Computational Mechanistic Analysis of the Formation of the Magnesium Silicate Monomers MgSiO3 and Mg2SiO4.

Abstract

Silicate grains comprise a large fraction of cosmic dust, motivating a need to understand how they form. The current body of work on silicates generally reflects the abundance of silicate grains, yet models for their formation often do not consider silicate chemistry on the smallest scale, which can form species available for dust grain nucleation processes. In order to expand upon previous attempts to bridge this gap in silicate chemistry, novel gas-phase reaction pathways for the magnesium silicate monomers enstatite (MgSiO₃) and forsterite (Mg₂SiO₄) from MgH, H₂O, and SiO are presently computed using highly accurate quantum chemical calculations. MgSiO₃ and Mg₂SiO₄ form through a series of reactions that initially excludes silicon addition, creating the elusive species MgOH and Mg₂O prior to further reaction. The formation of the two silicate monomers is expected to be efficient with the primary bottleneck being the amount of MgH available for reaction. The addition of these reactions to cosmic chemical networks will add further clarity to the processes that govern dust formation, most significantly for those occurring within stellar outflows of asymptotic giant branch stars.