The Mineralogy of Meteorites: Some General Issues in Meteoritic Mineralogy; Some
Last Updated: 14th May 2015By Lon Clay Hill
The Mineralogy of Meteorites:
Some General Issues in Meteoritic Mineralogy;
Some Special Issues in Terminology
Purpose: This and subsequent articles are intended to give scientific context to the mineral listings which are being posted in my blogs {First blog [Minerals in the Ivuna CI1 meteorite (CI prototype)], posted 24 Oct 2013}.
Caveat: This short article focuses on general issues in meteoritic mineralogy; later articles will focus on more group specific issues (beginning with Carbonaceous Chondrites and, then, Enstatite-rich meteorites).
This mini-essay is an attempt to provide a pedagogical and epistemological overview which will give context to some linguistic and scientific issues which arise when we study the minerals of meteorites. Scientifically speaking, minerals are naturally occurring ordered aggregates of atoms possessing a specific crystal form. To be fully recognized as a mineral by scientists the chemical and crystallographic entity must be stipulated along with at least one known instance of occurrence before the International Association of Mineralogists (IMA). While the author himself, a retired college science teacher, attempts to be as scientifically accurate and current as possible, a supplemental focus of this essay lies elsewhere. We will be specifically concerned with two entangled issues — One, what scientific questions arise naturally in the science of meteoritics and, two what terminological or linguistic tensions arise when we use language developed in the study of terrestrial rocks to describe objects which are from out of this world. In other words, while I am going to be using scientific words to describe scientifically important mineralogical entities, we will also be partially engaged in understanding some problems in disciplinary dialects that are more properly understood as linguistic problems than as strictly scientific issues.
CONTENTS:
I. Important Issues in Meteorite Mineralogy
II. Meteoritic Terminology (First pass)
III. Bibliography
I. Special Issues in Meteoritic Mineralogy
IA. Fundamental Definitions — Stones & Irons, Falls & Finds, etc.
IB. Sample Size & Target Size: Limitations and Restraints
IC. Meteorites & their Original Homeworlds (“Parent Bodies”)
ID. Special Mineralogical Emphases at a more granular Level.
IA. Fundamental Definitions — Stones and Irons, Falls and Finds, Conspicuous Constituents.
Meteorites — rocks and metallic hunks of material which “fall” from the sky, sometimes in quite spectacular fashion — are, mineralogically speaking, an intriguing combination of the familiar and the unknown. Most meteorites which are recovered immediately after reaching the ground (“Falls”) are primarily aggregates of well-known silicate minerals (esp. Olivine, the Pyroxenes, and Plagioclase). Most — but not all — of these meteorites usually have both (1) unusual textures (i.e. chondrules and chondrule relics) and (2) small amounts of Fe-Ni metal which are rarely seen in terrestrial rocks. Still, their overall mineralogy is generally familiar even if they occasionally contain minerals which are rarely or never observed in earth rocks. Indeed, over 90% of meteorite falls are called “stones.”
A large number of meteorites are recovered without having been seen or heard to fall (“Finds”). Most of these “Finds” are also ‘stones.’ However, approximately 5% of both Finds and Falls are characterized by conspicuous amounts of metallic iron accompanied by nickel (“Fe-Ni Metal”). Those meteorites whose composition is dominated by Fe-Ni metal are referred to as “Iron Meteorites” or, more simply, as Irons. Indeed, in the public eye, Irons dominate the public imagination. If one visits a science museum which displays meteorites, one’s eye is often drawn to one or two prominently arranged large iron meteorites. Their unusual appearance — often enhanced by fine polish revealing the intricacies of their unearthly mineralogy — draws immediate attention. Indeed, many iron finds are eventually recovered precisely because a meteorite’s appearance struck an uninitiated observer as ‘passing strange.’ There is a second reason one is more likely to see these large meteorites on display. Iron meteorites are sturdy objects which are more likely than most meteorites to survive their violent passage thru the earth’s atmosphere in relatively intact form. A recovered iron is much more likely to be found as a single large mass or as a few neighboring masses than a recovered stone. On the other hand, despite individual variations, stony meteoroids are much more likely to be broken into hundreds or even thousands of small pieces. Only a few intact stones have been recovered with a mass of a ton or more. On the other hand, several iron meteorites have masses of ten tons or more. The largest one, Hoba, is a relatively intact single mass of 60 tons found in Namibia in 1920. Several of the larger irons had lain in various desert for centuries before recovery in their still relatively unaltered shapes. Indeed, it should also be noted that the total mass of recovered irons is much greater than the recovered mass of the more numerous stones.
A small but still significant number of meteorites are roughly half stone-half Fe-Ni Metal. These meteorites, reasonably enough, are called “Stony-Irons.” Like the irons, the unusual mineralogy of stony-irons can easily command the attention of an uninitiated observer. And, likewise, a number of stony-irons are often on prominent display in museums.
A frequent and usual obvious problem with all finds is their propensity to weather in their new terrestrial environment. In most case, neither atmospheric oxygen or running water were part of their original homeland environment. Iron rusts when left outside. And, sooner or later, whether it is industrial iron or meteoritic iron which lies outside in the wind and rain, telltale brown stains and crumbly orange-brown aggregates of Goethite and other oxidation products begin to appear. Other meteoritic minerals are also quite vulnerable. The metallic iron and nickel may form separate minerals in their new terrestrial environment. So, for example, whether they originally formed large inclusions within irons or small dispersed blebs within stones — on earth the original meteoritic iron sulfides and iron phosphides become terrestrial iron sulfates and iron phosphates.
