A SYSTEMATIC CLASSIFICATION OF METEORITES
PART V

Weyrauch et al. (2018) analyzed the mineral and chemical data from 80 enstatite chondrites representing both EH and EL groups and spanning the full range of petrologic types for each group. They found that a bimodality exists in each of these groups with respect to both the Cr content in troilite and the Fe concentration in niningerite and alabandite (endmembers of the [Mn,Mg,Fe] solid solution series present in EH and EL groups, respectively). In addition, both the presence or absence of daubréelite and the content of Ni in kamacite were demonstrated to be consistent factors for the resolution of four distinct E chondrite groups: EHa, EHb, ELa, and ELb (see table below).

A few other E chondrites with intermediate mineralogy have been identified, including LAP 031220 (EH4), QUE 94204 (EH7), Y-793225 (E-an), LEW 87223 [EL3-an; abs], and PCA 91020 (possibly related to LEW 87223). Studies have determined that these meteorites were not derived from the EH or EL source through any metamorphic processes, and some or all of them could represent separate E chondrite asteroids.

A number of the meteorites in the tables above were subsequently determined by Mc Ardle et al. (2025) to be impact-melt rocks, and they have reevaluated the respective petrologic types as follows (see also their alternative quantitative subgroup classification scheme below):

• AhS MS-52, AhS MS-D, and EET 90102–from ELb6 to EL IMR
• Y-793246–from ELb4 to EL IMR
• AhS MS-7 [5/6]–from ELb5 to EL IMR
• PCA 91020 [3-anom; poss. rel. to LEW 87223] and NWA 974 [6]–from ungrouped to EL IMR
• Oudiyat Sbaa ('Not Yet Classified By Subtype')–from EH5 to EH IMR
• NWA 15359 [EL3], KLE 98300 [EH3], NWA 4945 [EL6], NWA 4780 [EL4], NWA 14141 [EL4], and Adrar Bous [EL5]–(these are not included in the tables above) to EL IMR

A comprehensive study was conducted by Mc Ardle et al. (2025) which utilized geochemical and mineralogical data for a large sampling of enstatite chondrites representing a broad range of petrologic types within the two established chemical groups, EL and EH. They first determined which of the meteorites had experienced shock melting followed by either quenching or slow cooling, as well as possible annealing. These impact-melt rocks were eliminated from their study, while the remainder, which experienced only parent body thermal heating and related metamorphism, were reevaluated as to petrologic type.

Mc Ardle et al. (2025) concluded that previous classification schemes proposed by Weyrauch et al. (2018) and others had included shock-melted meteorites among the samples, and therefore could only distinguish the quenched impact-melt rocks on the one hand (ELb and EHb), and the slowly cooled impact-melt rocks (ELa and EHa) together with the nonimpact-melted samples on the other hand. After excluding all shock-melted meteorites, and considering only those which experienced parent body thermal metamorphism, certain trends became evident such as the following (see all newly defined EC petrologic type classification parameters proposed by Mc Ardle et al. [2025]):

• both the EL and EH groups show a positive correlation between Ti concentration in troilite and petrologic type
• the EL group shows a positive correlation between Si content in kamacite and petrologic type
• the EH group, and likely also the EL group, shows a positive correlation between glass/feldspar abundance and petrologic type

A new quantitative petrologic type classification scheme was proposed by Mc Ardle et al. (2025) that only applies to those enstatite chondrites (EL and EH) which experienced parent body thermal metamorphism without shock melting. Their classification scheme uses mineralogic, petrographic, and geochemical insights utilizing the glass/feldspar, olivine, enstatite, sulfide, and metal components present in each meteorite (see their Table 11 and 12).

In addition, after a thorough investigation of both the pros and cons of all previously proposed subgrouping schemes for type 3 ECs, Mc Ardle et al. (2025) developed a simplified subtype classification based on the Cr₂O₃ content of olivine, which is presently applicable to the EH3 group. Two distinct subtypes were considered to be resolvable within reason, with the boundary between them established at 0.25 wt% average Cr₂O₃ content in olivine (see diagram below):

1. more primitive EH3_Low subtype:
   (e.g. Y-691, NWA 13299, SAH 97072)
2. less primitive EH3_High–EH4 subtype:
   (e.g. DOM 14021, Qingzhen, ALHA77295, EET 83322, LAR 12252, Indarch, MIL 07028)

PART I: CHONDRITES, METACHONDRITES
PART II: PRIMITIVE ACHONDRITES, ACHONDRITES, STONY-IRONS, IRONS
PART III: MARTIAN METEORITES—GEOCHEMICAL CLASSIFICATION
PART IV: DIOGENITES—IUGS TAXONOMY
PART VI: STONY METEORITES—METAMORPHIC FACIES SERIES

You must collect things for reasons you don't yet understand.
Daniel J. Boorstin–Librarian of Congress

© 1997–2025 by David Weir