Proposed Required Classification Elements for Each Meteorite Type

Classification Handbook for the Meteoritical Society Nomenclature Committee
DRAFT Apr. 2011


Nomenclature Notes and Abbreviations

  • Meteorite name and number requests and submission requirements are unchanged and can be found at http://www.meteoriticalsociety.org.
  • Detailed “write-ups” are required for all Martian, lunar, anomalous, and ungrouped meteorites.
  • Oxygen isotopes are generally not required, but may be asked for if the acquired information is ambiguous and does not lead to a clear classification.
  • For unequilibrated meteorites and subtypes, mineral compositions should be given as the composition mean and range with the number of analyses. For example, for an UOC L subtype, olivine Fa24.5±12, N = 14. Standard deviations (SDs) are optional.
  • Textural descriptions should be brief (unless in a detailed write-up), e.g. protogranular, equigranular, igneous, polygonal, poikilitic, etc.
  • For consistency, olivine composition should be given as Fa, not Fo; pyroxene compositions as Fs, not En; and plagioclase compoitions as An, not Ab.
  • Inclusion of unusual minor element contents are encouraged, e. g., CaO in olivine = 1.78 wt. %, Cr2O3 in magnetite = 2.45 wt. %, or Al2O3 in enstatite = 2.6 wt. %.
  • Ungrouped – Describes meteorites which have been sufficiently characterized to determine that they do not fit into any of the established groups.
  • Anomalous – Designates meteorites that have been determined to be a member of a specific group, but have certain properties that are unusual or distinctive.

Additional information, links, and photographs are available at:

Abbreviations and Nomenclature Notes

  • opx = orthopyroxene
    • Fs = ferrosillite (FeSiO3) molecule
    • Wo = wollastonite (CaSiO3) molecule
    • Orthopyroxene is orthorhombic and is distinguished from pigeonite (monoclinic) by optical extinction angle and CaO content. Opx has < 3 wt% CaO or Wo<5, pigeonite has 3-7 wt% CaO or Wo5-14.
  • cpx = Ca-pyroxene (molecule designations same as opx)
  • olv = olivine
    • Fa = fayalite (Fe2SiO4) molecule
  • FeO/MnO = molecular ratios in olivine and pyroxenes, also given as Fe/Mn.
  • plag = plagioclase, (Na,Ca)(Al,Si)Si2O8
    • An = anorthite CaAl2Si2O8 molecule
    • Ab = albite (NaAlSi3O8) molecule
    • Or = orthoclase (KAlSi3O8) molecule
  • Metal = Ni-Fe metal
  • N/R = not require

AUBRITES

Minerals and compositions:

  • Orthopyxene (nearly Fe-free)
  • Metal (mostly Si-bearing kamacite)
  • Plagioclase (Na-rich, An<24)
  • Sulfides present (troilite with minor amounts of Ni, Ti, Cr; alabandite; daubreelite; niningerite; oldhamite)

Mineral Modes (vol%): N/R

Texture:

  • Igneous, brecciated, unbrecciated
  • grain size: fine- to coarse-grained

ACAPULCOITE-LODRANITE Family

Minerals and compositions:

  • Othopyroxene: Fs5-15Wo<3.5; FeO/MnO = 9-17
  • Olivine: Fa4-16; FeO/MnO = 12-31
  • Diopside: Fs3-8.2Wo42-48; FeO/MnO = 10-14, commonly Cr-rich
  • Plagioclase: sodic, An8-32Or2-4

Mineral Modes (vol%): N/R

Texture:

  • Polygonal, metamorphic (120° triple junctions)
  • Mean grain size: <0.4 mm for acapulcoites; >0.5 mm for lodranites. Important! Grain size is the critical criterion that distinguishes acapulcoites from lodranites.

