An experimental study of enstatite formation reaction between forsterite and Si-rich gas

Experiments on the reaction between forsterite and Si-rich gas were carried out in a vacuum furnace using a newly designed double-capsule technique. A thin layer of enstatite was formed on the surface of the forsterite by the reaction. The thickness of the enstatite layer changes in proportion to the square root of time (parabolic rate law), indicating that the rate is controlled by diffusion in the enstatite layer. The activation energy and the logarithm of the pre-exponential term of the parabolic rate constant were obtained at505 ± 188 kJ/mol and3.6 ± 5.6 cm 2/s, respectively. It is difficult to definitely determine the mechanism of the reaction from these values, but consideration of strain generation due to volume change during the reaction may suggest that the counter-diffusion of silicon and magnesium takes place along the grain boundary in the enstatite layer. The degree of the reaction of forsterite with Si-rich residual gas in the primordial solar nebula is discussed based upon the measured reaction rate, and it is suggested that very little enstatite was formed by this reaction in the cooling nebula, if the grain size of the forsterite crystals was controlled by homogeneous nucleation theory.