THERMODYNAMIC PROPERTIES OF STOICHIOMETRIC STAUROLITE H2FE4AL18SI8O48AND H6FE2AL18SI8O48

Recent studies have shown that all natural and synthetic staurolite cr ystals are nonstoichiometric. In an Fe-Al-Si-O-H (FASH) system, the pr edominant hypothetical stoichiometric end-members are H2Fe4Al18Si8O48 (2H) and H6Fe2Al18Si8O48 (6H). End-member molar volume (V), specific h eat (C-p), and calorimetric entropy (S) are presented on the basis of previous crystallographic and calorimetric work. Compositional data, i ncluding 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. U sing (1) the above values for V and C-p, (2) thermodynamic data for ad ditional phases in the prograde staurolite + quartz reaction, (3) expe rimental data, including new data on the reaction of staurolite + quar tz to sillimanite + almandine, (4) mole fraction models and compositio n estimates, and (5) natural data from Black Mountain, New Hampshire, and Maine, we have determined Delta H-f (-23961.25 +/- 20 kJ/mol, -239 92.86 +/- 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 stau rolite as a function of P, T, and coexisting phases. These values of S are substantially lower than corrected calorimetric values. These com bined thermodynamic data for stoichiometric staurolite reproduce most of the experimental reversals, and show that synthetic and natural sta urolite near staurolite + quartz breakdown conditions vary from H = 2. 8 to >4.6 apfu with increasing P. Within the staurolite + quartz stabi lity field, the composition of staurolite coexisting with almandine ca n be contoured for H content, which increases with P. Likewise, stauro lite coexisting with aluminum silicate can be contoured for H content, which increases with P and decreases with T. For excess-SiO2 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 occ urs with aluminum silicate and quartz. Those experimental data points that do not fit the calculated curve result mainly from use of H-poor staurolite as starting material under conditions at which H-rich staur olite is stable. Discrepancies between individual experimental data po ints and the curve calculated from the thermodynamic data are thus mai nly the result of problems with experimental studies. These results pr ovide an improved data set for calculation of staurolite equilibria.