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

The thermodynamic basis for describing the variation with pressure of the s tate of cation order within a single phase is presented, and existing exper imental data on the volume change associated with ordering are reviewed. To gether, these show that the pressure effect on cation order can, within exp erimental uncertainties, be subsumed into a term Delta V-dis = V-disordered - V-ordered, the difference in volumes between the fully disordered and th e fully ordered states. Direct measurements of Delta V-dis show that it rar ely exceeds 1% of the molar volume at room conditions, and that the applica tion of high pressure at constant temperature leads to a reduction in the m agnitude of Delta V-dis. Two possible experimental methodologies for determining the variation of ca tion order with pressure are reviewed. The indirect method is to determine the variation with pressure and temperature of the volumes of a suite of sa mples of different states of order, and thereby obtain Delta V-dis as a fun ction of P and T. The direct method is one of equilibration of samples at v arious pressures and the subsequent characterisation of the resulting state of order to obtain the variation with pressure. Because the effect of temp erature changes on the state of cation order are far greater than those imp osed by pressure changes, it is concluded that current experimental uncerta inties preclude this direct technique from being usefully employed to deter mine the pressure dependence of cation ordering.