Article

Physics and Chemistry of Minerals

, Volume 11, Issue 3, pp 113-124

A new solid state diffusion model applied to inverse zoning and diffusion rims in minerals

  • Friedemann FreundAffiliated withDepartment of Physics, Arizona State University
  • , Bruce V. KingAffiliated withDepartment of Physics, Arizona State University

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Abstract

Any oxide and silicate mineral which is nominally anhydrous but crystallized in the presence of H2O incorporates traces of H2O in solid solution. In the case of MgO it can be shown that OH pairs convert into H2+O 2 2− . If the H2 molecules are lost, the O 2 2− remain in the lattice as excess oxygen stabilized by excess cation vacancies. When the O 2 2− anions decay either thermally or by decompression unbound O states (positive holes) are generated which lead to surface charges and subsurface space charge layers. Calculated space charge profiles are presented. O concentrations as small as 10–20 ppm suffice to create electric surface fields of the order of 4·107 V·m−1. The diffusion mechanism which derives from these premises incorporates novel features: the cation diffusion is coupled to the counterdiffusion of unbound and vacancy-bound O states. The cation diffusion is predicted to be very fast because first, it is field-enhanced (electrochemically driven) and second, it is not rate-limited by the intrinsic cation vacancy concentration nor by the counter-diffusion of other cations. The model may apply to cases of inverse zoning and diffusion rim formation in minerals under certain P-T conditions.