Fluorite and pyrite occur together in a wide variety of hydrothermal mineral deposit types. Bacterially-assisted pyrite weathering (oxidation) in mine dumps and tailings generates sulfuric acid (H₂SO₄); this in turn can generate highly toxic hydrofluoric acid (HF) in any deposit that also contains fluorite. The balanced chemical reaction is

CaF₂ (fluorite) + H₂SO₄ + H₂O=CaSO₄·2H₂O (gypsum) + 2HF

This reaction is, in fact, the low temperature analog to the reaction used in the commercial production of hydrofluoric acid. Field evidence that this reaction occurs under natural conditions includes (1) the presence of gypsum coating fluorite on dumps of the silver mines near Guadalcazar, San Luis Potosi, Mexico and the Wagon Wheel Gap fluorspar mine, Colorado, (2) the presence of highly anomalous fluoride concentrations (up to 600 mg/L) in seeps draining pyritic/fluoritic tailings at the Tungstona Mine near Bagdad, Arizona, and (3) experimental formation of gypsum coatings on fluorite placed in acid mine seeps for 14 weeks (and longer) at the Black Pearl Mine, also near Bagdad, Arizona.

Laboratory experiments were undertaken in sealed Teflon flasks in order to quantify reaction rates under temperatures (0-50^oC), pH levels (0-4), and fluorite grain sizes (coarse to powdered) common to pyritic mine dumps and tailings. Reaction rates were determined mainly by weight loss of the fluorite reactant over time, supplemented by ion chromatography of the fluoride-containing product. Results show the expected dependence of reaction rate on sulfuric acid concentration, fluorite surface area, and temperature. Less expected was the fact that the reaction is comparatively rapid even for coarse crystals at room temperature (e.g., 1.2% wt loss in only 24 hr for a 1 cm fluorite octahedron in 1M acid at 23^oC). The implication is that large-scale generation of hydrofluoric acid is possible in nature on oxidizing mine dumps and tailings that contain abundant pyrite and fluorite.