GYPSUM SOLUBILITY AT SUBZERO TEMPERATURES

Gypsum is an important mineral in many terrestrial and marine environm ents. Despite numerous studies that estimated the gypsum solubility pr oduct at temperatures greater than or equal to 0 degrees C, there are no estimates at temperatures <0 degrees C. The objectives of this stud y were to estimate, for the temperature range from -25 to 25 degrees C : (i) the gypsum solubility product and (ii) the Fitter-equation param eters needed to define Ca-SO4 interactions to high solute concentratio ns (ionic strengths >10 M). The gypsum solubility product was extrapol ated to subzero temperatures by integrating the van't Hoff equation us ing a published equation for the standard enthalpy of reaction for gyp sum. A least-squares regression technique was developed to estimate ei ght temperature-dependent Fitter-equation parameters that describe ion interactions among Ca, SO4, Na, and CI, The calculated Fitter-equatio n parameters were in reasonable agreement with published values. Calcu lated gypsum activity products were independent of ionic strength to c oncentrations >17 M, The resulting Fitter-equation parameters coupled with gypsum solubility data fit the theoretical gypsum solubility prod uct equation reasonably well. The calculated eutectic concentration an d temperature for a pure gypsum solution are 0.01293 M and -0.06 degre es C. Gypsum is a common mineral in many cold region soils, According to the current paradigm for seawater freezing, sulfate should exclusiv ely precipitate as mirabilite; however, there is evidence that gypsum ran also precipitate during seawater freezing, The parameters develope d in this study will allow, for the first time, the theoretical explor ation of mechanisms such as freezing on gypsum geochemistry.