Ultramafic rocks deficient in silica (SiO2) have relatively low fluoride content (100 mg/kg), whereas intermediate rocks have an average concentration of ~400 mg/kg and felsic igneous rocks, which contain high concentrations of SiO2, tend to have even higher fluoride concentrations (around 1000 mg/kg). Fluoride tends to be associated with micas in igneous rocks, has a strong association with phosphates in both primary and secondary minerals and sorbs to clays and aluminium hydroxides secondary weathering products (Wedepohl 1978). Geochemical conditions largely control the concentration of fluoride in aquifer systems. Figure "Geochemical processes that control fluoride solubility. a) The precipitation of fluorite" illustrates the dependence of fluoride concentration on calcium concentration in a system where fluorite (CaF2(s)) precipitation controls the amount of dissolved fluoride. Assuming the source of calcium to be calcite at atmospheric PCO2, a calcium concentration of 20 mg/L would be expected. Calcium concentrations are higher at higher partial pressures of CO2, which further drive down fluoride concentrations that may be additionally limited by sorption to sediment minerals. In poorly pH-buffered systems with low pH, such as in association with low-calcium felsic rocks, there may be little control of dissolved fluoride content. For example, high-fluoride minerals in granitic rocks cause elevated dissolved fluoride concentrations in India and Sri Lanka.
NOTE: Article from the Geogenic Contamination Handbook