Abstract
The reaction 2 talc + 2 forsterite = 5 enstatite + 2 H2O has been investigated between 1 and 3.5 GPa. Here, we report the results from 2 sets of experiments: (1) “conventional”. static quenching experiments performed several years ago in non-end-loaded and end-loaded piston-cylinder apparatus, and (2) a new set of experiments conducted in a novel type of apparatus, a “rocking” piston cylinder. This new technique was employed for the reinvestigation to overcome segregation problems encountered during static experiments. The run-products of the latter experiments are homogenous throughout the entire capsule. The reaction was bracketed to 680.690 °C at 1 GPa, to 660.680 °C at 1.7 GPa, to 650.660 °C at 2.5 GPa, to 600.640 °C at 3 GPa, and to 550.600 °C at 3.5 GPa. The location of the reaction in P-T-space is consistent with low-pressure experimental data of Chernosky et al. (1985), and is in reasonable agreement with the “conventional” high-pressure experiments of this study. The new experimental results were used to refine the existing thermodynamic properties of talc within the framework of the Holland and Powell (1998) database. The standard state enthalpy and entropy amount to Hof298= 5912.358 kJ/mol and So298 = 239.1 J/(K·mol). The refined standard entropy is about 9% lower than the entropy obtained calorimetrically. When the clinoenstatite-orthoenstatite transition is taken into account, So 298 of talc improves by ~6 J/K. We assume that a further increase of the standard state entropy of talc can be achieved by varying the activity of H2O in the fluid as a function of pressure and temperature.
© 2015 by Walter de Gruyter Berlin/Boston
