Thermodynamic properties of tremolite; a correction and some comments

Microprobe analysis of tremolite from “St. Gotthard,” Switzerland, shows that it is close to endmember in composition (Ca = 1.97 apfu), and not hypercalcic (Ca = 2.16 apfu), as a previous wetchemical analysis suggested. The latter formula, and a corresponding thermodynamic activity (0.67), were used...

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Bibliographic Details
Published inThe American mineralogist Vol. 85; no. 3-4; pp. 466 - 472
Main Authors Evans, Bernard W., Ghiorso, Mark S., Kuehner, Scott M.
Format Journal Article
LanguageEnglish
Published Washington Mineralogical Society of America 01.04.2000
Walter de Gruyter GmbH
Subjects
amphibole group
cell dimensions
Central Europe
chain silicates
chemical composition
clinoamphibole
crystal structure
electron probe data
Equilibrium
Europe
Mineralogy
Minerals
Saint Gotthard Switzerland
silicates
Switzerland
thermodynamic properties
Thermodynamics
tremolite
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Summary:Microprobe analysis of tremolite from “St. Gotthard,” Switzerland, shows that it is close to endmember in composition (Ca = 1.97 apfu), and not hypercalcic (Ca = 2.16 apfu), as a previous wetchemical analysis suggested. The latter formula, and a corresponding thermodynamic activity (0.67), were used in the experimental database for the extraction of an optimal set of thermodynamic properties for tremolite (Chernosky et al. 1998). We derive a revised enthalpy of formation of tremolite (-12307.9 kJ/mol) based on bracketing experiments for the breakdown reactions of tremolite and tremolite + forsterite, and the assumption of time-averaged effective equilibrium compositions of phases in the experimental charges. Mg-cummingtonite substitution in tremolite is accommodated during free-energy minimization calculations by allowing the stoichiometry of the standard-state reaction to vary with the 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 activities of pyroxene and amphibole phasecomponents with P and T.
ISSN:0003-004X
1945-3027
DOI:10.2138/am-2000-0407