The crystal structure of CoMg olivine has been investigated by in situ neutron and synchrotron X-ray powder diffraction at temperatures between 23°C and 1050°C. The olivine sample was prepared at 1450°C by solid-state reaction. During quench, the structure acquired a partially anti-ordered Co,Mg site occupancy, XCoM1>XCoM2⁠, which upon ensuing heating persisted up to 550°C due to slow exchange kinetics. Above 600°C, internal equilibrium was readily obtained. The temperature variation of the Co,Mg site occupancies was modelled by RT ln(KD) = −12080(±620) = 1.60(±0.58) T [J mol−1]. The pronounced negative exchange enthalpy strongly favours the anti-ordered state, as does the positive exchange entropy so that CoMg olivine will remain in this state up to the melting point.

The anisotropy of thermal expansion was analysed using an isothermal equation-of-state (EoS) (Kumar, 1995) coupled with the thermal Mie-Grüneisen EoS. This coupling allowed for relating the anisotropic expansivity to an anisotropic thermal pressure. Both properties are linked to the temperature-dependent rotations of the M1 and M2 octahedra in the [001] plane. The rotations increase from Mg via CoMg to Fe olivine. The Kumar and Birch-Murnaghan EoS’s gave virtually indistinguishable results upon modelling thermal expansion.

The increase of the mean bond distances <M1-O> and <M2-O> with temperature changes at ~550°C when the redistribution of Co and Mg begins. In terms of a distortion parameter defined as a normalized standard deviation, the bond length distortion of the M2 octahedron is larger than that of M1. Conversely, M1 is more distorted than M2, if the distortion is defined by the variation of the octahedral faces as well as by the variation of the O-M-O angles subtended at the M sites. The former distortion relates to the exchange entropy.

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