Crystallographic and Chemical Studies of a Bronzite and Chromite in the Yamato (b) Achondrite

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X-ray single crystal diffraction studies, supplemented by electron microprobe analyses, have been made of pyroxene and chromite in the Yamato (b) achondrite. The major phases in the meteorite are homogeneous grains of bronzite (Ca_2Mg_<74>Fe_<24>) with no exsolution lamellae of augite, and euhedral to subhedral grains of chromite with high Cr and Mg contents. Troilite and metallic iron are also present. The pyroxene is identical in composition to those of the hypersthene achondrites but the portion of the meteorite examined does not have the very coarse-grained and brecciated texture of most hypersthene achondrites. X-ray diffraction study of an octahedral chromite grain shows that it consists of portions with slightly different orientations. Measurement of the magnetic moment of the chromite shows no abrupt change in magnetization and the absence of a distinct thermal hysteresis between 20 and 400℃, consistent with the magnetic properties of chromite. Yamato (b) is a unique hypersthene achondrite with a non-brecciated granoblastic texture. We suggest that Yamato (b) may have undergone a reheating event that caused recrystallization of the pyroxene, without change in composition.
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