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Gypsum Solubility at Subzero Temperatures

U.S. Army Cold Regions Research and Engineering Lab., 72 Lyme Rd., Hanover, NH 03755

* Corresponding author ( gmarion{at}hanover-crrel.army.mil).

ABSTRACT

Gypsum is an important mineral in many terrestrial and marine environments. Despite numerous studies that estimated the gypsum solubility product at temperatures 0°C, there are no estimates at temperatures <0°C. The objectives of this study were to estimate, for the temperature range from -25 to 25°C: (i) the gypsum solubility product and (ii) the Pitzer-equation parameters needed to define Ca-SO₄ interactions to high solute concentrations (ionic strengths >10 M). The gypsum solubility product was extrapolated to subzero temperatures by integrating the vant's Hoff equation using a published equation for the standard enthalpy of reaction for gypsum. A least-squares regression technique was developed to estimate eight temperature-dependent Pitzer-equation parameters that describe ion interactions among Ca, SO₄, Na, and Cl. The calculated Pitzer-equation parameters were in reasonable agreement with published values. Calculated gypsum activity products were independent of ionic strength to concentrations >17 M. The resulting Pitzer-equation parameters coupled with gypsum solubility data fit the theoretical gypsum solubility product equation reasonably well. The calculated eutectic concentration and temperature for a pure gypsum solution are 0.01293 M and –0.06°C. Gypsum is a common mineral in many cold region soils. According to the current paradigm for seawater freezing, sulfate should exclusively precipitate as mirabilite; however, there is evidence that gypsum can also precipitate during seawater freezing. The parameters developed in this study will allow, for the first time, the theoretical exploration of mechanisms such as freezing on gypsum geochemistry.

Received for publication January 15, 1997.