Meteorite taxonomy (the practice and science of classification) or systematics is a topic of interest to both scientists and collectors. There is sort of a generally accepted taxonomic scheme for meteorites, but it is continually being revised and re-examined as we learn more about the origin of these rocks. Necessarily, history and the capability of scientific examination have played important roles. As we have moved from purely macroscopic observations--i.e.,say the distinctions between irons and stones--to more subtle distinctions--i.e., the petrographic character of certain minerals--to the analysis of trace element ratios and isotopes, meteoriticists have added to and changed the scheme. Here are diagrams of the existing scheme and one possible genetic scheme taken from Weisberg et al.: Systematics and Evaluation of Meteorite Classification:
Click on the diagrams for a larger image.
I have constructed the following table is from information adapted from Rocks from Space by O. Richard Norton, the Catalogue of Meteorites, Fifth edition, by Monica Grady, The Handbook of Iron Meteorites by Vagn Buchwald, (University of California Press, 1975), and Meteorites and Their Parent Planets by Harry McSween, Weisberg, and other sources as noted,
The links in the table will lead you to pictures of meteorites or to explanatory material. If you click on a mineral name you will be linked to the Meteorite Mineral page. Click back to return to the table.
Meteorite Classification Table
(Click on letter designations for photo catalogs and information pages for specific meteorites.)
Distinguishing Features/ Chondrule Character
Stony Meteorites are characterized by chondrules--small spheres (average diameter of 1 mm) of formerly melted minerals that have come together with other mineral matter to form a solid rock. Chondrites are believed to be among the oldest rocks in the solar system. Click here to link to an article on the Nature and Origin of Chondrules. Click here to see a close-up picture of chondrules. 82 percent of meteorite falls are chondrites.
Number designations for chondrites: Most classified chondrites are given a number designation to catagorized their "petrologic type." The number refers to the alteration of the chondrules. A designation of "3" indicates unaltered chondrules. Numbers greater than 3 indicate increasing thermal metamorphism. A designation of 7 indicates complete obliteration of chondrules. Numbers less than three indicate increasing aqueous alteration. A designation of 1 indicates that chondrules have been obliterated by aqueous alteration. A lack of numeric designation means that no alteration designation has been assigned. Chondrites of Petrologic Type 3 are referred to as "unequibrated" because of their lack of metamorphism. Here is a table from Weisberg indicating the criteria for petrologic type"
Further reading, King, ed., Chondrules and Their Origins. 1983
Note on type 7 Chondrites. There is no firm agreement among meteorite researchers on the definition of type 7 chondrites. Many meteorites previously described as type 7 are either type 6 or are impact melts. Impact melting is not considered a proper basis for type 7 classification. Future research is expected to clear up this issue.
Here is a diagram from Weisberg showing petrologic types for each group:
Letter Designations: The
E stands for Enstatite. H indicates high metallic iron
and L indicates low metallic iron. Very Rare--1.5% of
falls. Minerals: Enstatite, metal, sulfides,
plagioclase, and occasional olivine (EH).
|Abundant||E3, EH3, EL3|
|Distinct||E4, EH4. EL4|
|Less Distinct||E5, EH5, EL5|
|Indistinct||E6, EH6, EL6|
The most common meteorite from observed falls--73.5%.
The initial letter designation for ordinary Chondrites, "O", is usually deleted. Brecciated meteorites with clasts of more than one type of alteration are designated with the range of alteration present in the meteorite. The letter designation refers to the iron content for the meteorite as a whole. E.g., Zag bears the designation H3-6.
Some meteorites are classified as transitional between the main groups, e.g., H/L, L/LL
When a researcher is not sure of a class, it may be designated with a notation like L(LL) with the less certain class being in parentheses.
|Melted [* these rocks can be considered primitive achondrites and not Ordinary Chondrites]||LL7|
These rare meteorites contain elemental carbon, a basic building block for life. 3.6% of falls.