The weathering issues will be discussed further as we proceed (Section IA). However, here we make a preliminary note to remind is that when we find meteorites from homeworlds which themselves have had episodes of aqueous activity — whether as frost, running water, or as percolating water-containing liquids — it may not always be easy to discern whether the ‘weathering’ occurred here on earth or long ago and far way when the meteorite dwelled in a strange land. And, indeed, such is the case with some of the minerals in Martian meteorites and several groups of Carbonaceous chondrites.
IB. Sample Size and Mineralogical Targets:
Limitations and Restraints.
As a general rule, meteorite specimens are much smaller than the kinds of specimens obtained from a geological outcrop, a mining pit or even a cave. Furthermore, destructive analysis is always severely limited if not impossible. Sufficient material to prepare powder for X-ray analysis, thin sections or polished sections for a definitive mineral identity is not always possible. Thus, in meteoritical studies we often find significant chemical, physical or petrographical information or physical information about important phases presented without the definitive mineralogical identifications we expect to find in high quality reports about terrestrial rocks and minerals. In the our mineralogical listings we will frequently report instances of Chemically Defined Phases which fall somewhat outside the customary categories. Thus, most MinDat Mineral listing report ‘Mineral Groups’, ‘valid minerals’ and mineral varieties. The Mineral groups contain IMA recognized minerals (with known crystallographical properties). In turn, these minerals themselves may be expressed in several varieties. Of course, many important geological and mineralogical studies of terrestrial bodies extend beyond the customary mineralogical definitions. Glasses, for example, often receive extensive coverage in geological studies. However, our listings of meteorite mineralogy is more flexible than is the usual case.
There is a second and independent reason why, in spite of its limitations, microscopic detail is important for meteorite studies. A goodly number of meteorites are breccia — rocks assembled from fragments of previous rocks and minerals. These breccia are found in many meteorite classes because — like the moon — many meteoritic homeworlds were airless worlds which have provided no protection for surface rocks against the incessant rain of asteroids, comets, and meteoroids which have struck these small bodies thruout the Solar System’s past 4.5 billion years. The countless tiny rock and mineral fragments found in brecciated meteorites can be studied and characterized only if one has access to microscopic detail which may lack useful macroscopic context so helpful in studies of terrestrial rocks and minerals.
IC. Meteorites and Their Original Homeworlds (“Parent Bodies”)
When one studies terrestrial rocks and minerals we know the home world and, more importantly, we almost always already have some factually grounded ideas about their geographical and geological provenance. A meteorite, however, which is found upon the surface of the earth is not only “from out of this world” — it is in fact a visitor from (initially at least) an unknown world. Over the past few centuries we have made real progress in determining the original homeworlds of many meteorites. We are certain or virtually certain that a small percentage of meteorites have come from the moon and Mars; we are confident that a number of meteorites came from the asteroid Vesta; and, we are also confident that most of the other meteorites were once on or within various asteroidal bodies. Indeed, we sometimes venture to assert that some of the Carbonaceous Chondrites were produced in the Outer Asteroid Belt and, conversely, that that the original homeworlds of the Enstatite-rich meteorites were within the Inner Asteroid Belt or even closer to the sun. However, when we make these assertions our hypotheses are generally based upon a combination of well-established theory, reasonable hypothesis, and possible explanations whose intellectual boundaries are not easily determined.
A central tenet of meteoritics is that many — almost certainly most — meteorites which have been parked on small and airless worlds which have experienced very little of the geological activity (vulcanism, plate tectonics, metamorphism …) which have shaped and altered the earth’s surface over the past 4 billion years. Most meteorites, then, have originated on geologically dead worlds — worlds which have experienced virtually no ‘geological activity’ besides occasional asteroidal and asteroidal impacts. Of course, these impacts must be accounted for. The Criseyde surfaces of asteroids, comets and and moons remind us of their considerable effects. And, when we examine closely the moonrocks and meteorites in our possession almost all of them are noticeably altered and/or brecciated to the naked eye and at the microscopic level.
Still, once we take account of any alterations due to impacts on the home world, additional impacts during the duration of the meteorites ejection from its parent body, and — of course — the short, but nontrivial passage thru the earth’s atmosphere which ended with a thud, once we recover a meteorite we may have a visitor from another world. In some cases the rock has come from a moderately large asteroid, the moon or Mars — but it is even more likely that we will have before us a visitor from a small or very small world of the early solar system [Diameter < 100 km]. The chemistry and mineralogy of such meteorites will inform us about the ambient physical conditions of at least one region in the young or even very young solar system. That is the Holy Grail and the promise of much contemporary research. And, indeed, it is an objective which has already been met in part. Meteorites have already provided us with fundamental information and clues about the age and composition of the earth, the sun and the solar system. How we untangle the diverse clues within meteorites to improve our understanding of the early Solar System, however, requires us to take a closer look at the issue of the pertinence of mineral identification.
