Natural Halides

a group of minerals of saltlike compounds; they are simple or complex derivatives of the hydrogen halide acids HF, HC1, HBr, and HI. As part of their mineral structure, complex halides contain O²⁻, (OH)⁻ (so-called oxy and hydroxy halides), or molecular water of the crystal hydrate type (hydrous halides), along with anionic halogens. The sharp difference between the crystallochemical properties (the ion radius and ionization potential) of the F⁻ ion and those of the other halogen ions—Cl⁻, Br⁻;, and I⁻—necessitates the division of natural halogens into two large classes: (1) the fluorides and (2) the chlorides, bromides, and iodides. About 30 mineral forms, most of which are rare, are known in the fluoride class. Those most frequently found in deposits are the simple fluorides villiaumite NaF, fluorite CaF₂, and fluocerite (Ce, La)F₃, and the complex fluorides cryolite Na₃AlF₆, cryolithionite Na₃Al₂[LiF₄]₃, thomsenolite NaCaAlF₆·H₂O, gearksutite CaAlF₄(OH)·H₂O, and creedite Ca₃Al₂F₈(OH)₂[SO₄]·2H₂O. Geochemically, the fluorine compounds are distinguished by greater chemical stability, the presence of an essentially ionic bond in the crystal structure of the minerals, and the capacity of fluorine in the minerals to form complex groups of the [AlF₆] and [SiF₆] type. Fluorides are formed mainly in pegmatites (acidic and alkaline rock), pneumatolytic-hydro-thermal veins, greisens, skarns, and other deposits of metasomatic origin.

More than 70 mineral forms are known in the chloride, bromide, and iodide classes. Minerals containing cations of sodium, potassium, magnesium, and iron, as well as silver, copper, lead, mercury, and bismuth, are the most widespread: rock salt (halite) NaCl, potassium chloride (sylvite) KCl, carnallite KClMgCl₂·6H₂O, bischofite MgCl₂·6H₂O, cerargyrite AgCl, Pb₃Cu₃AgCl₇(OH)₆, bismoclite BiClO, and cotunnite PbCl₂. Natural bromides and iodides are represented by the rare minerals bromargyrite AgBr, embolite Ag(Cl, Br), marshite CuI, and iodargyrite AgI. Chlorides, bromides, and iodides are found mainly in mineral associations of supergene processes, where chlorine plays the primary geochemical role, forming the minerals of the chemical precipitates in natural salt deposits, as well as rarer compounds with typical metallic cations (silver, copper, lead, and mercury) in some types of oxidation zones of complex deposits.