Thermodynamic properties of tremolite: A correction and some comments

Microprobe analysis of tremolite from "St. Gotthard," Switzerland, shows th at it is close to end member in composition (Ca = 1.97 apfu), and not hyper calcic (Ca = 2.16 apfu), as a previous wet-chemical analysis suggested. The latter formula, and a corresponding thermodynamic activity (0.67), were us ed in the experimental database for the extraction of an optimal set of the rmodynamic properties for tremolite (Chernosky et al. 1998). We derive a re vised enthalpy of formation of tremolite (-12307.9 kJ/mol) based on bracket ing experiments for the breakdown reactions of tremolite and tremolite + fo rsterite, and the assumption of time-averaged effective equilibrium composi tions of phases in the experimental charges. Mg-cummingtonite substitution in tremolite is accommodated during free-energy minimization calculations b y allowing the stoichiometry of the standard-state reaction to vary with th e changing bulk composition of the amphibole. The breakdown temperature of tremolite is lowered with increasing cummingtonite component. Our procedure also allows continuous variation in the equilibrium compositions and activ ities of pyroxene and amphibole phase-components with P and T.