Crystal field effect on the olivine-spinel transformation

Abstract

The olivine-spinel transformation in silicates and germanates is discussed in terms of ionic radius ratio and crystal field effect and is shown to be understood in a most systematic way. The pressure for the transition to spinel is found to increase linearly with increasing ionic radius ratio r R r M , where R and M are divalent and tetravalent cations, if no contribution from crystal field effect is expected. In case of transition metal ions, the excess crystal field stabilization in spinel over olivine remarkably lowers the transition pressure to cubic spinel below the general trend. It is suggested that without the crystal field stabilization the modified spinel structure becomes more stable than the cubic normal spinel structure at high pressure when the ionic radius ratio has high values.