THE OCCURRENCE OF GROSSITE (CAAL4O7) IN CHONDRITES

The mineral grossite (CaAl4O7; Ca-dialuminate, also termed CA2) is a rare phase in terrestrial and extraterrestrial rocks. In meteorites, grossite was found in Ca,Al-rich inclusions (CAIs) from Allende (CV3), Vigarano (CV3), Leoville (CV3), Efremovka (CV3), Colony (CO3), Murchison (CM2), ALH 85085 (CH), Acfer 182 (CH), Acfer 059-El Djouf 001 (CR2), and the anomalous C-chondrite Adelaide. In Ca,Al-rich inclusions from most of these meteorites, grossite is an accessory phase; however, in many CAIs from ALH 85085, Acfer 182, and Acfer 059-El Djouf 001 it is a major phase. In this paper, we summarize the results of literature reports on grossite-bearing inclusions and focus, in detail, on the occurrence, mineralogy, and chemistry of CaAl4O7-containing Ca,Al-rich inclusions in Acfer 182 and Acfer 059-El Djouf 001. In these carbonaceous chondrites, abundant CAIs with high modal abundances of grossite (>30 vol%, often >60 vol%) were discovered. To classify Ca,Al-rich inclusions with such high modal abundances of CaAl4O7 we suggest to introduce a new type of Ca,Al-rich inclusion: grossite-rich inclusions. In Acfer 059-El Djouf 001 and Acfer 182, grossite-containing inclusions occur with different petrographic characteristics. The most common type consists of a core of grossite rimmed by melilite, Ca-pyroxene +/- spinel +/- hibonite. In an other type, anhedral, polycrystalline grossite complexes of irregular shape are enclosed in melilite. In some spherical CA2-inclusions abundant euhedral to subhedral grossite and/or hibonite crystals are embedded in melilite. In two inclusions, a core of hibonite is mantled by grossite. Additionally, some grossite-bearing inclusions with unique texture and/or mineralogy were found. One of these inclusions is rich in Zr,Y-rich perovskite and contains phases rich in the ultrarefractory elements Sc and/or Zr. Almost all of the grossite-containing inclusions are rich in Al2O3 (>50 wt%), even if grossite is only a minor phase. Based on textural appearance and/or on REE contents in some inclusions (core minerals with Group II patterns) a condensation history for most inclusions is suggested. A single inclusion rich in perovskite and other phases containing high concentrations of ultrarefractory elements appears to have formed as an early condensate from the solar nebula.