The system Fe–Nb–S and its geological implications

Abstract

Phase relations within the system Fe–Nb–S were studied experimentally by the technique of evacuated silica glass tubes in the temperature interval 400–1000°C. The dominant field in the system is a broad solid-solution FexNbyS2 (0 < x < 0.5, 1.0 < y < 1.4) that includes the composition of the mineral edgarite. Also, it forms stable assemblages with pyrrhotite and pyrite. The iron sulfides dissolve limited amounts of Nb: pyrrhotite dissolves up to 1.13 at.% Nb, and pyrite, only 0.02 at.% Nb. We refined the crystal structure of five samples of the FexNbyS2 solid solution, and established the relation between the unit-cell volume and Fe/(Fe + Nb). The occurrence of edgarite can be expected in associations with other sulfides crystallizing under extremely low fugacity of oxygen.