FLUORINE 



225 



O 2000 



Ol-= 

 1900 1910 1920 1930 1940 1950 1960 1970 



Figure 26. — World production, 1913-70, U.S. production, 



1900-70, and U.S. consumption, 1911-70, of fluorspar 



(CaF,). 



amounts of gaseous fluorine are given off in some 

 active volcanic areas. Most fluorine in nature, how- 

 ever, is found combined in solid form. The radius 

 of the fluoride ion F- (1.33 A) is very close to that 

 of the hydroxyl ion (OH) - (1.33 A) and the oxygen 

 ion 0-^ (1.32 A), and with proper valence condi- 

 tions, diadochic substitution of F- for (OH) - or 

 0-^ is relatively simple. Fluorine in the lithosphere 

 therefore is tied up as an essential element in fluor- 

 ide minerals or as a substitute in other minerals, 

 mainly silicates and phosphates. Fluorite, CaFz, is 

 geologically the most abundant and economically the 

 most important fluoride. Some other fluorides of 

 possible economic significance are cryolite, NasAlFg ; 

 sellaite, MgFs ; villiaumite, NaF ; and bastnaesite, 

 (Ce,La) (C03)F, whereas other, still less common 

 fluorides will probably be of little economic signifi- 

 cance in the future. Fluorine occurs in many sili- 

 cate minerals, including tourmaline, topaz, and the 

 micas phlogopite, zinnwaldite, lepidolite and biotite, 

 and in the phosphates monazite, xenotime, fluorapa- 

 tite, and amblygonite. Fluorapatite, Ca5(P04,C03)3F, 

 and topaz, Al2Si04(F,OH)2, are of economic sig- 

 nificance. 



The organic chemistry of fluorine in terms of 

 fluorosis and fluorocarbons is well understood; the 

 geochemistry of fluorine is not. The distribution of 

 fluorine in the earth's crust can be noted but not ade- 

 quately explained. Crustal abundance is difficult to 

 determine. Fleischer and Robinson (1963, p. 67) 

 reported a value of 650 ppm (parts per million) 

 fluorine for the average content of the continental 

 rocks of the earth's crust. They also gave the follow- 

 ing mean values, in parts per million, for various 

 rock types : basalt, 360 ; andesite, 210 ; rhyolite, 480 ; 

 phonolite, 930 ; gabbro and diabase, 420 ; granite and 

 granodiorite, 870; alkalic rocks, 1,000; limestone, 

 220; dolomite, 260; sandstone and graywacke, 180; 

 shale, 800 ; oceanic sediment, 730 ; and soil, 285. The 

 geochemical cycle of fluorine and the problems as- 

 sociated with fluorine determinations were discussed 

 by Fleischer and Robinson (1963) and Barth 

 (1947). 



Although fluorine is widespread throughout the 

 lithosphere, biosphere, and hydrosphere, it tends to 

 be concentrated in fairly specific environments. In 

 igneous activity fluorine is considered as a charac- 

 teristic component of the volatile phase of magmatic 

 differentiation. More specifically fluorine is concen- 

 trated in alkalic and silicic hypabyssal and ex- 

 trusive rocks and related hydrothermal deposits, in 

 complex pegmatites, in carbonatites, and in altera- 

 tion zones, including greisens, associated with al- 

 kalic, silicic and carbonatite intrusive rocks. 

 Fluorine becomes concentrated in sedimentary en- 

 vironments largely by precipitation as fluorite, other 

 fluorides, or fluorapatite during accumulation of the 

 sediments or during their subsequent diagenesis. 

 These minerals are not common as detrital com- 

 ponents of the rocks. Five general groups of sedi- 

 mentary deposits contain concentrations of fluorine : 

 volcaniclastic, lacustrine, evaporite, marine carbon- 

 ate, and marine phosphate beds. 



Fluorine is a common but variable constituent of 

 gases and waters of volcanic origin, mainly as HF 

 but also as Fj, SiFi, and HjSiFe. Volcanic exhala- 

 tions contribute large quantities of fluorine to the 

 surface environment. For example, Zies (1929) esti- 

 mated that at one time the Valley of Ten Thousand 

 Smokes, Alaska, was releasing HF at the rate of 

 200,000 tons annually. 



The fluoride content of ground water ranges from 

 zero to a maximum recorded value of 67.2 ppm, and 

 few waters contain more than 10 ppm. Large areas 

 of the world, however, locally have ground waters 

 containing more than 1.5 ppm F. The fluoride con- 

 tent of ground water depends in part upon the 

 nature of the bedrock. Alkalic igneous rocks, dolo- 



