The solubility of anhydrite (CaSO 4) in NaCl-H 2O from 100 to 450°C and 1 to 1000 bars

Abstract

The solubility of anhydrite was determined at temperatures ranging from 100 to 450°C, pressures from 1 to 1000 bars, and in solutions ranging in NaCl concentration from dilute to 6 moles NaCl per kg of H 2O. Inreased pressure causes the solubility of anhydrite to become greater at all temperatures and NaCl concentrations. The effect of temperature and NaCl concentration increase is more complicated; the solubility diminishes with temperature rise to a minimum value at elevated temperatures. The location of the temperature minimum depends on NaCl concentration and is at progressively lower temperatures in solutions of increasing NaCl concentration. The solubility of anhydrite as a function of NaCl concentration rises to a maximum, the NaCl concentration at which depends on temperature and pressure. The maxima become progressively less pronounced at higher temperatures and at about 180°C the solubility continuously becomes greater at a lessening rate with increasing NaCl concentration. At about 300°C, anhydrite solubility rises linearly with increasing NaCl concentration and above 300°C the solubility rises at an increasing rate with increasing NaCl concentration. In nature, most H 2O and NaCl-H 2O solutions saturated with anhydrite would become undersaturated on migrating surfaceward and supersaturated on moving downward. An important exception to this generalization would be solutions subjected to changes above the conditions at which solubility minima occur. At such elevated temperatures and pressures, upward rising solutions would tend to become supersaturated. Highly saline solutions would be more effective in depositing anhydrite at shallow depths in response to temperature and pressure changes than would dilute saline or simple water solutions. Solutions undergoing pressure drop at nearly constant temperature, as might happen to solutions moving from regions of lithostatic pressure (zones of nearly impermeable rocks) to regions of lower pressure (fractures, permeable beds or drill holes) could precipitate anhydrite. Specialized conditions are required in general for the deep-seated precipitation of anhydrite. Therefore, the presence of anhydrite in igneous rocks or ore bodies can be used as evidence of previous existence of unusual temperature-pressure-compositional conditions. Anhydrite in sedimentary rocks could have formed in many ways, ranging from direct precipitation from sea water to the action of externally derived solutions flowing through the rooks. Some anhydrite would tend to precipitate from connate brines as sediments are subjected to temperature and pressure changes accompanying progressive burial and lithification.