich pyroxene] that are used much more extensively in the meteoritic literature than in other scientific and technical disciplines which study rocks and geology.

Ordinary Chondrite (OC) — A stony meteorite which has been classified by the Meteoritical Society as an 'ordinary chondrite' and posted as such at The Meteoritical Bulletin Database <http://www.lpi.usra.edu/meteor/metbull.php >. Ordinary chondrites represent the great majority of meteorite falls. Most of them contain chondrules, small spheroidal silicate-rich droplets. The others are believed to have once had chondrules that have been lost due to thermal metamorphism.

Ordinary Chondrite Group (e.g., H-,L-,LL-group). Almost all ordinary chondrites are classified as belonging to either the H-group (relatively high in total iron, L-group (relatively low in total iron, or LL-group (relatively very low in total iron compared to other chondrites. A few ordinary chondrites have been classified with as H/L meteorites and L/LL meteorites. It is suspected (believed by some, thought likely by others, and merely entertained by yet others) that most or all ordinary chondrites were derived from 3 distinct original parent bodies (OPBs). Whether the H/L and/or L/LL meteorites were derived from distinct OPBs, are meteorites of mingled parentage, or are products of some other circumstance is — as far as this author can see — totally up for grabs.

Original Parent Body (OPB) — A putative homeworld from which the material now found in a meteorite was derived. As a general rule it is believed/hoped (a working hypothesis, at least) that all or most of the meteorites in a defined meteorite group are fragments of a single OPB. [Historically, outliers within such groups have often been found to be fragments from different worlds.
CAVEAT: Original Parent Body (OPB). The vast majority of meteorites whose meteoroidal orbital characteristics can be determined from observations of the bolide produced by the meteoroid's atmospheric entry were derived from bodies in the asteroid belt. It also seems quite likely that the OPBs of these meteorites were also asteroids, whether those OPBs still exist or not. However, there is at least tentative evidence that some meteorites have reached the earth from somewhere outside the asteroid belt — such origins have, for example, been suggested for both CM carbonaceous chondrites and H/L ordinary chondrites.

Planetochemical — Relating to chemical signatures that characterize planetological objects within the solar system. In this work these considerations are operationally confined mostly to the terrestrial worlds, asteroids, and meteoroids of the inner solar system along with the putative original planet bodies (OPBs) of recovered meteorites. Prime Exemplar: Planetochemical parameters (Fe/Mn ratios in basalts and oxygen isotopes) are characteristically different in terrestrial, achondritic, and Martian stones. Likewise, relative populations of sulfides (pyrite, troilite, daubréelite) vary greatly within the inner solar system planets and within meteorites from the asteroid belt.

Unequilibrated Ordinary Chondrite (UOC)— An ordinary chondrite which is 'unequilibrated'. 'Unequilibrated' is a term of art as all ordinary chondrites are at some level unequilibrated. For starters, the term can perhaps be best understood, however, as referring to a chondrite's dominant silicates. The composition of olivine and pyroxenes grains in UOCs are quite variable indicating that they are 'out of equilibrium' with each other. UOCs are classified as belonging to petrologic type 3. UOCs normally have more distinct species and varieties of minerals than do the equilibrated ordinary chondrites (EOCs) as well as primary glass.

Weatherate — (1) Normal definition: A mineralogical product of (terrestrial) weathering. A mineralite which has been produced by the terrestrial alteration of meteorite material.

Weatherate — (2) Extended definition: A mineralogical product of terrestrial or extra-terrestrial weathering. In Martian meteorites and carbonaceous chondrites and, occasionally, in ordinary chondrites phases are sometimes observed that were produced by oxidation and/or hydration in a preterrestrial environment.




Article has been viewed at least 461 times.

Comments

Thanks Lon! There sure is a lot to digest!

Scott Rider
7th Sep 2017 10:46pm

In order to leave comments to this article, you must be registered
Mineral and/or Locality  
Mindat.org is an outreach project of the Hudson Institute of Mineralogy, a 501(c)(3) not-for-profit organization.
Copyright © mindat.org and the Hudson Institute of Mineralogy 1993-2017, except where stated. Mindat.org relies on the contributions of thousands of members and supporters.
Privacy Policy - Terms & Conditions - Contact Us Current server date and time: September 23, 2017 11:49:43
Go to top of page