Natural Fluorides

a class of minerals comprising natural compounds of fluorine with the elements Na, K, Ca, Mg, Al, rare-earth elements (TR), and, more rarely, Cs, Sr, Pb, Bi, and B.

Approximately 35 natural fluorides are known, which are divided into simple and complex fluorides. The simple natural fluorides include the villiaumite group, NaF, the fluorite group, CaF₂, the sellaite group, MgF₂, and the fluocerite group, (Ce,La)F₃. The complex natural fluorides, in which the complex-forming elements are B, Al, Mg, TR, or Si and the complexing agent is fluorine, include the avogadrite group, (K,CS)[BF₄], the cryolite group, Na₃[AlF₆], the gagarinite group, NaCa[TRF₆], the neighborite group, Na[MgF₃], the weberite group, Na₂[MgAlF₇], the thomsenolite group, NaCa[AlF₆]·H₂O, and the malladrite group, Na₂ [SiF₆]. The most widely distributed natural fluoride is fluorite (seeFLUORITE).

Natural fluorides are colorless or light-colored, transparent or translucent minerals, with a vitreous luster. They have a low hardness on Mohs’ scale, ranging from 2 to 5, a density that varies from 2,000 to 3,180 kg/m³ (with the exception of the rare-earth fluorides), and extremely low refractive indexes, ranging between 1.30 and 1.50 (1.61 in fluocerite).

Natural fluorides are found in volcanic sublimates (ferruccite, avogadrite, cryptohalite, malladrite). They also occur as accessory minerals in granites and alkaline rocks and in their effusive analogues (fluorite). They are characteristic of the late stages in the development of carbonatites (fluorite), granitic pegmatites, greisens, and hydrothermal formations (fluorite); they are also characteristic of alkaline granites and their associated metasomatites (cryolite, fluocerite, gagarinite) and of lujauvrites, foyaites, and urtites (villiaumite). Many aluminofluorides are formed during the hydrothermal alteration of cryolite (thomsenolite, ralstonite, pachnolite, weberite, chiolite). In the oxidation zone, hy-pergenic natural fluorides often form along the endogenic natural fluorides: gearksutite, creedite, fluellite, and fluorite (ratofkite), the last being characteristic of sedimentary strata.

Of the natural fluorides, only fluorite and cryolite are of practical importance.