ID. Disciplinary Emphasis at a more Granular Level: Meteoritics v. earth-inspired Terrestrial Mineralogy. Sulfide and Weatherite alert.
There are, of course, many features of meteorite minerals that require attention. Surprise and familiarity will accompany us in studying the minor minerals — the oxides, the carbonates, the phosphates, etc. — which are found in meteorites. I mention two additional items which may require special awareness — sulfide mineralogy and weathering products.
Sulfide mineralogy is especially pregnant in its importance for meteoritic mineralogy. Most meteorite - stones and irons - contain Troilite, a hexagonal stoichiometric form of FeS, which is exceedingly rare on earth. The ubiquitous nature of meteoritic Troilite (usually, much more common in meteorites than common terrestrial iron sulfides such as Pentlandite, Pyrrhotite, or Pyrite) reminds us that the environment in which meteorite mineralogies developed was indeed “Out of this world”. A number of uniquely meteoritic sulfides are found in Enstatite Chondrites and in (Enstatite-rich) Aubrites. The iron meteorite Mundrabilla is ~30 vol% Troilite. There has been a great deal of speculation about the earth’s core composition — some of it driven by meteorite studies — which suggest that cosmochemically speaking — there may be more sulfur in the earth’s core and elsewhere than our surface limited perspective suggests.
Once a meteorite reaches the surface of the earth, it is subject to weathering influences which would not have existed in space or (usually) on its original homeworld (usually a small airless, geologically inactive world). The most obvious instance of this erosion is found in the iron oxides and iron and nickel hydrates that begin to form on and within the irons and, at a smaller scale, on the surface and within the ordinary stone meteorites. Even when at first glance the meteorite looks to be its normal ‘unearthly’ self — weathering products are readily apparent in a hand lens or in a microscope.
Such weathering products are not restricted to the metallic phases. Carbonates, phosphates, and sulfates are commonly produced on earth by interactions of wind and water with various minerals — producing or altering carbonates, phosphates and sulfates. Once a meteorite reaches the earth, its phosphides and sulfides are transformed into phosphates and sulfates and while its original complement of carbonates, phosphates and sulfates sulfates are altered new suites of different carbonates, phosphates and sulfates. Much of this alteration process can be minimized by storing new falls in dry storage areas and containers. However, particularly in the case of sulfates, it has been discovered that this can be a very problematic concern. Some anhydrous or nearly anhydrous sulfates can become altered (Hydrated) even when stored in relatively dry containers.
These problems become especially important when we study Martian meteorites and some carbonaceous meteorites. These meteorites were once in an aqueously active environment. When we observe their minerals it can be very difficult indeed to know whether the ‘weathering’ occurred on their original home world or here on earth.
II. TERMINOLOGY: Selected Mineralogically Significant Constituents of Meteorites
We list here a number of mineralogically significant constituents found in meteorite studies - but which are usually absent or rare in terrestrial rocks and minerals. [In other words, this is not, however, a ‘Mineralogical Glossary. Olivine is important in meteoritical studies - but it is also important in studies of terrestrial geology and so we ignore olivine for present purposes. Here we have listed a sample of rock types, minerals, glasses, and chemically defined phases of special importance for many classes of meteoritics.
Achondrites
Chondrites
Chondrules
‘Fe-Ni metal’
Kamacite [= Iron: variety Kamacite.]
Martensite
Merrillite
Plessite
Silica [SiO2]
Silicon carbide [SiC]
Stony-Iron
Taenite
Troilite
Definitions and Descriptions
Achondrites
Achondrites are stony meteorites which do not contain chondrules. In the past various achondrites have been called ‘basaltic meteorites’ and other terms because their silicate mineralogy is quite similar to basalts and other igneous rocks familiar to terrestrial geologists. However, among meteorites, the achondrites stand out precisely because they do not exhibit any evidence of the chondrules so prevalent in most stony meteorites.
Chondrites
Chondrites are meteorites which usually contain chondrules (or evidence of past chondrules which have been altered by metamorphic heating). Some meteorites (e.g., certain Carbonaceous Chondrites) are called chondrites because they appear to be chemically and/or genetically similar to chondrites with chondrules.
Chondrules
Chondrules are small, roughly spherical rock droplets of hot vapors which condensed rather quickly into glass and minerals. Most stony meteorites are called Chondrites because they usually contain chondrules or evidence of past chondrules. The word is derived from the Greek ‘ξονδροσ’ - grain.
Fe-Ni metal
Because the unoxidized or ‘free’ metal found in most stony meteorites and in iron meteorites is almost always mostly iron accompanied by nickel, this metal is referred to as simply “Fe-Ni metal”. This Fe-Ni metal has several mineralogical expressions but the term is indispensable when it is unnecessary or impossible to be more specific. When one finds free iron in a meteorite with very little Nickel (e.g., in the Renazzo meteorite) then we know we are dealing with a truly anomalous meteorite!
There are a number of synonyms for “Fe-Ni Metal” (usually self-explanatory), but we will try to stick to this expression.
