Thermal pressure in the laser-heated diamond-anvil cell; an X-ray diffraction study

The P changes induced by laser heating on samples compressed in a diamond-anvil cell have been studied by comparing experimentally observed phase transitions in forsterite and wadsleyite with well-constrained phase diagrams and equations of state. The results show a clear increase in P in the laser hot spot with respect to the nominal P measured from the ruby fluorescence at room T. At 2200 + or - 100 K, for example, wadsleyite has been produced from forsterite at a nominal P of 11 GPa, which is 4 GPa lower than the reported transition P. Also, the measured high-P/high-T molar volumes of forsterite and wadsleyite appear much smaller than those calculated from available thermoelastic data. This P increase may reconcile the conflicting experimental determinations of the coesite-stishovite transition made with multi-anvil press and diamond-anvil cell. The P increase induced by laser heating is a function of the thermoelastic coefficients alpha (thermal expansion) and K (sub T) (bulk modulus) of the sample. The need for an internal P standard in such work is stressed.