KUSHIROITE, CaAl2SiO6, A NEW MINERAL IN CARBONACEOUS CHONDRITES: ITS FORMATION CONDITIONS AND GENETIC SIGNIFICANCE IN Ca-Al- RICH REFRACTORY INCLUSIONS

Introduction: Ca-Tschermak (hereafter CaTs), CaAl 2 SiO 6 , component has been one of the most important hypothetical members of pyroxene, and CaTs-rich pyroxene was abundantly reported from refractory inclusions in carbonaceous chondrites [e.g., 1]. However, the nature of such pyroxene has not been characterized in detail because of their tiny grain size. For the first time, we succeeded to identify the exact nature of highly CaTs-rich pyroxene [2, 3], and named it kushiroite, which was approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA2008-059) on January, 2009. Here we discuss the formation condition and significance of kushiroite in refractory inclusions. Petrography of kushiroite-bearing inclusions: Kushiroite was first identified, occurring with grossite, in an inclusion (#186) of ALH 85085 (CH) [1-3]. Later possible kushiroite has been reported from inclusions in CH and other carbonaceous chondrites [e.g., 4-6]. These inclusions are mostly spherical in shape. Kushiroite is always encountered in the outer peripheral parts of inclusions, surrounding grossite, hibonite and other minerals. The bulk composition of such inclusion is highly refrac-tory: 18.6 wt. Discussion: Although CaTs is stable under high-pressure conditions [7], no obvious evidence for impact in CAI #186 indicates that the metastable crystallization of this pyroxene took place from refractory melt in the solar nebula. The bulk composition of #186 CAI plots just on the liquidus surface of hibonite, not grossite or others. This strongly indicates that this inclusion never experienced crystallization process under equilibrium conditions. We suggest that grossite and kushiroite in #186 did not simultaneously or continuously crystallized from the same melt droplet, but formed separately. Textural observation and bulk chemistry indicate that precursor of kushiroite plastered grossite and other minerals which had been pre-existed condensates. Coexisting grossite-bearing CAIs in the type specimen ALH 85085 show 26 Mg excesses with inferred initial 26 Al-27 Al ratios between 2.1x10-6 to 3.9x10-5 [1], thus evidencing that condensation , melting and crystallization took place in the solar nebula when 26 Al was still extant.