Iron (e.g. Native iron)
Native iron (i.e. ‘free’ or unoxidized iron) is extremely rare on or near the earth’s surface. The most important source of (low nickel) native iron is Disko Island, Greenland. When found as a single mineralogical phase it has a body-centered cubic crystallographic structure (Im3m). One is actually more likely to find this mineralogical phase in certain iron meteorites called ‘Hexahedrites.’ Indeed, in these meteorites the iron can exist as single crystals with a mass of a ton or more. One is even more likely to find body-centered free iron as one component of a two component mixture of Fe-Ni metal in the more common iron meteorites. Metallurgists, working with artificially produced iron, had labeled this body-centered cubic iron ‘α-Fe’ well before encountering natural free iron in anything other than a meteorite.
Iron Meteorite (or, Iron)
An Iron Meteorite is a meteorite whose composition is dominated by Fe-Ni metal. Mineralogically, Irons, are usually a mixture of Kamacite and Taenite. A few Irons with Ni < 5-6% (“Hexahedrites’) are almost entirely monominerallic iron. The mineralogy of a significant number of irons has also been distorted by pre-terrestrial shocks.
Kamacite [= Iron: variety Kamacite.]
Kamacite is free iron (Im3m), usually coexisting with Taenite (Fm3m), when it is found in meteorites. When Kamacite is found without Taenite, it is also called ‘Kamacite’ in most meteoritical literature. Kamacite without Taenite has apparently been deemed ‘Iron’ by the IMA. Except in those rare cases where the nickel content is extremely low, however, this usage violates both custom and scientific relevance.
Martensite
Martensite refers to disordered iron (or iron-and-nickel) which has cooled too rapidly to become kamacite or kamacite + taenite. This metallurgical term is used in studies of meteoritic metal to what might be more properly called a ‘metallic glass.’
Merrillite
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!
Plessite
Plessite is a term for very fine intergrowths of kamacite and taenite. It is encountered with some frequency since on numerous occasions it appears that the Fe-Ni metal cooled rather quickly, but not too quickly to prevent a partial separation of the more Ni-rich Taenite from the Kamacite.
Silica [SiO2]
‘Silica’ is a conceptually simple example of an important Chemically Defined Phase. Several minerals — trigonal Quartz, hexagonal Tridymite, isometric Cristobalite, monoclinic Coesite, and tetragonal Stishovite — are all forms of silica. One can also find silica as a glass. Even when explicit mineralogical identification is missing, the presence of silica is important because the presence of the silica constrains the mineralogy and composition of surrounding phases. Except for Stishovite and Coesite, the ‘Silica’ polymorphs found in meteorites are not particularly unusual. However, in terrestrial studies it is usually easier to make a definitive mineralogical assay
Silicon carbide [SiC]
Silicon carbide [SiC] exists in nature as both as a hexagonal mineral and [β−SiC] and as an isometric polymorph [β−SiC]. The isometric mineral [β−SiC] is also known as Moissanite. In some meteorites silicon carbide is found as extremely small grains which are apparently remnants from the gas cloud which produced the solar nebula and the Solar System (aka ‘pre-solar’).
Stony-Iron
A Stony-Iron (meteorite) is roughly half silicate-rich stone and half Fe-Ni metal. At the present time, almost all meteorites designated as stony-irons belong either to the Mesosiderite or Pallasite meteorite groups. (Some anomalies: The Bencubbin, Lodran, and Steinbach meteorites have been classified as ‘Stony-irons” in the past, but they have been reclassified because of their clear chemical and isotopic affinities with various other non-stony iron meteorites.)
Taenite
Taenite is a form of Fe-Ni metal with a cubic face-centered structure and can be quite Ni-rich (up to 48%).
Troilite
Troilite is a hexagonal and stoichiometric form of FeS found in most meteorites and many lunar rocks as well. It is non-magnetic. One might be tempted to identify Troilite as an end-member variety of Pyrrhotite ( Fe1-xS) as Pyrrhotite also sometimes crystallizes in the hexagonal system, but that seems to be stretching things a bit. Pyrrhotite is magnetic. Troilite is quite rare on earth.
SECTION III. Provisional Bibliography
This Bibliography is merely indicative of the various sources used in this article and the Meteorite Lists which this article supports. All reports of specific minerals found in specific meteorites are referenced in the appropriate blogs. We begin here with some general reference books, some review articles, and some WEB sites. As the author does not have access to most paid subscription Journals and their limited access WEB sites, there is no pretense here of completeness. However, the sources below can certainly help the enterprising amateur, student or teacher make a serious foray into the fascinating world of meteorite mineral studies.
BOOKS
Monica M. Grady (2000). Catalogue of Meteorites (5/e). Cambridge University Press: Cambridge; New York; Oakleigh; Madrid; Cape Town. 689 pages.
Ivan Kostov (1968). Mineralogy. Oliver and London: Edinburg & London. 587 pages.
ARTICLES
Alexander N. Krot, Anders Meibom, Michael K. Weisberg & Klaus Keil
(2002). Invited Review: The CR chondrite clan: Implications for early solar system processes. Meteoritics & Planetary Science 37, #11, 1451-1490.
Alan E. Rubin (1997). Mineralogy of Meteorite Groups. Meteoritics & Planetary Science 32, #4, 231-247.
WEB RESOURCES
The Meteoritical Society’s Meteoritical Bulletin Database: [
http://www.lpi.usra.edu/meteor/metbull.php ]
The Mineralogy Database. [
http://webmineral.com/]
Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association:
[
http://www.ima-mineralogy.org/Minlist.htm]
The IMA uses awkward PDF files and is also constrained by proprietary considerations. Both features often delay create delays to instant access.
