Recent contributions to the crystal chemistry of staurolite have been aided by Miissbauer studies on 35 natural and synthetic samples and structure determinations on 42 single crystals. The combination of these data with chemical studies enables us to understand better the crystal chemistry of this mineral relative to its use as a petrogenetic indicator. In reduced rocks about 3.5% of Fe is Fe3+, and in oxidized rocks about 7% is Fe3+. It is now possible to recast a staurolite chemical analysis in terms of site occupancies using reasonable guidelines and intersite partitioning. Whereas the formula for hypothetical stoichiometric end-member iron staurolite is H2Fe4Al18Si8O48, chemical end-member formulas such as H3Fe4.35Al17.90Si7.65O48 are useful for retrieving thermodynamic data from experimental studies on simple systems. Other formulas may be written to account for appropriate amounts of the R2+H−2 substitution. In the absence of analyses for H, stoichiometry may be estimated from chemical analyses by assuming Si + Al − ⅓ Li + ⅔ Ti + Fe3+ = 25.55 ions pfu. In normal staurolite Li may be assumed to be 0.2. H may then be estimated by subtracting total cation charge from 96. H estimates for any group of related natural staurolite samples are normally a function of mineral assemblage. The preferred thermodynamic mole fraction (activity) model for phase equilibrium calculations takes into account dilution on all cation sites except H. Applied to staurolite-chloritoid pairs, this mole fraction model does not provide an explanation for the variable Kd values between these two minerals. Knowledge of the ferric content, formula, and mole fraction model of staurolite enables meaningful retrieval and use of thermochemical quantities from experimental studies.

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