EVAPORATION OF FORSTERITE IN H-2 GAS

Kinetics of evaporation of forsterite in hydrogen gas was investigated by high temperature vacuum experiments in the pressure range plausibl e for the solar nebula. The evaporation rate at total pressure (P-tot) 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 P-tot. The e vaporation rate, J(exp)(Fo), is fitted by J(exp)(Fo) = 1.72P(tot)(1.19 ) + 9.87 x 10(-7) (g . cm(-2). s(-1)) for P-tot below 10(-4) bar. The condensation coefficient, alpha, which is a factor related to kinetics of surface reactions, is evaluated by using the Hertz-Knudsen equatio n for the kinetic theory of gas molecules. The ratio of the experiment ally obtained evaporation rate to that calculated from chemical equili brium in the system Mg2SiO4-H-2 gives the a value of 0.06 in vacuum, w hich increases up to 0.2 with increasing P-tot from 10(-3) to 10(-4) b ar. The apparent increase of forsterite evaporation rate with increasi ng H-2 abundance is due mainly to increase of the equilibrium vapor pr essure, which corresponds to increase in the driving force, and partly to increase in a (reduction of the kinetic barrier) for evaporation. The experimental results were applied to understand behavior of forste rite dusts with time in an abruptly heated model nebula mostly compris ing forsterite and H-2. The nebular system can be divided into complet e and partial evaporation regimes, which is defined by a dust enrichme nt factor. For the complete evaporation regime (low dust enrichment), the minimum time for forsterite grains to totally evaporate is estimat ed as a function of total pressure, temperature, and initial grain siz e. The Lifetime of forsterite grains (<10 mu m in size) could be less than 1 h at 1700 degrees C. The experimental results were further appl ied to examine the possibility of isotopic fractionation for forsterit e grains in the solar nebula. By evaluating the competition between ev aporation from surface and elemental diffusion in forsterite, it is sh own that forsterite grains could have isotopically fractionated to be heavier only for Mg, but not for Si and O.