THE STABILITY OF TREMOLITE - NEW EXPERIMENTAL-DATA AND A THERMODYNAMIC ASSESSMENT

The equilibria: tremolite + forsterite = 2diopside + 5enstatite + H2O (1) tremolite + 3calcite + 2quartz = 5diopside + 3CO(2), + H2O (4) hav e been reversed experimentally at P-fluid = P-H2O = 0.5 kbar, 1.0 kbar , and 5.0 kbar and at P-fluid = P-H2O + P-CO2 = 5 kbar, respectively. Starting materials consisted of natural tremolite (St. Gotthard, Switz erland) and quartz (Brazil), and synthetic calcite, forsterite, diopsi de, and enstatite mixed in stoichiometric proportions. Reaction direct ion was determined by comparing XRD patterns of reactant and product a ssemblages and by examining surface features of experimental products with an SEM. Our new experimental data for Equilibrium 1 are consisten t with the natural-tremolite results of Skippen and McKinstry (1985), who used St. Gotthard tremolite, whereas the new bracket for Equilibri um 4 is approximate to 25 degrees C lower than that of Slaughter et al . (1975) who also used St. Gotthard tremolite. Comparison of our resul ts with other studies indicates that use of the St. Gotthard tremolite in place of synthetic tremolite in the starting material displaces th ese equilibria toward higher temperatures by about 25 and 5 degrees C, respectively. Tremolite stability differences reflected in these data , as well as in phase equilibrium data for nine additional equilibria involving synthetic and natural tremolite can be accounted for with a simple ideal on-site mixing model to describe tremolite compositional differences. Our analysis leads us to conclude, however, that tremolit e growth in some experiments near the equilibrium boundary occurs with respect to metastable end-member pyroxenes used in starting materials , whereas pyroxene-stable half-brackets involve growth of stable pyrox ene compositions. Thermodynamic properties for end-member tremolite, r etrieved by mathematical programming analysis of the experimental phas e equilibrium data with these assumptions, provide the most sound basi s for prediction of calcic amphibole stability relationships in natura l assemblages, as well as improved calibration of quantitative amphibo le geothermobarometers. Our success in extracting consistent thermodyn amic properties for end-member tremolite from experimental data obtain ed with both synthetic and natural tremolite, assuming the former to c ontain 10 mol% magnesiocummingtonite component (Jenkins 1987), can be taken either as support for the validity of this assumption or as an i ndication that chain multiplicity faults (Maresch et al. 1994) produce a similar degree of stabilization as this solid solution.