If fluorite dissolves to equilibrium, CaF2 ↔
Ca2+ + 2 F- ,
the law of mass action yields:
K = 10-10.6 =
[Ca2+] [F-] 2
or
[F-] =
(10-10.6 / [Ca2+])½.
This hyperbolic relation predicts that high fluoride waters will have low Ca concentrations and vice versa.
The Ca concentration in water is often regulated by calcite equilibrium. In high-alkalinity, high-pH waters the Ca concentration is low.
The pH and alkalinity may increase by dissolution of alumino-silicates (weathering), or by evaporation and escape of CO2 gas.
These two processes, in combination with calcite and fluorite equilibria, probably explain the high F concentrations
found in many African Rift Valley waters.
The PHREEQC input file ca_f.txt models the effect of feldspar dissolution on the
Ca and F concentrations.
In the model, protons dissolve Na from the feldspar (keyword REACTION), Ca and F react according to equilibrium with
calcite and fluorite (keyword EQUILIBRIUM_PHASES).
The pH and alkalinity increase by the reaction, causing calcite to precipitate and Ca to be removed from the initially high-Ca water.
As the result, fluorite dissolves and the F concentration increases.
Another process that can lower the Ca concentration in water is ion exchange with Na.
F replaces OH in the mineral apatite ( Ca5(OH,F)(PO4)3 ) which makes the mineral harder and brittler. Apatite is part of the bones and teeth of humans. Ingestion of too much F is the cause of fluorosis, an embrittling of the cartilage that leads to painful joints and easily breakable bones in older people. The MAC value for F in drinking water is 1.5 mg/L (depending on the consumption rate since the body cumulatively stores the element).