The dehydration reactions anthophyllite = enstatite + quartz + fluid (1) and talc = anthophyllite + quartz + fluid (2) have been bracketed, with reversed experiments and PН2O = Ptotal. Smooth curves drawn through the bracketing data for reactions (1) and (2) pass through the coordinates 2 kbar, 770°; 1.5 kbar, 755°; 1 kbar, 730°; 0.5 kbar, 680° and 3 kbar, 738°; 2 kbar, 711°; 1.5 kbar, 697°; 1 kbar, 678°; 0.5 kbar, 655°C, respectively. Synthetic quartz, anthophyllite, enstatite, and talc were used as starting materials. Reversibility was established by determining the relative growth or diminution (as judged by examining relative intensities of X-ray reflections) of the high- with respect to the low-temperature assemblages. Both curves are consistent with the solubility data of Hemley et al. (1977b) at 1 kbar but are not entirely compatible with the hydrothermal data of Greenwood (1963). Our experimental data indicate that the phase boundaries for reactions (1) and (2) intersect the phase boundary for the reaction (3) talc = 3 enstatite + quartz + H2O at low pressure (below 200 bars) and are, therefore, more consistent with the anthophyllite phase diagram proposed by Hemley et al. than with the diagram proposed by Greenwood.

Gibbs energy difference functions calculated using bracketing data for reactions (1), (2), and (4) talc + forsterite = 5 enstatite + H2O permit simultaneous evaluation of the free energies of formation of talc (−5513.69±4.69 kJ mol−1), enstatite (−1456.38±1.93 kJ mol−1), and anthophyllite (−11,323.26±5.35 kJ mol−1) from the elements at 298K and 1 bar.

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