Some General Issues in Meteoritic Mineralogy;
Some Special Issues in Terminology
Purpose: This and subsequent articles are intended to give scientific context to the mineral listings which are being posted in my blogs {First blog [Minerals in the Ivuna CI1 meteorite (CI prototype)], posted 24 Oct 2013}.
Caveat: This short article focuses on general issues in meteoritic mineralogy; later articles will focus on more group specific issues (beginning with Carbonaceous Chondrites and, then, Enstatite-rich meteorites).
This mini-essay is an attempt to provide a pedagogical and epistemological overview which will give context to some linguistic and scientific issues which arise when we study the minerals of meteorites. Scientifically speaking, minerals are naturally occurring ordered aggregates of atoms possessing a specific crystal form. To be fully recognized as a mineral by scientists the chemical and crystallographic entity must be stipulated along with at least one known instance of occurrence before the International Association of Mineralogists (IMA). While the author himself, a retired college science teacher, attempts to be as scientifically accurate and current as possible, a supplemental focus of this essay lies elsewhere. We will be specifically concerned with two entangled issues — One, what scientific questions arise naturally in the science of meteoritics and, two what terminological or linguistic tensions arise when we use language developed in the study of terrestrial rocks to describe objects which are from out of this world. In other words, while I am going to be using scientific words to describe scientifically important mineralogical entities, we will also be partially engaged in understanding some problems in disciplinary dialects that are more properly understood as linguistic problems than as strictly scientific issues.
CONTENTS:
I. Important Issues in Meteorite Mineralogy
II. Meteoritic Terminology (First pass)
III. Bibliography
I. Special Issues in Meteoritic Mineralogy
IA. Fundamental Definitions — Stones & Irons, Falls & Finds, etc.
IB. Sample Size & Target Size: Limitations and Restraints
IC. Meteorites & their Original Homeworlds (“Parent Bodies”)
ID. Special Mineralogical Emphases at a more granular Level.
IA. Fundamental Definitions — Stones and Irons, Falls and Finds, Conspicuous Constituents.
Meteorites — rocks and metallic hunks of material which “fall” from the sky, sometimes in quite spectacular fashion — are, mineralogically speaking, an intriguing combination of the familiar and the unknown. Most meteorites which are recovered immediately after reaching the ground (“Falls”) are primarily aggregates of well-known silicate minerals (esp. Olivine, the Pyroxenes, and Plagioclase). Most — but not all — of these meteorites usually have both (1) unusual textures (i.e. chondrules and chondrule relics) and (2) small amounts of Fe-Ni metal which are rarely seen in terrestrial rocks. Still, their overall mineralogy is generally familiar even if they occasionally contain minerals which are rarely or never observed in earth rocks. Indeed, over 90% of meteorite falls are called “stones.”
A large number of meteorites are recovered without having been seen or heard to fall (“Finds”). Most of these “Finds” are also ‘stones.’ However, approximately 5% of both Finds and Falls are characterized by conspicuous amounts of metallic iron accompanied by nickel (“Fe-Ni Metal”). Those meteorites whose composition is dominated by Fe-Ni metal are referred to as “Iron Meteorites” or, more simply, as Irons. Indeed, in the public eye, Irons dominate the public imagination. If one visits a science museum which displays meteorites, one’s eye is often drawn to one or two prominently arranged large iron meteorites. Their unusual appearance — often enhanced by fine polish revealing the intricacies of their unearthly mineralogy — draws immediate attention. Indeed, many iron finds are eventually recovered precisely because a meteorite’s appearance struck an uninitiated observer as ‘passing strange.’ There is a second reason one is more likely to see these large meteorites on display. Iron meteorites are sturdy objects which are more likely than most meteorites to survive their violent passage thru the earth’s atmosphere in relatively intact form. A recovered iron is much more likely to be found as a single large mass or as a few neighboring masses than a recovered stone. On the other hand, despite individual variations, stony meteoroids are much more likely to be broken into hundreds or even thousands of small pieces. Only a few intact stones have been recovered with a mass of a ton or more. On the other hand, several iron meteorites have masses of ten tons or more. The largest one, Hoba, is a relatively intact single mass of 60 tons found in Namibia in 1920. Several of the larger irons had lain in various desert for centuries before recovery in their still relatively unaltered shapes. Indeed, it should also be noted that the total mass of recovered irons is much greater than the recovered mass of the more numerous stones.
A small but still significant number of meteorites are roughly half stone-half Fe-Ni Metal. These meteorites, reasonably enough, are called “Stony-Irons.” Like the irons, the unusual mineralogy of stony-irons can easily command the attention of an uninitiated observer. And, likewise, a number of stony-irons are often on prominent display in museums.
A frequent and usual obvious problem with all finds is their propensity to weather in their new terrestrial environment. In most case, neither atmospheric oxygen or running water were part of their original homeland environment. Iron rusts when left outside. And, sooner or later, whether it is industrial iron or meteoritic iron which lies outside in the wind and rain, telltale brown stains and crumbly orange-brown aggregates of Goethite and other oxidation products begin to appear. Other meteoritic minerals are also quite vulnerable. The metallic iron and nickel may form separate minerals in their new terrestrial environment. So, for example, whether they originally formed large inclusions within irons or small dispersed blebs within stones — on earth the original meteoritic iron sulfides and iron phosphides become terrestrial iron sulfates and iron phosphates.
