The CaSO4-Na2SO4-H2O system was studied at temperatures of 25°, 40°, 55°, 70°, 85°, and 100°C. and at salinities of 0.0, 0.5, 1.0, 2.0, and 4.0 molal NaCl. The penta salt (5CaSO4·Na2SO4·3H2O) appeared at 70° C. at all salinities studied and at 85° C. at 0.0 and 0.5 molal NaCl. This salt was only obtained by starting with calcium sulfate in the gypsum form, and could not be formed from the anhydrite. Glauberite (CaSO4·Na2SO4) was found at all temperatures and all salinities except at 0.0 to 2.0 molal NaCl at 25° C. It formed only at salinities and temperatures at which thenardite was the stable sodium sulfate phase. Under these conditions (of thenardite stability), glauberite can be formed from calcium sulfate in either the gypsum or the anhydrite phase. The “labile salt” (CaSO4·2Na2SO4·2H2O) was found at 40° C. at all salinities studied and at 25° C. at 3.5 and 4.0 molal NaCl. This salt is generally considered metastable, but there was no evidence of its disappearance after as much as 3 months of reaction time. Two forms of calcium sulfate were observed, gypsum (CaSO4·2H2O) and the insoluble anhydrite (CaSO4). Only at 85° C. at salinities of 1.0 molal NaCl and above, and 100° C. was any conversion of gypsum to anhydrite observed. Sodium sulfate occurred as two phases, mirabalite (Na2SO4·10H2O) and thenardite (Na2SO4). Mirabalite, found only at 25° C. from 0.0 to 2.0 molal NaCl, changed to thenardite at 3.5 and 4.0 molal NaCl. © 1968, American Chemical Society. All rights reserved.
