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Recent studies have shown that all natural and synthetic staurolite crystals are nonstoichiometric. In an Fe-Al-Si-O-H (FASH) system, the predominant hypothetical stoichiometric end-members are H 2 Fe 4 Al 18 Si 8 O 48  (2H) and H 6 Fe 2 Al 18 Si 8 O 48  (6H). End-member molar volume (V), specific heat (C P ), and calorimetric entropy (S) are presented on the basis of previous crystallographic and calorimetric work. Compositional data, including H content, are not available for any experimental staurolite. Because of the number of variables involved, it is necessary to use a trial and error approach to enthalpy (H) and entropy (S) retrieval. Using (1) the above values for V and C P , (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, we have determined ΔH f  (- 23961.25 ± 20 kJ/mol, - 23992.86 ± 40 kJ/mol), S [918 ± 20 J/(mol· K), 850 ± 40 J/(mol·K)] for 2H and 6H staurolite. We have also determined H content in staurolite as a function of P, T, and coexisting phases. These values of S are substantially lower than corrected calorimetric values. These 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 &gt;4.6 apfu with increasing P. Within the staurolite + quartz stability field, the composition of staurolite coexisting with almandine can be contoured for H content, which increases with P. Likewise, staurolite coexisting with aluminum silicate can be contoured for H content, which increases with P and decreases with T. For excess-SiO 2  rocks at any given P and T, the total range of FASH staurolite composition is between that which occurs with almandine and quartz and that which occurs with aluminum silicate and quartz. Those experimental data points that do not fit the calculated curve result mainly from use ofH-poor staurolite as starting material under conditions at which H-rich staurolite is stable. Discrepancies between individual experimental data points and the curve calculated from the thermodynamic data are thus mainly the result of problems with experimental studies. These results provide an improved data set for calculation of staurolite equilibria.

Thermodynamic properties of stoichiometric staurolite H 2 Fe 4 Al 18 Si 8 O 48 and H 6 Fe 2 Al 18 Si 8 O 48