The weathering issues will be discussed further as we proceed (Section IA). However, here we make a preliminary note to remind is that when we find meteorites from homeworlds which themselves have had episodes of aqueous activity — whether as frost, running water, or as percolating water-containing liquids — it may not always be easy to discern whether the ‘weathering’ occurred here on earth or long ago and far way when the meteorite dwelled in a strange land. And, indeed, such is the case with some of the minerals in Martian meteorites and several groups of Carbonaceous chondrites.
IB. Sample Size and Mineralogical Targets:
Limitations and Restraints.
As a general rule, meteorite specimens are much smaller than the kinds of specimens obtained from a geological outcrop, a mining pit or even a cave. Furthermore, destructive analysis is always severely limited if not impossible. Sufficient material to prepare powder for X-ray analysis, thin sections or polished sections for a definitive mineral identity is not always possible. Thus, in meteoritical studies we often find significant chemical, physical or petrographical information or physical information about important phases presented without the definitive mineralogical identifications we expect to find in high quality reports about terrestrial rocks and minerals. In the our mineralogical listings we will frequently report instances of Chemically Defined Phases which fall somewhat outside the customary categories. Thus, most MinDat Mineral listing report ‘Mineral Groups’, ‘valid minerals’ and mineral varieties. The Mineral groups contain IMA recognized minerals (with known crystallographical properties). In turn, these minerals themselves may be expressed in several varieties. Of course, many important geological and mineralogical studies of terrestrial bodies extend beyond the customary mineralogical definitions. Glasses, for example, often receive extensive coverage in geological studies. However, our listings of meteorite mineralogy is more flexible than is the usual case.
There is a second and independent reason why, in spite of its limitations, microscopic detail is important for meteorite studies. A goodly number of meteorites are breccia — rocks assembled from fragments of previous rocks and minerals. These breccia are found in many meteorite classes because — like the moon — many meteoritic homeworlds were airless worlds which have provided no protection for surface rocks against the incessant rain of asteroids, comets, and meteoroids which have struck these small bodies thruout the Solar System’s past 4.5 billion years. The countless tiny rock and mineral fragments found in brecciated meteorites can be studied and characterized only if one has access to microscopic detail which may lack useful macroscopic context so helpful in studies of terrestrial rocks and minerals.
IC. Meteorites and Their Original Homeworlds (“Parent Bodies”)
When one studies terrestrial rocks and minerals we know the home world and, more importantly, we almost always already have some factually grounded ideas about their geographical and geological provenance. A meteorite, however, which is found upon the surface of the earth is not only “from out of this world” — it is in fact a visitor from (initially at least) an unknown world. Over the past few centuries we have made real progress in determining the original homeworlds of many meteorites. We are certain or virtually certain that a small percentage of meteorites have come from the moon and Mars; we are confident that a number of meteorites came from the asteroid Vesta; and, we are also confident that most of the other meteorites were once on or within various asteroidal bodies. Indeed, we sometimes venture to assert that some of the Carbonaceous Chondrites were produced in the Outer Asteroid Belt and, conversely, that that the original homeworlds of the Enstatite-rich meteorites were within the Inner Asteroid Belt or even closer to the sun. However, when we make these assertions our hypotheses are generally based upon a combination of well-established theory, reasonable hypothesis, and possible explanations whose intellectual boundaries are not easily determined.
A central tenet of meteoritics is that many — almost certainly most — meteorites which have been parked on small and airless worlds which have experienced very little of the geological activity (vulcanism, plate tectonics, metamorphism …) which have shaped and altered the earth’s surface over the past 4 billion years. Most meteorites, then, have originated on geologically dead worlds — worlds which have experienced virtually no ‘geological activity’ besides occasional asteroidal and asteroidal impacts. Of course, these impacts must be accounted for. The Criseyde surfaces of asteroids, comets and and moons remind us of their considerable effects. And, when we examine closely the moonrocks and meteorites in our possession almost all of them are noticeably altered and/or brecciated to the naked eye and at the microscopic level.
Still, once we take account of any alterations due to impacts on the home world, additional impacts during the duration of the meteorites ejection from its parent body, and — of course — the short, but nontrivial passage thru the earth’s atmosphere which ended with a thud, once we recover a meteorite we may have a visitor from another world. In some cases the rock has come from a moderately large asteroid, the moon or Mars — but it is even more likely that we will have before us a visitor from a small or very small world of the early solar system [Diameter < 100 km]. The chemistry and mineralogy of such meteorites will inform us about the ambient physical conditions of at least one region in the young or even very young solar system. That is the Holy Grail and the promise of much contemporary research. And, indeed, it is an objective which has already been met in part. Meteorites have already provided us with fundamental information and clues about the age and composition of the earth, the sun and the solar system. How we untangle the diverse clues within meteorites to improve our understanding of the early Solar System, however, requires us to take a closer look at the issue of the pertinence of mineral identification.
ID. Disciplinary Emphasis at a more Granular Level: Meteoritics v. earth-inspired Terrestrial Mineralogy. Sulfide and Weatherite alert.
