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

Makoto Kimura, Takashi Mikouchi, Akio Suzuki, Masaaki Miyahara, Eiji Ohtani, Ahmed El Goresy

Research output: Contribution to journalArticlepeer-review

25 Citations (Scopus)

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 Ca1.008(Mg0.094Fe 0.034Al0.878)(Al0.921Si1.079)O 6, 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 CaAlAlSiO 6 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.

Original languageEnglish
Pages (from-to)1479-1482
Number of pages4
JournalAmerican Mineralogist
Volume94
Issue number10
DOIs
Publication statusPublished - 2009 Oct

Keywords

  • CH chondrite
  • Ca-Tschermak component
  • Kushiroite
  • Pyroxene
  • Refractory inclusion

ASJC Scopus subject areas

  • Geophysics
  • Geochemistry and Petrology

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