Explanation of Letter Designations:
The "C" stands for Carbonaceous Chondrite. The second letter refers to type localities (except the H designation). The number refers to alteration.
|Ivuna||Friable, more water. Minerals: Phyllosilicates, magnetite||CI|
|Mighei||Friable, less water. Minerals: Phyllosilicates, tochilinite, olivine||CM1-CM2|
|Vigarano.||Fe rich olivine, CAIs||CV2-CV3.3|
|Renazzo||Minerals: Phyllosilicates, pyroxene, olivine, metal||CR|
|Ornans||Minerals: Olivine, Pyroxene, CAIs, metal.||CO3-CO3.7|
|Karoonda||Minerals: Olivine, CAIs||CK|
|Bencubbin||Minerals: Metal, Pyroxene. [ongoing research suggests the product of asteroidal collisions.]||CB|
|High Iron||Minerals: Pyroxene, metal, olivine. [May be related to Bencubbinites]||CH|
|Tagish Lake||This is a unique meteorite that samples the D asteroid family.||TAG|
|Rumurutiites||Minerals: Olivine, pyroxene, plagioclase, sulfide.||R|
Stony Meteorites without chondrules. Scientists believe that some of these meteorites originated on the surface of the Moon or Mars. 7.8 percent of meteorite falls are achondrites.
Note on Groupings:
Howardites, Eucrites, and Diogenites have been grouped as HED meteorites. These types may originate on the asteroid Vesta. Chassignites, Shergottites, and Nakhlites are grouped as SNC meteorites. These types are believed to originate on Mars.
See H. McSween, Meteorites and Their Parent Planets for and excellent explanation.
|Howardites||Vesta regolith||Eucrite-diogenite mix||HOW|
|Eucrites||Vesta basaltic crust||Anorthite-pigeonite||EUC|
|Nakhlites||Martian plutonic rock||Diopside-olivine||NAK|
|Chassignite||Martian plutonic rock||Olivine||CHA|
|Lunar||The Moon||Basalt and Regolith||LUN|
|Aubrites||Melted E Chondrite||Enstatite||AUB|
|Acapulcoite||Remelted chondrite||Olivine, Pyroxene||ACAP|
|Lodranite||Same as ACAP--more melt||Olivine, Pyroxene||LOD|
|Ureilites||Melted C-chondrite body||Olivine-pigeonite||URE|
|Angrite||Non-HED Basalt||Olvn, Pyrx., Plagioclase||ANGR|
|Brachinite||A or S type Asteroids||Olivine||BRACH|
|Winonaite||Like IAB & IIICD incl.||WIN|
Irons (structural classification)
These meteorites are made of a crystalline iron-nickel alloy. Scientists believe that they resemble the outer core of the Earth. 4.8 percent of meteorite falls are irons.
|Hexahedrites <6% Ni. Contains kamacite, but not taenite||>50mm||H|
|Plessitic||0.2mm Kamacite spindles||Opl|
|Ataxites High Ni content||(no structure)||D|
Irons (Chemical Classification)
A second scheme for classifying iron meteorites is by their chemistry. The determining factors are groupings of meteorites with similar ratios of trace elements to nickel. Generally, the higher the Roman numeral of the classification, the lower the concentration of trace elements. The casual observer cannot see this as one can with the Widmanstatten bandwidth that is the determining factor for structural classification. Chemical classification is important because it suggests that certain iron meteorites share a common origin or were formed under similar conditions. The Handbook of Iron Meteorites by Vahn Buchwald has a more complete description. Here is a diagram taken from Weisberg, et al.
|kamacite, taenite, silicates, carbides||Om-Og||IAB|
|kamacite, taenite, silicates, carbides||Om-Og||IC|
|kamacite, taenite, (daubreelite)||Ogg, H||IIAB|
|kamacite, taenite, silicates||Off-Og||IIE|
|kamacite, taenite,||Plessitic Oct., Atax.||IIF|
|kamacite, taenite, troilite, phosphides||Om-Og||IIIAB|
|kamacite, taenite, carbides||Off-D||IIICD|
|kamacite, taenite, carbides, graphite||Og||IIIE|
|kamacite, taenite, silicates, graphite||All||Anom|
These meteorites are mixtures of iron-nickel alloy and non-metallic mineral matter. Scientists believe that they are like the material that would be found where the Earth's core meets the mantle. 1.2 percent of meteorite falls are stony irons.
|Pallasites||Main-group Pallasites||iron, olivine||PAL|
|Eagle Station grouplet||iron, olivine, pyroxene|
|Pyroxene Pallasite grouplet||iron, pyroxene|
Mesosiderites have been divided into a classification grid that resembles that for chondrites. There are three broad petrologic groups designated A, B, and C. Within these groups there is a numeric classification of metamorphic grade. Grade 1 is fine grained and fragmental, 2 and 3 show progressive recrystallization, and 4 is a melt breccia.
|Class A (Basaltic)||iron, Ca pyroxene, plagioclase||1A|
|Class B (Ultramafic)||iron, Ca pyroxene, plagioclase, Orthopyroxene||1B|
|Class C (Orthopyroxene)||orthopyroxene||2C|
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