Thortveitite (Sc_2Si_2O_7), the First Solar Silicate?

Abstract

In condensation calculations for a cooling gas of solar composition, or dust enriched variants, melilite is invariably the earliest condensing silicate except at very high dust-to-gas ratios [e.g., 1]. The possibility that silicates of the highly refractory lithophiles, such as Sc and Zr, actually form first is not tested because of a lack of thermodynamic data. Meteoritic occurrences may, therefore, provide the best clues to the earliest evolution of Si in nebular solids. During a nano-mineralogy investigation of the Murchison CM2 carbonaceous chondrite, we identified thortveitite (Sc_2Si_2O_7), along with davisite (CaScAlSiO6), panguite [(Ti,Sc,Al,Mg)_(1.8)O_3], spinel, and Sc-rich diopside, in the Sc-enriched ultrarefractory inclusion MUR1. This is the first meteoritic occurrence of thortveitite and the second natural occurrence of panguite, a Ti-rich oxide recently discovered in Allende [2]. Field-emission SEM with EDS, electron back-scatter diffraction, and electron microprobe were used to characterize the compositions and structures of these two minerals and associated phases.

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