There are, of course, many features of meteorite minerals that require attention. Surprise and familiarity will accompany us in studying the minor minerals — the oxides, the carbonates, the phosphates, etc. — which are found in meteorites. I mention two additional items which may require special awareness — sulfide mineralogy and weathering products.
Sulfide mineralogy is especially pregnant in its importance for meteoritic mineralogy. Most meteorite - stones and irons - contain Troilite, a hexagonal stoichiometric form of FeS, which is exceedingly rare on earth. The ubiquitous nature of meteoritic Troilite (usually, much more common in meteorites than common terrestrial iron sulfides such as Pentlandite, Pyrrhotite, or Pyrite) reminds us that the environment in which meteorite mineralogies developed was indeed “Out of this world”. A number of uniquely meteoritic sulfides are found in Enstatite Chondrites and in (Enstatite-rich) Aubrites. The iron meteorite Mundrabilla is ~30 vol% Troilite. There has been a great deal of speculation about the earth’s core composition — some of it driven by meteorite studies — which suggest that cosmochemically speaking — there may be more sulfur in the earth’s core and elsewhere than our surface limited perspective suggests.
Once a meteorite reaches the surface of the earth, it is subject to weathering influences which would not have existed in space or (usually) on its original homeworld (usually a small airless, geologically inactive world). The most obvious instance of this erosion is found in the iron oxides and iron and nickel hydrates that begin to form on and within the irons and, at a smaller scale, on the surface and within the ordinary stone meteorites. Even when at first glance the meteorite looks to be its normal ‘unearthly’ self — weathering products are readily apparent in a hand lens or in a microscope.
Such weathering products are not restricted to the metallic phases. Carbonates, phosphates, and sulfates are commonly produced on earth by interactions of wind and water with various minerals — producing or altering carbonates, phosphates and sulfates. Once a meteorite reaches the earth, its phosphides and sulfides are transformed into phosphates and sulfates and while its original complement of carbonates, phosphates and sulfates sulfates are altered new suites of different carbonates, phosphates and sulfates. Much of this alteration process can be minimized by storing new falls in dry storage areas and containers. However, particularly in the case of sulfates, it has been discovered that this can be a very problematic concern. Some anhydrous or nearly anhydrous sulfates can become altered (Hydrated) even when stored in relatively dry containers.
These problems become especially important when we study Martian meteorites and some carbonaceous meteorites. These meteorites were once in an aqueously active environment. When we observe their minerals it can be very difficult indeed to know whether the ‘weathering’ occurred on their original home world or here on earth.
II. TERMINOLOGY: Selected Mineralogically Significant Constituents of Meteorites
We list here a number of mineralogically significant constituents found in meteorite studies - but which are usually absent or rare in terrestrial rocks and minerals. [In other words, this is not, however, a ‘Mineralogical Glossary. Olivine is important in meteoritical studies - but it is also important in studies of terrestrial geology and so we ignore olivine for present purposes. Here we have listed a sample of rock types, minerals, glasses, and chemically defined phases of special importance for many classes of meteoritics.
Achondrites
Chondrites
Chondrules
‘Fe-Ni metal’
Kamacite [= Iron: variety Kamacite.]
Martensite
Merrillite
Plessite
Silica [SiO2]
Silicon carbide [SiC]
Stony-Iron
Taenite
Troilite
Definitions and Descriptions
Achondrites
Achondrites are stony meteorites which do not contain chondrules. In the past various achondrites have been called ‘basaltic meteorites’ and other terms because their silicate mineralogy is quite similar to basalts and other igneous rocks familiar to terrestrial geologists. However, among meteorites, the achondrites stand out precisely because they do not exhibit any evidence of the chondrules so prevalent in most stony meteorites.
Chondrites
Chondrites are meteorites which usually contain chondrules (or evidence of past chondrules which have been altered by metamorphic heating). Some meteorites (e.g., certain Carbonaceous Chondrites) are called chondrites because they appear to be chemically and/or genetically similar to chondrites with chondrules.
Chondrules
Chondrules are small, roughly spherical rock droplets of hot vapors which condensed rather quickly into glass and minerals. Most stony meteorites are called Chondrites because they usually contain chondrules or evidence of past chondrules. The word is derived from the Greek ‘ξονδροσ’ - grain.
Fe-Ni metal
Because the unoxidized or ‘free’ metal found in most stony meteorites and in iron meteorites is almost always mostly iron accompanied by nickel, this metal is referred to as simply “Fe-Ni metal”. This Fe-Ni metal has several mineralogical expressions but the term is indispensable when it is unnecessary or impossible to be more specific. When one finds free iron in a meteorite with very little Nickel (e.g., in the Renazzo meteorite) then we know we are dealing with a truly anomalous meteorite!
There are a number of synonyms for “Fe-Ni Metal” (usually self-explanatory), but we will try to stick to this expression.
Iron (e.g. Native iron)
Native iron (i.e. ‘free’ or unoxidized iron) is extremely rare on or near the earth’s surface. The most important source of (low nickel) native iron is Disko Island, Greenland. When found as a single mineralogical phase it has a body-centered cubic crystallographic structure (Im3m). One is actually more likely to find this mineralogical phase in certain iron meteorites called ‘Hexahedrites.’ Indeed, in these meteorites the iron can exist as single crystals with a mass of a ton or more. One is even more likely to find body-centered free iron as one component of a two component mixture of Fe-Ni metal in the more common iron meteorites. Metallurgists, working with artificially produced iron, had labeled this body-centered cubic iron ‘α-Fe’ well before encountering natural free iron in anything other than a meteorite.