     


BRACHINITES

Minerals and compositions:

  • Olivine: Fa30-35; FeO/MnO = 68-82
  • Ca-pyroxene: Fs9-14Wo44-48; FeO/MnO = 38-45

Mineral modes (vol%):

  • Olivine >80, Ca-pyroxene 4-15, chromite up to 3

Texture:

  • Equigranular metamorphic (polygonal); grain size: mean <2 mm

CARBONACEOUS CHONDRITES (CI, CMCOCVCKCR, and CH)

Minerals and compositions:

  • Olivine
    • CI and CM, none
    • CO – variable Fa, FeO/MnO = 110-138
    • CV – Fa0-50, FeO/MnO >136
    • CK – Fa1-47, FeO/MnO = 105-140
    • CR – Fa1-3, FeO/MnO = 7-24
    • CH – Fa1-36
  • Orthopyroxene
    • variable in CM, CO, CV
    • CK – Fs22-29
    • CR – Fs4
    • CH – Fs<10
  • Magnetite in CK, report Cr2O3)
  • Metal in vol% (CO, 1-5; CV, 0-5; CR, 5-8; CH, 20)

Mineral modes (vol%): N/R

Texture: N/R

Petrologic types:

  • Petrologic subtypes for carbonaceous chondrites are limited mostly to the CO class, which is also limited to 3.0 to 3.9 subtypes with no further petrologic subdivisions, Similar to ordinary chondrites, the CO3 class also shows systematic variations for Fa and Cr2O3 content in olivine (Grossman, 2004), although the statistical data base for COs are limited thus, the subtype divisions are not well defined

CB CARBONACEOUS CHONDRITE

CBa and CBb chondrites (Bencubbin-like)

Minerals and compositions:

  • Olivine: Fa1.3-4.5
  • Orthopyroxene: Fs2-4.5Wo1.5-4.0
  • Metal: kamacite >60 vol%; Ni = 5-8 wt%
  • Troilite: 2-5 wt% Cr

Mineral Modes (vol%): N/R

Texture:

  • Typical absence of fine-grained matrix (low matrix/chondrule ratio); grain size: chondrules are coarse, cm-sized; CBa is distinguished from CBb by large chondrule size (cms compared to mms) and 60 vol% metal vs. 80%

ENSTATITE CHONDRITES

EH and EL chondrites

Minerals and compositions:

  • Orthopyroxene (predominantly clinoenstatite in types 3 and 4, mixed with disordered orthoenstatite in subtype 5 and all ordered orthopyroxene in type 6. Composition is essentially Fs<1, with rare examples up to Fs12 and most contain unusually high Al2O3 contents (<14 wt%). FeO/MnO ratios are unreliable due to low FeO and MnO contents)

Mineral modes (vol%): N/R

Texture:

  • Types 3-6 constrained mainly on decreasing amounts of clinoenstatite with increasing orthopyroxene (orthoenstatite) and increased recrystallization of chondrules so that EL6 and EH6 chondrites have few discernable chondrules and all enstatite is orthorhombic. EH chondrites have metal >25 vol%.

HED CLAN (HowarditesEucrites, and Diogenites)

Eucrites (basaltic and “cumulate” types)

Basaltic eucrites are typically brecciated (monomict or polymict). Either orthopyroxene or pigeonite will be present.

Minerals and compositions:

  • Orthopyroxene (Fs48-68Wo1-5; FeO/MnO = 25-38 (host))
  • Pigeonite (Fs51-67Wo5-14 FeO/MnO = 26-39 (host))

Mineral Modes (vol%): N/R

Texture:

  • Basaltic eucrite textures are highly variable, mostly subophitic and ophitic, but may include phyric and variolitic textures, in addition to shock-formed textures, e. g., microgranular, quench, and microphenocrystic. Cumulate eucrites are commonly protogranular with equal amounts of pyroxene and plagioclase.

Diogenites

Minerals and compositions:

  • Orthopyroxene (Fs20-34Wo2.1-4.2, FeO/MnO = 26-38)
  • Olivine (Fa28-39, FeO/MnO = 44-59)

Mineral Modes (vol%): N/R

Texture:

  • Igneous, protogranular, poikilitic; some are cataclastic. recrystallized-granular; grain size: millimeters to centimeters.

Howardites

Howardites do not have unique mineral characteristics like other meteorite classes and are considered to be regolith breccias that consist essentially of >10 modal % of diogenites and <90 modal % of eucrites, some of which are very dark, shock-melted clasts. If a suspected howardite contains <10 modal % of diogenite and >90 % eucrite material the stone is classified as a eucrite. On the other hand, a stone that is <10 % eucrite and >90 % diogenite, it is a diogenite. Howardites may also contain minor components that include carbonaceous chondrites, in addition to minor exotic clasts.


