Kushiroite, CaAlAlSiO₆: A new mineral of the pyroxene group from the ALH 85085 CH chondrite, and its genetic significance in refractory inclusions

Abstract

The new mineral kushiroite, belonging to the pyroxene group, was first discovered in a refractory inclusion in the CH group carbonaceous chondrite ALH 85085. The chemical formula is Ca_1.008(Mg_0.094Fe_0.034Al_0.878)(Al_0.921Si_1.079)O₆, containing 88% CaAlAlSiO₆ and 12% diopside components. We identified the exact nature of kushiroite by micro-Raman spectroscopy and electron backscatter diffraction (EBSD) analyses. The results are consistent with those obtained from the synthetic CaAlAlSiO₆ pyroxene, thus indicating a monoclinic structure (space group C2/c). Although CaAlAlSiO₆ has been one of the most important hypothetical components of the pyroxene group, it is here for the first time established to be a naturally occurring mineral. We named this pyroxene with >50% CaAlAlSiO₆ component kushiroite, which was recently approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA2008- 059). The name is for Ikuo Kushiro, Professor Emeritus at the University of Tokyo, Japan, and eminent experimental petrologist, for his outstanding experimental investigations on silicate systems involving the Ca-Tschermak component. There is no obvious evidence for impact in this inclusion. We suggest that metastable crystallization of this pyroxene took place from refractory melts in the solar nebula. Coexisting grossite-bearing refractory inclusions in the type specimen ALH 85085 show ²⁶Mg excesses with inferred initial ²⁶Al-²⁷Al ratios between 2.1 × 10^-6 to 3.9 × 10^-5, providing evidence that condensation, melting, and crystallization took place in the solar nebula when 26Al was still extant.