Experimental results on variations in the O content and cation diffusion in olivines, and especially fayalite, were considered. It was found that the cation tracer diffusivity for various Fe contents increased with increasing O activity. The observed O activity dependence in single crystals, for all Fe contents, was larger than expected on the basis of the cation vacancy concentration. However, it corresponded to the values which were expected in the case of polycrystalline samples. It was suggested that neutral associates, which formed by reactions between doubly negatively charged cation vacancies and Fe3+ ions on Me sites, contributed significantly to the cation diffusion at high O activities. For some reason, these neutral associates did not contribute to Fe tracer diffusion in polycrystalline samples. It was proposed that their concentrations were much lower near to boundaries than in the bulk. It was also observed that, in the case of very pure fayalite which was grown by using the float-zone method, the cation tracer diffusion depended strongly upon the orientation. Diffusion in single crystals was fastest in the <001> direction and slowest in the <010> direction; a behavior which was expected on the basis of the crystal structure. The O activity dependences which were observed for Fe diffusion in monocrystals were very similar for all of the orientations which were studied.

T.L.Tsai, R.Dieckmann: Materials Science Forum, 1997, 239-241, 399-402