Evaporation rates of forsterite in the system Mg₂SiO₄–H₂

Abstract

Evaporation experiments of forsterite single crystals were carried out in H₂ gas (1.4×10⁻⁵ bar H₂) at temperatures ranging form 1200 to 1450°C. Forsterite evaporated congruently. The measured evaporation rates of {010} surfaces were j_Fo=2480_{[mole cm⁻²sec⁻¹]}exp(−372_{[kJ mole⁻¹]}⁄RT). A model on the evaporation rates of forsterite in the system Mg₂SiO₄–H₂, where free evaporation-dominated (RED) and hydrogen reaction-dominated (HRD) regimes are included, was revised based on the Hertz-Knudsen equations with equilibrium vapor pressures. The measured evaporation rates are less than the ideal rates calculated by the present model, showing that kinetics at the evaporating surfaces constrain the evaporation rates. The evaporation coefficients, which are defined as the ratios between measured and calculated rates, were 0.04–0.12. Data on present and previous experiments of forsterite evaporation in H₂ gas and vacuum were compiled, and compared with the calculated rates. The evaporation coefficients varies from about 0.03 to 0.2 in a wide range of temperature and H₂ pressure. No specific relation with temperature or total pressure was recognized, and the evaporation coefficient can be roughly regarded as 0.1 both in the FED and HRD regimes.