In Co-Mg olivines, cations are distributed over two non-equivalent sites of octahedral coordination, M1 and M2. The kinetics of Co-Mg intersite exchange between M1 and M2 sites for (Co0.6Mg0.4)(2)SiO4 were studied from 500 to 680 degrees C by in situ optical spectroscopy using temperature-jump relaxation. The relaxation times were obtained from modeling relaxation curves with a single exponential behaviour. The apparent activation energy of about 193 +/- 9 kJ/mol for cation exchange between M1 and M2 sites in (Co0.6Mg0.4)(2)SiO4 was determined from the Arrhenius plots of the inverse relaxation times. (c) 2007 Elsevier B.V. All rights reserved.