Thermodynamic properties of stoichiometric staurolite H2Fe4 Al18 Si8O48 and H6Fe2Al18Si8O48

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In an Fe-Al-Si-O-H (FASH) system, the predominant hypothetical stoichiometric end-members are H2Fe4Al18Si8O48 (2H) and H6Fe2Al18Si8O48 (6H). End-member molar volume (V), specific heat (Cp), and calorimetric entropy (S) are presented. Using: 1) the above values for V and Cp, 2) thermodynamic data for additional phases in the prograde staurolite + quartz reaction, 3) experimental data, including new data on the reaction of staurolite + quartz to sillimanite + almandine, 4) mole fraction models and composition estimates, and 5) natural data from Black Mountain, New Hampshire, and Maine, ΔHf (-23 961.25 ± 20 kJ/mol, -23 992.86 ± 40 kJ/mol), S[918 ± 20 J/(mol.K), 850 ± 40 J/(mol.K)] were determined for 2H and 6H staurolite. H content in staurolite was also determined as a function of P, T, and coexisting phases. The combined thermodynamic data for stoichiometric staurolite reproduce most of the experimental reversals, and show that synthetic and natural staurolite near staurolite + quartz breakdown conditions vary from H = 2.8 to >4.6 apfu with increasing P. -from Authors

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American Mineralogist

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