The ternary system K 2SO 4MgSO 4CaSO 4

Abstract

Melting and subsolidus relations in the system K 2SO 4MgSO 4CaSO 4 were studied using heating-cooling curves, differential thermal analysis, optics, X-ray diffraction at room and high temperatures and by quenching techniques. Previous investigators were unable to study the binary MgSO 4CaSO 4 system and the adjacent area in the ternary system because of the decomposition of MgSO 4 and CaSO 4 at high temperatures. This problem was partly overcome by a novel sealed-tube quenching method, by hydrothermal synthesis, and by long-time heating in the solidus. As a result of this study, we found: (1) a new compound, CaSO 4·3MgSO 4 (m.p. 1201°C) with a field extending into the ternary system; (2) a high temperature form of MgSO 4 with a sluggishly reversible inversion. An X-ray diffraction pattern for this polymorphic form is given; (3) the inversion of β-CaSO 4 (anhydrite) to α-CaSO 4 at 1195°C, in agreement with grahmann; (1) (4) the melting point of MgSO 4 is 1136°C and that of CaSO 4 is 1462°C (using sealed tube methods to prevent decomposition of the sulphates); (5) calcium langbeinite (K 2SO 4·2CaSO 4) is the only compound in the K 2SO 4CaSO 4 binary system. This resolved discrepancies in the results of previous investigators; (6) a continuous solid solution series between congruently melting K 2SOP 4·2MgSO 4 (langbeinite) and incongruently melting K 2SO 4·2CaSO 4 (calcium langbeinite); (7) the liquidus in the ternary system consists of primary phase fields of K 2SO 4, MgSO 4, CaSO 4, langbeinite-calcium langbeinite solid solution, and CaSO 4·3MgSO 4. The CaSO 4 field extends over a large portion of the system. Previously reported fields for the compounds (K 2SO 4·MgSO 4·nCaSO 4), K 2SO 4·3CaSO 4 and K 2SO 4·CaSO 4 were not found; (8) a minimum in the ternary system at: 740°C, 25% MgSO 4, 6% CaSO 4, 69% K 2SO 4; and ternary eutectics at 882°C, 49% MgSO 4, 19% CaSO 4, 32% K 2SO 4; and 880°, 67·5% MgSO 4, 5% CaSO 4, 27·5% K 2SO 4.