Discovery of dmisteinbergite (hexagonal CaAl2Si2O8) in the Allende meteorite: A new member of refractory silicates formed in the solar nebula

Abstract

Dmisteinbergite, CaAl_2Si_2O_8 with P6_3/mcm structure, was identified in a rounded coarse-grained igneous Type B2 Ca-,Al-rich inclusion (CAI) STP-1 from the Allende CV3 carbonaceous chondrite. STP-1 belongs to a very rare type of refractory inclusions, Fractionation and Unknown Nuclear effects (FUN) CAIs, which experienced melt evaporation and crystallization at low total gas pressure (P 1200 °C) region, possibly near the proto-Sun and were subsequently radially transported away from region, possibly by a disk wind. The Allende dmisteinbergite occurs as irregular single crystals (100–600 μm in size) in contact with gehlenitic melilite and Al,Ti-diopside, poikilitically enclosing euhedral spinel, and rare anorthite. It is colorless and transparent. The mean chemical composition, determined by electron microprobe analysis, is (wt%) SiO_2 42.6, Al_2O_3 36.9, CaO 20.2, MgO 0.05, sum 99.75, giving rise to an empirical formula of Ca_(1.01)Al_(1.96)Si_(2.02)O_8. Its electron backscatter diffraction patterns are a good match to that of synthetic CaAl_2Si_2O_8 with the P6_3/mcm structure and the unit cell a = 5.10 A, c = 14.72 A, and Z = 2. Dmisteinbergite could have crystallized from a silicate melt at high temperature (~1200–1400 °C) via rapid cooling. Dmisteinbergite in Allende, the first find in a meteorite, is a new member of refractory silicates, among the first solid materials formed in the solar nebula.