ORDINARY CHONDRITES (HL, and LL)

Equilbrated Chondrites

Minerals and compositions:

  • Olivine
    • H (Fa16-20, FeO/MnO = 30-40)
    • L (Fa22-26, FeO/MnO = 46-55)
    • LL (Fa27-32, FeO/MnO = 50-63)
  • Orthopyroxene
    • H (Fs14.5-18.5)
    • L (Fs19-22)
    • LL (Fs23-2)

Mineral modes (vol%): N/R

Texture: N/R

Shock level:

  • S1 – unshocked
  • S2 – very weakly shocked, undulatory extinction, fractures
  • S3 – add planar fractures, opaque shock veins and melt pockets
  • S4 – mosaicism in olivine; plagioclase partially isotropic, PDFs
  • S5 – strong mosaicism and PDFs; maskelynite; many melt pockets and veins
  • S6 – solid state recrystallization, melting and hi P phases; plagioclase melted
  • S7 – whole rock melting

Weathering grade:

  • WO – typically fresh falls, no visible oxidation of metal or sulfide
  • W1 – minor oxide rims around metal/troilite; minor oxide veins
  • W2 – moderate metal oxidation (20-40 % of metal)
  • W3 – heavy oxidation (40- 96 % of metal)
  • W4 – nearly complete oxidation (>95 % of metal)
  • W5 – mafic silicate oxidation along cracks
  • W6 – massive replacement of silicates by clays and oxides

Unequilibrated Chondrites – Petrologic types

CriterionPetrologic Type
3456
Homogeneity (% mean deviation)px ≥5%; olivine ≥5%

 

<5% to homogeneoushomogeneous
Low-Ca pyroxenepredominately monoclinic>20% monoclinic<20% monoclinicorthorhombic
Feldsparminor primarysecondary, <2µm grainssecondary, 2-50µm grainssecondary, >50µm grains
Chondrule glassclear, isotropicdevitrifiedabsent
Matrixopaque to transparenttransparent microcrystallinerecrystallized
Chondrule-matrix boundarysharply definedmoderately diffusediffuse

STONY-IRON METEROITES

Mesosiderites

Minerals and compositions:

  • Orthopyroxene (Fs15-33Wo0.8-4.5; FeO/MnO = 16-35)

Mineral Modes (vol%):

  • Metal (typically about 50 vol%, but can range from 30 to 70 vol%)

Texture:

  • Silicate clasts/matrix
  • Igneous to recrystallized
  • metal (metal balls, nuggets, fine-grained blebs, poikilitic (non-metal inclusions), other)

Pallasites

Minerals and compositions:

  • Olivine (Fa12-18; FeO/MnO = 35-61)
  • Metal

RUMURUTI (R Class)

Minerals and Compositions:

  • Olivine (Fa36-40, FeO/MnO = 70-90; very abundant; Ni-bearing)
  • Sulfides (sulfide rich: pyrrhotite and pentlandite very common, minor troilite; pentlandite commonly contains Cr up to 2 wt%, troilite may contain Ni up to 3 wt%)
  • Plagioclase (sodic, An9-23Or3-6)

Mineral Modes (vol%): N/R


UREILITES

Minerals and compositions:

  • Olivine (cores: Fa14-23, FeO/MnO = 17-36; CaO = 0.3 to 0.8 wt%; Cr2O3 = 0.5 to 1.0 wt%; reduced rims: Fa1-10)
  • Pigeonite (Fs9-22Wo5-13; no exsolution (a few ureilites have opx instead of pig))
  • Graphite (see texture below)

Mineral Modes (vol%): N/R

Texture:

  • Typical ureilites and are easily recognized by the simple mineralogy (olivine and pyroxene), medium to coarse-grained equigranular textures (with triple junctions), the presence of fine-grained reduced margins on olivine (from the reduction of FeO in olivine), and interstitial ribbon-like graphite and metal
  • Bimodal ureilites, few in number, show extreme grain size and mineral contents
  • Polygonal ureilites, also few in number, are fragmental breccias that contain clasts of other ureilites and other meteorites classes.
  • Mosaicized ureilities are typified by very fine-grained (microns), recrystallized silicates from heavy shock loading.

WINONAITES

Minerals and compositions:

  • Orthopyroxene (Fs1-9; FeO/MnO = 1-8)
  • Ca-pyroxene (Fs2-9Wo43-46)
  • Plagioclase (An10-23)
  • Olivine (Fa1-8; FeO/MnO = 6-15)

Texture:

  • Equilibrated, metamorphic textures (triple junctions)