Abstract
Reversals for the reaction 2 annite+3 quartz=2 sanidine+3 fayalite+2 H2O have been experimentally determined in cold-seal pressure vessels at pressures of 2, 3, 4 and 5 kbar, limiting annite +quartz stability towards higher temperatures. The equilibrium passes through the temperature intervals 500–540° C (2 kbar), 550–570° C (3 kbar), 570–590° C (4 kbar) and 590–610° C (5 kbar). Starting materials for most experiments were mixtures of synthetic annite +fayalite+sanidine+quartz and in some runs annite+quartz alone. Microprobe analyses of the reacted mixtures showed that the annites deviate slightly from their ideal Si/Al ratio (Si per formula unit ranges between 2.85 and 2.92, AlVI between 0.06 and 0.15). As determined by Mössbauer spectroscopy, the Fe3+ content of annite in the assemblage annite+fayalite +sanidine+quartz is around 5–7%. The experimental data were used to extract the thermodynamic standard state enthalpy and entropy of annite as follows: H 0 f, Ann =−5125.896±8.319 [kJ/mol] and S 0 Ann=432.62±8.89 [J/mol/K] (consistent with the Holland and Powell 1990 data set), and H 0 f,Ann =−5130.971±7.939 [kJ/mol] and S 0 Ann=424.02±8.39 [J/mol/K] (consistent with the TWEEQ data base, Berman 1991). The preceeding values are close to the standard state properties derived from hydrogen sensor data of the redox reaction annite=sanidine+magnetite+H 2 (Dachs 1994). The experimental half-reversal of Eugster and Wones (1962) on the annite +quartz breakdown reaction could not be reproduced experimentally (formation of annite from sanidine+fayalite+quartz at 540° C/1.035 kbar/magnetite-iron buffer) and probable reasons for this discrepancy remain unclear. The extracted thermodynamic standard state properties of annite were used to calculate annite and annite+quartz stabilities for pressures between 2 and 5 kbar.
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Received: 20 December 1994/Accepted: 29 May 1995
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Dachs, E., Benisek, A. The stability of annite+quartz: reversed experimental data for the reaction 2 annite+3 quartz=2 sanidine+3 fayalite +2 H2O. Contrib Mineral Petrol 121, 380–387 (1995). https://doi.org/10.1007/s004100050103
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DOI: https://doi.org/10.1007/s004100050103