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The brand-new MCC Dhofar 225
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(a small 0.82g partial slice)
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A Full Slice of MCC Dhofar 735
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(a thinly cut 13g full slice)
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Another View of Dhofar 735
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(back side of the same slice)
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Synonyms: Meta-C group, metamorphosed CCs
General: This is a brand-new group of carbonaceous chondrites
that has been proposed, recently. Some members of this group have
been previously classified as anomalous CI or CM members, but new
studies show that they have not just been thermally metamorphosed in
their parent body, but also show common O-isotopic compositions,
distinct from the CI and the CM groups.
Description: The MCCs resemble CM and CI chondrites very much,
and belong to petrologic type 1 or 2. Their O-isotopic compositions can
be clearly resolved from the CI and CM fields, they show a low water
content, as well as unique infrared spectral characteristics of their matrix
phyllosilicates indicating dehydration.
Mineralogy: Mineralogic and chemical characteristics of the MCCs
correspond to those of CI and CM group members prior to thermal
metamorphism, but the MCCs are usually depleted of iron and sulfur
compared to CI and CM chondrites. CAIs are sparse, and mostly small.
Origin & Formation: All MCCs show comparably young CRE ages,
suggesting a near-Earth asteroid as their common parent body. The
spectral properties of binary asteroid 1998 ST27 actually provide a good
match for the unique spectral characteristics of the MCCs, and it is
possibly the source of these unusual chondrites.
Members: Originally, this small grouplet comprised just three
Antarctic finds:
Belgica 7904,
Yamato 82162, and
Yamato 86720. More recently,
Dhofar 225 and
Dhofar 735 have also been shown to belong to that rare class, adding
enough members to make it a full-fledged group, tentatively named the
MCC group. Some additional CC finds with affinities to the CM and CI
group have been suggested to be metamorphosed, also, but O-isotopic
compositions of these finds have yet to be determined. |
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