Abstract

Experiments with natural and synthetic hexagonal FeS + hexagonal Fe (sub 1-x) S assemblages indicate that FeS remains essentially stoichiometric in composition at all temperatures of heating, whereas the composition of co-existing hexagonal Fe (sub 1-x) S varies with temperature of heating as follows: at 25, 60, 105, 125, 140 degrees C its composition is 47.85, 48.17, 48.57, 49.18, 49.67 atomic % Fe, respectively. A comparison of the compositions of natural co-existing FeS and hexagonal Fe (sub 1-x) S with the compositions of these phases determined experimentally, suggests that the majority of the natural phases have attained equilibrium compositions at earth surface temperatures.Experiments involving hexagonal Fe (sub 1-x) S, monoclinic Fe (sub 1-x) S and pyrite indicate that monoclinic Fe (sub 1-x) S forms in charges containing <47.20 + or - 0.10 atomic % Fe heated at < or =304 + or - 6 degrees C. The anomalous behavior of monoclinic Fe (sub 1-x) S and the development of the non-equilibrium assemblage hexagonal Fe (sub 1-x) S + monoclinic Fe (sub 1-x) S + pyrite + vapor in charges heated at a variety of temperatures has led to the interpretation that monoclinic Fe (sub 1-x) S is metastable with respect to hexagonal Fe (sub 1-x) S + pyrite. The compositions of co-existing hexagonal Fe (sub 1-x) S and monoclinic Fe (sub 1-x) S at < or =304 + or - 6 degrees C are judged to be 47.20 + or - 0.10 and 46.75 + or - 0.05 atomic % Fe, respectively. The development of natural pyrrhotite assemblages in terms of the experimental results is briefly discussed.

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