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:
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.
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.
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 (has 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%
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%.
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.
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%)
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.