Occurrence

Due to the reactivity of the element, fluoride is found in the environment in many mineral complexes, including fluorspar (fluorite or calcium fluoride, CaF 2), cryolite (sodium aluminium fluoride, Na 3AlF 6), rock phosphate (naturally occurring salts rich with PO 4), apatite [Ca 5(F,Cl,OH)(PO 4) 3], mica (salts of aluminium silicate, hornblende [(Ca,Na) 2(Mg,Al,Fe) 5 (Al,Si) 8(OH, F) 2] and many others. It has been estimated that Earth’s crust, on an average, contains 300 mg/kg fluoride.

Luckily most of these fluoride containing minerals are of low solubility, so that the concentration of fluoride in natural waters is relatively low, mostly controlled by the occurrence and the solubility of fluorite and the slow weathering of the other fluoride bearing minerals, e.g. about 1 mg/L in sea water and 0.5 mg/L or less in most surface waters (rivers and lakes). The concentration of fluoride in ground water is often higher than in surface water, but still low or moderate in most (calcareous) undergrounds.

In poor calcium undergrounds the concentration of fluoride in ground water can be quite excessive. This is the case in the so called 5 global fluoride belts.

Five Global Belts

Belt 1:

Turkey, Syria, Jordan, Egypt, Sudan, Somali, Ethiopia Kenya, Tanzania, Mozambique + South Africa.

Belt 2:

Egypt , Libya , Algeria , Morocco , Western Sahara, Mauritania .

Belt 3:

Turkey, Iraq, Iran, Afghanistan, Pakistan, India, North Thailand, (parts of) China.

Belt 4:

Sierra Nevada, USA Rocky Mountain, Central America, Colombia, Peru, Bolivia, Andes Mountains.

Belt 5:

Japan, Philippines, Volcanic Indonesia.

 

 

 

 

 

 

 

 

 

 

It has to be mentioned that the fluoride concentration in natural water can vary quite a lot, even within short distances in the same type of area, whatever fluorotic or non-fluorotic.

In non-fluorotic areas it is normally a useful rule of thumb, that ground water sources are most preferred because of hygienic quality and reliability of the source. Thus one should select the borehole technology, wherever affordable. Whereas in fluorotic areas protected shallow dug wells are often preferred because they, in general, provide lower fluoride waters.

Written by Eli Dahi March 2009