Three crystalline phases are known in the CaSO₄-H₂O system: gypsum (CaSO₄·2H₂O), bassanite (CaSO₄·0.5H₂O), and anhydrite (CaSO₄). Gypsum and anhydrite often form massive evaporite deposits [1] but gypsum can also appear as large crystals in caves [2,3]. But, gypsum and bassanite are also economically important materials (e.g. construction industry). On the other hand, the precipitation of calcium sulfate in mining or desalination plants is often an unwanted by-product [4,5], leading to costly production reduction. Despite the important role, marked inconsistencies still exist between solubility measurement data, thermodynamic predictions of stability and the experimental data obtained from precipitation experiments in the laboratory.

A series of experiments were conducted in which precipitation was carried out by chemical reaction in a wide temperature range, 40-120 ^oC, at three different salinities (0.8, 2.8 and 4.3 M NaCl). The phase stability was tested by varying the reaction time of the precipitate with the mother solution, from 2 min to 10 months.

Fig. 1. Experimental results obtained by chemical reaction, as a function of salinity, temperature and time during which the precipitate was in contact with the solution.

The two main observations are: (I) Within its predicted stability field no primary anhydrite precipitation occurred from aqueous NaCl solutions (Fig. 1). (II) Bassanite occurs as an important metastable phase and the bassanite stability increases significantly with increasing salinity. At 4.3 M NaCl bassanite remains stable for more than two months at 80 ºC (Fig. 1). Previous studies [6,7] on growth dynamics of gypsum and anhydrite, show that anhydrite presents much slower kinetics than gypsum. Additionally, the significantly higher surface free energy of anhydrite leads to much longer induction times for nucleation. Both observations could be responsible for the observed metastable development of gypsum crystals for temperatures corresponding to the stability field of anhydrite. In a previous study [8] we have detected the presence of bassanite as a precursor phase during gypsum precipitation. The present observations show us that with increasing temperature and salinity the stability of this metastable phase is increased but at present the precise mechanism behind this stabilization is still unknown. 

[1] Melvin (1991) Evaporites, Petroleum and Mineral Resources, 556;

[2] García-Ruiz et al., (2007) Geology, 35:327–330;

[3] García-Ruiz et al., (2008) McGraw-Hill Yearbook of Science and Technology, 154–156;

[4] Kagawa et al., (1981) Inorg. Nucl. Chem. 43:917–920;

[5] Stumm and Morgan (1995) Aquatic Chemistry, Wiley;

[6] Van Driessche et al., (2010) Cryst. Growth Des. 10:3909–3916;

[7] Morales et al., (2012) Cryst. Growth Des. 12; 414–421;

[8] Van Driessche et al., (2012) Science 336:69–72

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