Order-Disorder Studies in Silicates by Means of Gamma-Ray Resonant Absorption of 57Fe
The occupancy of Fe+2 at the nonequivalent octahedral sites in pyroxenes (chain silicates) has been determined from 57Fe resonant absorption spectra. The accuracy of the occupancy numbers is better than 0.02. The nuclear quadrupole splittings and isomer shifts are not affected by the degree of Fe+2 disorder. Therefore, the intensities of the doublets yield intrinsic values, independent of short-range order effects. Employing simple thermodynamical considerations, conclusions can be made on Fe+2 equilibrium distribution constants, free Gibbs energy differences for the Fe+2 exchange reactions, activation energies for ordering and disordering, etc.
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