Fluorite solubility equilibria in selected geothermal waters

DARREL KIRK NORDSTROM and EVERETT A. JENNE
Geochimica et Cosmochimica Acta, 1977 41:175-188

Abstract:
Calculation of chemical equilibria in 351 hot springs and surface waters from selected geothermal areas in the western United States indicate that the soll3bility of the mineral fluorite, CaF₂, provides an equilibrium control on dissolved fluoride activity. Waters that are undersaturated have undergone dilution by non-thermal waters as shown by decreased conductivity and temperature values, and only 2% of the samples are supersaturated by more than the expected error. Calculations also demonstrate that simultaneous chemical equilibria between the thermal waters and calcite as well as fluorite minerals exist under a variety of conditions.
Testing for fluorite solubility required a critical review of the thermdynamic data for fluorite. By applying multiple regression of a mathematical model to selected published data we have obtained revised estimates of the pK (10.96), δG°_ƒ, (-280.08 kcal/mole), δH°_ƒ (-292.59 kcal/mole). S° (16.39 cal/deg/mole) and C°_P (16.16 cal/deg/mole) for CaF₂ at 25°C and 1 atm. Association constants and reaction enthalpies for fluoride complexes with boron, calcium and iron are included in this review. The excellent agreement between the computer-based activity products and the revised pK suggests that the chemistry of geothermal waters may also be a guide to evaluating mineral solubility data where major discrepancies are evident.

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