Hydrothermal experiments have been conducted at 2000 bars in order to determine the composition of coexisting biotite and pyrrhotite. The sulfide-silicate compositional pairs were bracketed by using three layers within a single capsule: (1) pyrite + Fe-rich biotite in the top, (2) sanidine + magnetite in the middle, and (3) troilite + Mg-rich biotite in the bottom. At the conclusion of an experiment, the biotite in the top was found to have become more Mg-rich and the biotite in the bottom more Fe-rich, while a pyrrhotite of apparently homogeneous composition was found throughout the capsule.

Five equilibrium determinations at 700°C have been located:

(N)Po = 0.928, [Fe/(Fe + Mg)]B1 = 0.496 
0.936 0.530 
0.945 0.541 
0.947 0.581 
0.950 0.585 
(N)Po = 0.928, [Fe/(Fe + Mg)]B1 = 0.496 
0.936 0.530 
0.945 0.541 
0.947 0.581 
0.950 0.585 
and demonstrate that biotites become more Mg-enriched with increasingly sulfur-rich compositions of coexisting pyrrhotite.

The results can be applied to natural systems in which a pyrrhotite-biotite-magnetite-K-feldspar assemblage is present. Since sulfides are much more susceptible than Fe-Mg silicates to re-equilibration with decreasing temperature, silicate compositions can potentially be used as a measure of fS2 and fO2 during ore formation and metamorphism. This knowledge permits further calculation of chemical species in the vapor present during petrogenesis.

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