Subsolidus Phase Relations in the Systems BaCO3-SrCO3, SrCO3-CaCO3, and BaCO3-CaCO3

Abstract

The solid solubility and order-disorder relations in the systems , and were investigated at temperatures of from 400° to 1,000° C. with sufficient pressure to prevent decomposition of any carbonates involved. A high-temperature X-ray furnace and cold-seal bombs were used for most of the investigation. The known orthorhombic-rhombohedral phase changes in both were observed. Reactions are reversible and abrupt at a temperature of 806° C. in . The rhombohe-dral form becomes stable at lower temperatures in the binary system, which shows a complete series of solid solutions with a minimum temperature of transition at 745° C. near the composition . In the system there is a two-phase region consisting of orthorhombic and rhombohedral solid solutions. The maximum amount of calcium taken up by orthorhombic (Sr, is about 47 mole per cent at 550° C. The transition to the anion disordered form of was obtained by extrapolation at 955° In the system complete solid solubility exists above 850° C. The solubility of in is 3.5 mole per cent at 400° C. and increases to approximately 5.0 mole per cent at 700° C. The solubility of in is very low at temperatures below 600° C. A binary compound barytocalcite, undergoes a transformation above 520° C. to a disordered calcite form.