Internal reduction of an iron-doped magnesium aluminosilicate melt

Optical and electron microscopies are used to analyze the mechanism and kinetics of internal reduction of an Fe2+-doped magnesium aluminosilicate melt. Melt samples are heated to temperatures in the range of 1300degrees-1400degreesC under a flowing gas mixture of CO/CO2, which corresponds to a p(O2), range of 1 x 10(-13)-4 x 10(-13) atm. The melt experiences an internal reaction in which a dispersion of nanometer-scale iron-metal precipitates forms at an internal interface. The metal precipitates show no signs of coarsening within the samples; however, the crystals at the surface (which formed in the initial part of the reaction) do grow via vapor phase transport. The overall reaction is characterized by parabolic kinetics, which is indicative of chemical diffusion being the rate-limiting step. The diffusion of network-modifier divalent cations-particularly Mg2+ cations-is demonstrated to be the rate-limiting factor, and its diffusion coefficient is calculated to be similar to1 x 10(-6) cm(2)/s within the temperature range of the experiments.