The effect of pressure on cation ordering in minerals: Problems and perspectives

The thermodynamic basis for describing the variation with pressure of the state of cation order within a single phase is presented, and existing experimental data on the volume change associated with ordering are reviewed. Together, these show that the pressure effect on cation order can, within experimental uncertainties, be subsumed into a term Delta V-dis = V-disordered - V-ordered, the difference in volumes between the fully disordered and the fully ordered states. Direct measurements of Delta V-dis show that it rarely exceeds 1% of the molar volume at room conditions, and that the application of high pressure at constant temperature leads to a reduction in the magnitude of Delta V-dis. Two possible experimental methodologies for determining the variation of cation order with pressure are reviewed. The indirect method is to determine the variation with pressure and temperature of the volumes of a suite of samples of different states of order, and thereby obtain Delta V-dis as a function of P and T. The direct method is one of equilibration of samples at various pressures and the subsequent characterisation of the resulting state of order to obtain the variation with pressure. Because the effect of temperature changes on the state of cation order are far greater than those imposed by pressure changes, it is concluded that current experimental uncertainties preclude this direct technique from being usefully employed to determine the pressure dependence of cation ordering.