ENVIRONMENTAL SIGNIFICANCE OF GYPSUM COATING FLUORITE ON PYRITIC MINE DUMPS

In August 1990 one of us (DMB), on a visit to the old metal mines at Guadalcazar, San Luis Potosi, Mexico, noted a coating of soft white gypsum on some of the purple and green fluorite from the dumps. This observation suggested that the gypsum coating might have been produced by the action of acid sulfate waters (generated by pyrite weathering and decay) on the fluorite. The implied reaction is CaF2 (Fluorite) + H2SO4 + 2H2O=CaSO4*2H2O (Gypsum) + 2HF. Note that the by-product of this reaction is HF, hydrofluoric acid, widely known for its toxicity (it can cause extremely painful burns). In fact, this reaction is the low-temperature analog to how hydrofluoric acid is produced commercially, by the action of sulfuric acid on fluorite in heated steel kilns.

In an attempt to discover how common and environmentally significant this reaction might be in nature, the first author (JWS), on a June 2000 research visit to southwestern Colorado, likewise discovered gypsum coating fluorite at the old Wagon Wheel Gap fluorite mine, for which pyrite is a major gangue mineral. The gypsum from both occurrences has been verified optically and by powder x-ray diffraction. Inasmuch as both fluorite and pyrite are common in a wide variety of hydrothermal mineral deposits, pyrite weathering in mine dumps or tailings could be generating hydrofluoric acid in any deposit that also contains fluorite. Work in progress is designed to measure the kinetics and potential environmental significance of this reaction. We would obviously be interested in hearing of other occurrences of gypsum coating fluorite on mine dumps.