A detailed mineralogical, petrographic, and geochemical study of the highly reduced chondrite, Acfer 370

Among the many ungrouped meteorites, Acfer 370, NWA 7135, and El Medano 301—probably along with the chondritic inclusion in Cumberland Falls and ALHA 78113— represent a homogeneous grouplet of strongly reduced forsterite-rich chondrites characterized by common textural, chemical, mineralogical, and isotopic features. All of these meteorites are much more reduced than OCs, with a low iron content in olivine and low-Ca pyroxene. In particular, Acfer 370 is a type 4 chondrite that has olivine and low-Ca pyroxene compositional ranges of Fa 5.2–5.8 and Fs 9.4–33.4, respectively. The dominant phase is low-Ca pyroxene (36.3 vol%), followed by Fe-Ni metal (16.3 vol%) and olivine (15.5 vol%); nevertheless, considering the Fe-oxyhydroxide (due to terrestrial weathering), the original metal content was around 29.6 vol%. Finally, the mean oxygen isotopic composition D17O = +0.68& along with the occurrence of a silica phase, troilite, Ni-rich phosphides, chromite, and oldhamite confirms that these ungrouped meteorites have been affected by strong reduction and are different from any other group recognized so far.