Kinetics and thermodynamics of cation site-exchange reaction in olivines

Abstract

In many complex oxides cations are found to occupy structurally non-equivalent sites. The internal distribution equilibria determine the structure, as well as physical and chemical properties. Olivine is one of such complex oxides where divalent cations occupy two non-equivalent octahedral sites. The kinetics of cation exchange between non-equivalent sites in cobalt-containing (Co x Mg 1- x ) 2SiO 4 olivines have been studied by means of temperature-jump optical relaxation spectroscopy. Results derived from the modeling of the experimental absorbance relaxation curves show that the kinetics of cation site-exchange are strongly temperature- and composition-dependent with activation energies ranging from 196 to 221 kJ/mol in (Co x Mg 1- x ) 2SiO 4 mixed crystals for x between 0.6 and 0.1. The dependence of kinetics on oxygen activity supports a local defect mechanism for cation site-exchange involving vacancies. Finally, literature data reported for cation distributions in quenched samples are analyzed in respect to the temperature dependence of the cation distribution in cobalt-containing olivines yielding relevant thermodynamic parameters.