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Values of Chemical Thermodynamic Properties," Part I, Natl. Bur. Standards Tech. Note 270-1 (1965).

The change in solubility in water with time of a and ß calcium sulfate hemihydrate and of /3-soluble anhydrite and "insoluble" anhydrite were determined over the temperature range of 35°to 110°C. The solubility of the hemihydrates goes through a maximum with time, which is related to modifications in the calcium sulfate phase present. The a-hemihydrate is less soluble but more stable than the ß form. The stability of a and ß hemihydrate increase as the transition temperature, 99°C. for gypsum-a-hemihydrate and 103°C. for gypsum-/3-hemihydrate, is approached. At temperatures above 45°C. the solute content in contact with "insoluble" anhydrite, prepared by igniting gypsum at 890°C., surprisingly increased with time well beyond the values previously reported for anhydrite solubility.Three FORMS of calcium sulfate have long been recognized, CaS04-2H20 (gypsum), CaS04-M2H2O (hemihydrate), and CaS04 (anhydrite). Various modifications of these forms have also been postulated, but after a careful study and review of previous work, Kelley, Southard and Anderson (9) in 1941 concluded that the only unique and reproducible forms were: CaS0v2H20, a-CaS04• V2H2O, ß-CaSOv V2H2O, a-and /3-soluble anhydrites, and insoluble anhydrite. The -hemihydrate is prepared, for example, by dehydration of gypsum in an atmosphere of saturated steam. The /3-hemihydrate is formed by dehydration of gypsum under a lower water vapor pressure, and it shows a higher solubility than the -hemihydrate. a-and ß-anhydrites are prepared by further dehydration of a-and /3-hemihydrates, respectively; both rehydrate

mental results. Unfortunately, there is little information about the kinetics of such a reaction.Neither a pair of complexing agents nor one precipitant is superior throughout the rare earth series. One or more selective processes are operating. Somehow one rare earth cation is preferentially released in solution while the other tends to remain complexed. How this selectivity is achieved has not been deduced.