High-temperature stability of San Carlos olivine

A suite of heat-treatment experiments have been performed to test the high-temperature stability of San Carlos olivine within the theoretical {Mathematical expression} stability field at one atmosphere total pressure. Exsolution or contamination products did not form on the surfaces of samples which were surrounded by olivine, magnesia, or alumina. In contrast, silica-rich phases developed on the surfaces of samples which were in line-of-sight contact with silica or platinum. These silica-rich phases result from interaction of the olivine with silica or platinum in the furnace environment via vapor phase transport or surface diffusion, rather than from diffusion of excess silica from the bulk to the surface as suggested by Jaoul et al. (1984, 1985). This conclusion is consistent with the reported slow rate of diffusion of silicon in olivine and with the lack of internal precipitation of silica-rich phases. Consequently, it is concluded that San Carlos olivine does not contain silica in excess of the solubility limit under conditions which are within the {Mathematical expression}-T stability field for this (Mg,Fe,Ni)-olivine.