A series of hydrothermal experiments at 2000 bars have been conducted in order to investigate the systematic relation between biotite composition on the join phlogopite-annite and the composition of pyrrhotite. In an attempt to bracket the sulfide-silicate compositional pairs, three layers within a single capsule were used: 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 a bracketing run, the biotite in the top was found to have become more Mg-rich and the biotite in the bottom was found to have become 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))_Bi = 0.496 0.936 0.530 0.945 0.541 0.947 0.581 0.950 0.585 The data demonstrate that biotites tend to become more Mg-enriched with increasingly sulfur-rich compositions of pyrrhotite. This study can be applied to natural systems in which a pyrrhotite-biotite-magnetite-K-spar assemblage is present. Since sulfides have been observed to re-equilibrate at lower temperatures while the composition of the Fe-Mg silicates still mirror the conditions of metamorphism, silicate compositions can potentially be used as a measure of f_S₂ and f_O₂. From this, one can attempt to calculate the properties of the vapor during the alteration of the silicates and ore genesis. / Master of Science