Fluorine is the lightest of the halogen elements and the most electronegative. Fluoride ions have the same charge and very similar ionic radius to OH– so that F– substitutes readily for hydroxyl positions in minerals (Munoz, 1984). Fluorine occurs as an essential component in around 300 minerals, including some halides, phosphates, oxides, carbonates, borates, sulphates and silicates. However, the most important fluorine-bearing minerals are fluorite, CaF2, and fluorapatite, Ca5(PO4)3F. Fluorite occurs in felsic igneous rocks, sediments and as a gangue mineral in hydrothermal deposits including epithermal deposits, porphyry Cu and Mo deposits and pegmatites. Fluorapatite is a principal mineral of sedimentary phosphorites. Other F-bearing minerals include topaz, Al2(SiO4)(F,OH)2; villiaumite, NaF; bastnaesite, (Ca, La, Nd)(CO3)(F); sellaite, MgF2; and cryolite, Na3AlF6 (Table 3). Topaz occurs in pegmatites and hydrothermal deposits, sellaite in hydrothermal assemblages in association with Mg-rich rocks. Cryolite occurrence is usually restricted to pegmatite deposits. Bastnaesite, the REE-rich mineral, occurs in association with carbonatites and other alkaline igneous rocks. Villiaumite is found in association with trona in alkaline igneous provinces (Hayes et al., 2017).
Phyllosilicate minerals, including biotite, K(Mg, Fe)3(AlSi3O10)(OH,F)2 (Table 2), other micas, amphiboles and 2:1 layer clay minerals such as smectites, chlorites and illites also contain variable amounts of F (Hayes et al., 2017). In the phyllosilicate minerals, F occurs by hydroxyl substitution. Presence of F in clay minerals is responsible for increased atmospheric emissions associated with the brick firing industry (Chipera and Bish, 2002; Fuge, 2019).
|Fluorite||CaF2||Felsic igneous rocks, hydrothermal deposits, sedimentary rocks|
|Fluorapatite||Ca5(PO4)3F||Igneous rocks, metamorphic rocks, high-temperature hydrothermal deposits, marine sediments|
|Francolite||(Ca,Mg,Sr,Na)10(PO4,SO4,CO3)6F2–3||Diagenetic deposits in marine sedimentary rocks, skarns|
|Topaz||Al2(SiO4)(F,OH)2||Felsic igneous rocks, pegmatite|
|Bastnaesite||(Ca, La, Nd)(CO3)(F)||Carbonatites, other alkaline ultramafic igneous rocks, hydrothermal deposits|
|Cryolite||Na3AlF6||Granite pegmatite (rare, main occurrence Greenland)|
|Villiaumite||NaF||Alkaline igneous rocks|
|Biotite||K(Mg,Fe)3(AlSi3O10)(OH,F)2||Felsic igneous rocks, hydrothermal deposits|
|Hornblende||(Ca,Na)2(Mg,Fe,Al)5(Al,Si)8O22(OH,F)2||Igneous and metamorphic rocks|
Sedimentary phosphorite deposits contain F as fluorapatite and its carbonate variant, francolite (Ca, Mg, Sr, Na)10(PO4, SO4, CO3)6F2–3 (Benmore et al., 1983; Baghdady et al., 2016). Francolite, sometimes called carbonate-fluorapatite, is the primary mineral in phosphate ore and in the Florida deposits it contains 4 to 5 by weight percent F (Van Kauwenbergh et al., 1990). With substitution of CO32– for PO43– in the crystal structure, F– associates with the CO32– to balance the charge and accounts for the higher F content of francolite compared to that dictated by the structural formula of fluorapatite (McClellan and Lehr, 1969). Francolite is also present in some limestones. In marine mudstones, F adsorbs to clays. Most sandstones have a relative paucity of F-bearing minerals.
Substitution of F for hydroxyl ions is also important to the mineral structure of teeth and bones. Continuously variable solid solutions between calcium hydroxyapatite Ca5(PO4)3(OH) and fluorapatite can occur, the substitution of F resulting in reduced mineral solubility. This property and the increased resistance to acid attack is beneficial for protection against dental caries (Abou Neel et al., 2016; Chow and Markovic, 1998) and has been the rationale for increased use of F toothpastes, mouth washes and varnishes and for water fluoridation. Nonetheless, fluorapatite is mechanically weaker than hydroxyapatite and incorporation of F in the apatite structure increases tooth brittleness, a factor implicated in dental and skeletal fluorosis (Johnston and Strobel, 2020).