EVAPORATION OF FORSTERITE IN H2 GAS

Abstract

Kinetics of evaporation of forsterite in hydrogen gas was investigated by high temperature vacuum experiments in the pressure range plausible for the solar nebula. The evaporation rate at total pressure (Ptot) below 10-6 bar is nearly constant and is similar to that in vacuum, whereas the rate at 10-6 to 10-3 bar is dependent on Ptot. The evaporation rate, JFoexp, is fitted by JFoexp = 1.72P1.19tot + 9.87 x 10-7 (g.cm-2.s-1) for Ptot below 10-4 bar. The condensation coefficient, α, which is a factor related to kinetics of surface reactions, is evaluated by using the Hertz-Knudsen equation for the kinetic theory of gas molecules. The ratio of the experimentally obtained evaporation rate to that calculated from chemical equilibrium in the system Mg2SiO4-H2 gives the α value of 0.06 in vacuum, which increases up to 0.2 with increasing Ptot from 10-3 to 10-4 bar. The apparent increase of forsterite evaporation rate with increasing H2 abundance is due mainly to increase of the equilibrium vapor pressure, which corresponds to increase in the driving force, and partly to increase in α (reduction of the kinetic barrier) for evaporation.The experimental results were applied to understand behavior of forsterite dusts with time in an abruptly heated model nebula mostly comprising forsterite and H2. The nebular system can be divided into complete and partial evaporation regimes, which is defined by a dust enrichment factor. For the complete evaporation regime (low dust enrichment), the minimum time for forsterite grains to totally evaporate is estimated as a function of total pressure, temperature, and initial grain size. The lifetime of forsterite grains (<10 μm in size) could be less than 1 h at 1700oC. The experimental results were further applied to examine the possibility of isotopic fractionation for forsterite grains in the solar nebula. By evaluating the competition between evaporation from surface and elemental diffusion in forsterite, it is shown that forsterite grains could have isotopically fractionated to be heavier only for Mg, but not for Si and O.