Abstract

Equilibria between Fe-Mg amphiboles and pyrrhotites have been determined in the presence of magnetite and quartz at 2 kbar and 650°, 675°, 700°, 725°C. The assemblages produced may be expressed as the simultaneous equilibria:  
amphibole+O2magnetite+quartz+H2O
(1)
 
amphibole+S2pyrrhotite+quartz+H2O+O2
(2)

At 700°C, amphiboles of 29, 41, 49, and 57 mole percent Fe end-member coexist with pyrrhotites of N (2 × atomic fraction Fe) = 0.928, 0.934, 0.943, and 0.950, respectively. Compositions of coexisting amphibole-pyrrhotite pairs apparently are not seriously affected by temperature over the range investigated, although scatter of the amphibole data does not allow a rigorous analysis. Sulfur fugacity for runs was determined from pyrrhotite compositions, while fo2 was known from an experimentally determined magnetite-pyrrhotite curve. Knowledge of these two fugacities allowed calculation of fugacities of all species, including H20, assuming an H-O-S vapor, and thus reactions (1) and (2) were located in terms of fo2 and fs2.

Several models for the amphibole solid solution were used to explain the variation in composition of coexisting amphibole-pyrrhotite pairs at 700°C. The precision of measurement of both the amphibole compositions and the fugacities of volatile species does not justify other than an ideal solution model. A standard-state enthalpy of formation of (H°298°;1 atm) of Fe7Si8022(0H)2 amphibole from the elements of —2262 kcal/mole was calculated from a log Keq vs. l/T plot for reaction (1).

Application of the results is limited by the scarcity of data on natural amphibole-sulfide assemblages.

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