Phase Relations in the System Cr-Fe-S

Abstract

The systems Cr-S and Cr-Fe-S are being systematically investigated by silica tube experi- ments in which vapor is an inherent phase, as well as by collapsible tube experiments under high confining pressures. At 700 °C a hexagonal NiAs structure type solid solution, isostructural with high-temperature Fe,-xS solid solution, exists in the CrS-CrO.86S composition range, whereas a rhombohedral structure type solid solution spans the Cr.S.-Cr •. lS. composition region. Measure- ments of the delo.) reflection of CrO.9.S-CrO.86S solid solution members show increasing 2(J values with decreasing chromium content similar to the relations observed in the FeS-Fe,-xS solid solution series. The crystal structure of Cr,S4 is not similar to those of either smythite or greigite (Fe.S.); its powder diffraction pattern shows strong structural similarities to that of Cr2.1S., explaining the extensive solid solution between CraS4 and Cr2.1Sa. At high temperatures complete solid solution exists between the Fe,-xS and Cr,-xS end members. This ternary solid solution manifests itself as a broad field (varying in width from 3.5 to 5.5 wt. % sulfur) spanning the system, and is thus not narrow as reported in the literature. This monosulfide solid solution is intersected at about 700°C by a solvus which results in two coexisting extensive solid solutions. At lower temperatures (650° ± 50°C) an invariant reaction takes place whereby tie lines between the two monosulfide solid solutions are replaced by tie lines permitting coexistence of daubreelite (Cr.FeS4) and metallic aFeCrss. Daubreelite occurs in two crystallographic modifications; low-temperature cubic (spinel), and high-temperature hexagonal. 1.