Octahedral Cation Exchange in (Co0.21Mg0.79)(2)SiO4 Olivine at High Temperatures: Kinetics, Point Defect Chemistry, and Cation Diffusion

Abstract

The potential applications of transition metal-containing olivines, (M,Mg)(2)SiO4, in environmental sustainability and renewable energy rely on a better understanding of their internal structure, defect chemistry, and sublattice processes at high temperatures. In the olivine crystal structure, divalent cations occupy two nonequivalent octahedral sites. The kinetics of octahedral cation exchange between the two sites in a (Co0.21Mg0.79)(2)SiO4 single crystal has been studied from 500 to 700 degrees C by means of time-resolved optical relaxation spectroscopy upon rapid temperature-jumps. Our experiments show that the cation distribution in the two octahedral sites changes toward a random distribution with increasing temperature and that the exchange kinetics is strongly temperature dependent. Modeling of experimental relaxation data using a kinetic equation of cation exchange yields relaxation times of about 12 300 s at 500 degrees C and about 6 s at 700 degrees C, respectively, and an activation energy of about 230 +/- 12 kJ/mol for the cation exchange reaction in the (Co0.21Mg0.79)(2)SiO4 olivine. Calculations of vacancy concentrations based on a defect model and impurity levels in the (Co0.21Mg0.79)(2)SiO4 single crystal have been used to interpret the experimentally observed dependence on oxygen activity. We developed as well a formula to correlate experimental relaxation times to the cation diffusion coefficient along the b-axis in olivines. Such a relation allows one to estimate Mg self-diffusion coefficients D-b(Mg) as well as Co-Mg interdiffusion coefficients D-b along the b-axis, especially at low temperatures, for example, D-b(Mg) = 4.78 x 10(-23) m(2)/s and D-b = 9.33 x 10(-23) m(2)/s at 600 degrees C. Cation interdiffusion coefficients from the extrapolation of our diffusion data to high temperatures are in agreement with available literature data from Co-Mg interdiffusion experiments.