The system K2Mg2(SO4)3 (langbeinite)-K2Ca2(SO4)3 (calcium-langbeinite)

The join between the compositions K₂Mg₂(SO₄)₃ and K₂Ca₂(SO₄)₃ was studied by means of high-temperature equilibrium quenching techniques and by means of a heating stage mounted on an X-ray diffractometer. Complete solid solution exists in the system, but at 25°C members of the solid solution series are isometric only in the composition range 0–73·5 wt. per cent K₂Ca₂(SO₄)₃. At compositions richer in K₂Ca₂(SO₄)₃ than 73·5 wt. per cent, members of the series are optically biaxial. At higher temperatures members of the solid solution series are isometric at successively more calcium-rich compositions and pure K₂Ca₂(SO₄)₃ is isometric above about 200 ± 2°C. The system is not binary, as mixtures richer in K₂Ca₂(SO₄)₃ than 42 wt. per cent decompose with the formation of liquid and CaSO₄.