Crystallization experiments of SiO2-rich amorphous silicate: Application to SiO2-rich circumstellar dust and GEMS

Abstract

Crystallization experiments of relatively SiO2-rich amorphous silicates using the mean chemical composition of the silicate portions in GEMS (glass with embedded metal and sulfide), which is a major component in anhydrous interplanetary dust particles and a primitive material of the early solar system, were performed to understand the presence of crystalline silica around young stars and crystallization in GEMS. Olivine crystallized at 900-1400K, probably prior to pyroxene. Three different polymorphs of pyroxene, protopyroxene, orthopyroxene, and clinopyroxene, were identified at ≥1000K. Cristobalite, which is one of the silica polymorphs, crystallized only at high temperatures (≥1500K). We obtained time-temperature-transformation (TTT) crystallization diagrams. These results suggest that crystallization of a silica polymorph is kinetically difficult in a day or so at 900-1400K even for the SiO2-saturated composition, while the crystallization might be possible after metastable olivine crystallization if duration is long enough. The TTT diagram also indicates that the GEMS cooling timescale was 10 5 s if they condensed at 1000K as amorphous silicates and annealed during cooling after the condensation. © 2012. The American Astronomical Society. All rights reserved..