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Fluorite solubility equilibria in selected geothermal waters

January 1, 1977

Calculation of chemical equilibria in 351 hot springs and surface waters from selected geothermal areas in the western United States indicate that the solubility 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 thermodynamic 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), ΔGof (−280.08 kcal/mole), ΔHof (−292.59 kcal/mole), S° (16.39 cal/deg/mole) and CoP (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.




Publication Year 1977
Title Fluorite solubility equilibria in selected geothermal waters
DOI 10.1016/0016-7037(77)90224-1
Authors D. Kirk Nordstrom, E. A. Jenne
Publication Type Article
Publication Subtype Journal Article
Series Title Geochimica et Cosmochimica Acta
Index ID 70010852
Record Source USGS Publications Warehouse
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