Iron Meteorite (or, Iron)
An Iron Meteorite is a meteorite whose composition is dominated by Fe-Ni metal. Mineralogically, Irons, are usually a mixture of Kamacite and Taenite. A few Irons with Ni < 5-6% (“Hexahedrites’) are almost entirely monominerallic iron. The mineralogy of a significant number of irons has also been distorted by pre-terrestrial shocks.
Kamacite [= Iron: variety Kamacite.]
Kamacite is free iron (Im3m), usually coexisting with Taenite (Fm3m), when it is found in meteorites. When Kamacite is found without Taenite, it is also called ‘Kamacite’ in most meteoritical literature. Kamacite without Taenite has apparently been deemed ‘Iron’ by the IMA. Except in those rare cases where the nickel content is extremely low, however, this usage violates both custom and scientific relevance.
Martensite
Martensite refers to disordered iron (or iron-and-nickel) which has cooled too rapidly to become kamacite or kamacite + taenite. This metallurgical term is used in studies of meteoritic metal to what might be more properly called a ‘metallic glass.’
Merrillite
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!
Plessite
Plessite is a term for very fine intergrowths of kamacite and taenite. It is encountered with some frequency since on numerous occasions it appears that the Fe-Ni metal cooled rather quickly, but not too quickly to prevent a partial separation of the more Ni-rich Taenite from the Kamacite.
Silica [SiO2]
‘Silica’ is a conceptually simple example of an important Chemically Defined Phase. Several minerals — trigonal Quartz, hexagonal Tridymite, isometric Cristobalite, monoclinic Coesite, and tetragonal Stishovite — are all forms of silica. One can also find silica as a glass. Even when explicit mineralogical identification is missing, the presence of silica is important because the presence of the silica constrains the mineralogy and composition of surrounding phases. Except for Stishovite and Coesite, the ‘Silica’ polymorphs found in meteorites are not particularly unusual. However, in terrestrial studies it is usually easier to make a definitive mineralogical assay
Silicon carbide [SiC]
Silicon carbide [SiC] exists in nature as both as a hexagonal mineral and [β−SiC] and as an isometric polymorph [β−SiC]. The isometric mineral [β−SiC] is also known as Moissanite. In some meteorites silicon carbide is found as extremely small grains which are apparently remnants from the gas cloud which produced the solar nebula and the Solar System (aka ‘pre-solar’).
Stony-Iron
A Stony-Iron (meteorite) is roughly half silicate-rich stone and half Fe-Ni metal. At the present time, almost all meteorites designated as stony-irons belong either to the Mesosiderite or Pallasite meteorite groups. (Some anomalies: The Bencubbin, Lodran, and Steinbach meteorites have been classified as ‘Stony-irons” in the past, but they have been reclassified because of their clear chemical and isotopic affinities with various other non-stony iron meteorites.)
Taenite
Taenite is a form of Fe-Ni metal with a cubic face-centered structure and can be quite Ni-rich (up to 48%).
Troilite
Troilite is a hexagonal and stoichiometric form of FeS found in most meteorites and many lunar rocks as well. It is non-magnetic. One might be tempted to identify Troilite as an end-member variety of Pyrrhotite ( Fe1-xS) as Pyrrhotite also sometimes crystallizes in the hexagonal system, but that seems to be stretching things a bit. Pyrrhotite is magnetic. Troilite is quite rare on earth.
SECTION III. Provisional Bibliography
This Bibliography is merely indicative of the various sources used in this article and the Meteorite Lists which this article supports. All reports of specific minerals found in specific meteorites are referenced in the appropriate blogs. We begin here with some general reference books, some review articles, and some WEB sites. As the author does not have access to most paid subscription Journals and their limited access WEB sites, there is no pretense here of completeness. However, the sources below can certainly help the enterprising amateur, student or teacher make a serious foray into the fascinating world of meteorite mineral studies.
BOOKS
Monica M. Grady (2000). Catalogue of Meteorites (5/e). Cambridge University Press: Cambridge; New York; Oakleigh; Madrid; Cape Town. 689 pages.
Ivan Kostov (1968). Mineralogy. Oliver and London: Edinburg & London. 587 pages.
ARTICLES
Alexander N. Krot, Anders Meibom, Michael K. Weisberg & Klaus Keil
(2002). Invited Review: The CR chondrite clan: Implications for early solar system processes. Meteoritics & Planetary Science 37, #11, 1451-1490.
Alan E. Rubin (1997). Mineralogy of Meteorite Groups. Meteoritics & Planetary Science 32, #4, 231-247.
WEB RESOURCES
The Meteoritical Society’s Meteoritical Bulletin Database: [
http://www.lpi.usra.edu/meteor/metbull.php ]
The Mineralogy Database. [
http://webmineral.com/]
Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association:
[
http://www.ima-mineralogy.org/Minlist.htm]
The IMA uses awkward PDF files and is also constrained by proprietary considerations. Both features often delay create delays to instant access.
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