Order-Disorder Studies in Silicates by Means of Gamma-Ray Resonant Absorption of ⁵⁷Fe

Abstract

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.