Solubility equilibria in the ternary systems Rb 2 SO 4 – CaSO 4 – H 2 O and Cs 2 SO 4 – CaSO 4 – H 2 O at ambient temperature, and the crystal structure of rubidium syngenite

For the ternary systems, Rb 2 SO 4 – CaSO 4 – H 2 O and Cs 2 SO 4 – CaSO 4 – H 2 O, solubility data have been determined at ambient temperatures (22 and 25 °C). In both systems, gypsum is found as the stable CaSO 4 -phase in the presence of low alkali metal sulfate concentrations. In addition, two double salts exist in the Rb-system: Rb-disalt (Rb 2 SO 4 ·2CaSO 4 ) and Rb-syngenite (Rb 2 SO 4 ·CaSO 4 ·H 2 O) in addition to gypsum and Rb 2 SO 4 . For Rb-syngenite, the crystal structure was determined by single crystal structure analysis (space group P 21/ m (11), a = 6.323 Å, b = 7.223 Å, c = 10.044 Å, β = 102.96°, Z = 2, V = 447.03 Å 3 ), which is isomorphous to potassium syngenite (K 2 SO 4 ·CaSO 4 ·H 2 O). In the Cs-system, besides gypsum and Cs 2 SO 4 , only the double salt Cs 2 SO 4 ·2CaSO 4 exists. Graphical abstract