Fluorite

Fluorite
Deep green isolated fluorite crystal resembling a truncated octahedron, set upon a micaceous matrix, from Erongo Mountain, Erongo Region, Namibia (overall size: 50 mm × 27 mm, crystal size: 19 mm wide, 30 g)
General
Category	Halide mineral
Formula (repeating unit)	CaF2
IMA symbol	Flr[1]
Strunz classification	3.AB.25
Crystal system	Isometric
Crystal class	Hexoctahedral (m3m) H–M symbol: (4/m 3 2/m) (cF12)
Space group	Fm3m (No. 225)
Unit cell	a = 5.4626 Å; Z = 4
Identification
Color	Colorless, although samples are often deeply colored owing to impurities; Purple, lilac, golden-yellow, green, blue, pink, champagne, brown.
Crystal habit	Well-formed coarse sized crystals; also nodular, botryoidal, rarely columnar or fibrous; granular, massive
Twinning	Common on {111}, interpenetrant, flattened
Cleavage	Octahedral, perfect on {111}, parting on {011}
Fracture	Subconchoidal to uneven
Tenacity	Brittle
Mohs scale hardness	4 (defining mineral)
Luster	Vitreous
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	3.175–3.184; to 3.56 if high in rare-earth elements
Optical properties	Isotropic; weak anomalous anisotropism; moderate relief
Refractive index	1.433–1.448
Fusibility	3
Solubility	slightly water soluble and in hot hydrochloric acid
Other characteristics	May be fluorescent, phosphorescent, thermoluminescent, and/or triboluminescent
References	[2][3][4][5]

Fluorite (also called fluorspar) is the mineral form of calcium fluoride, CaF₂. It belongs to the halide minerals. It crystallizes in isometric cubic habit, although octahedral and more complex isometric forms are not uncommon.

The Mohs scale of mineral hardness, based on scratch hardness comparison, defines value 4 as fluorite.^[6]

Pure fluorite is colourless and transparent, both in visible and ultraviolet light, but impurities usually make it a colorful mineral and the stone has ornamental and lapidary uses. Industrially, fluorite is used as a flux for smelting, and in the production of certain glasses and enamels. The purest grades of fluorite are a source of fluoride for hydrofluoric acid manufacture, which is the intermediate source of most fluorine-containing fine chemicals. Optically clear transparent fluorite has anomalous partial dispersion, that is, its refractive index varies with the wavelength of light in a manner that differs from that of commonly used glases, so fluorite is useful in making apochromatic lenses, making it valuable, particularly in photographic optics. Fluorite optics are also usable in the far-ultraviolet and mid-infrared ranges, where conventional glasses are too opaque for use. Fluorite also has low dispersion, and a high refractive index for its density, which can make it useful for some specialized purposes in optics.