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Fluorite

[floor-ahyt, flawr-, flohr-]

Fluorite (also called fluorspar) is a mineral composed of calcium fluoride, CaF2. It is an isometric mineral with a cubic habit, though octahedral and more complex isometric forms are not uncommon. Cubic crystals up to 20 cm across have been found at Dalnegorsk, Russia. Crystal twinning is common and adds complexity to the observed crystal habits.

The word fluorite is derived from the Latin root fluo, meaning "to flow" because the mineral has a relatively low melting point and was used as an important flux in smelting. Fluorite gave its name to its constitutive element fluorine.

Occurrence

Fluorite may occur as a vein deposit, especially with metallic minerals, where it often forms a part of the gangue (the worthless "host-rock" in which valuable minerals occur) and may be associated with galena, sphalerite, barite, quartz, and calcite. It is a common mineral in deposits of hydrothermal origin and has been noted as a primary mineral in granites and other igneous rocks and as a common minor constituent of dolostone and limestone.

Fluorite is a widely occurring mineral which is found in large deposits in many areas. Notable deposits occur in Germany, Austria, Switzerland, England, Norway, Mexico, and Ontario in Canada. Large deposits also occur in Kenya in the Kerio Valley area within the Great Rift Valley. In the United States, deposits are found in Missouri, Oklahoma, Illinois, Kentucky, Colorado, New Mexico, Arizona, Ohio, New Hampshire, New York, Alaska and Texas. Illinois has historically been the largest producer of fluorite in the United States, however, the last of the mines closed in 1995. The Illinois General Assembly passed a resolution in 1965 declaring fluorite as the official state mineral.

Blue John

One of the most famous of the older-known localities of fluorite is Castleton in Derbyshire, England, where, under the name of Derbyshire Blue John, purple-blue fluorite was extracted from several mines/caves, including the famous Blue John Cavern. During the 19th century, this attractive fluorite was mined for its ornamental value. The name derives from French "bleu et jaune" (blue and yellow) characterising its color. Blue John is now scarce, and only a few hundred kilograms are mined each year for ornamental and lapidary use. Mining still takes place in the nearby Treak Cliff Cavern. Recent deposits in China have produced fluorite with coloring and banding similar to the classic Blue John stone.

Fluorescence

Many samples of fluorite fluoresce under ultra-violet light, a property that takes its name from fluorite. Many minerals, as well as other substances, fluoresce. Fluorescence involves the elevation of electron energy levels by quanta of ultra-violet light, followed by the progressive falling back of the electrons into their previous energy state, releasing quanta of visible light in the process. In fluorite, the visible light emitted is most commonly blue, but red, purple, yellow, green and white also occur. The fluorescence of fluorite may be due to impurities such as yttrium or organic matter in the crystal lattice. It is not surprising, therefore, that the color of visible light emitted when a sample of fluorite is fluorescing appears dependent on where the original specimen was collected, different impurities having been included in the crystal lattice in different places. Neither do all fluorites fluoresce equally brightly, even from the same locality. Therefore ultra-violet light is not a reliable tool for the identification of specimens, nor for quantifying the mineral in mixtures. For example, among British fluorites, those from Northumberland, County Durham, and Eastern Cumbria are the most consistently fluorescent, whereas fluorites from Yorkshire, Derbyshire, and Cornwall, if they fluoresce at all, are generally only feebly fluorescent.

Fluorite also exhibits the property of thermoluminescence.

Uses

There are three principal types of industrial use for fluorite, corresponding to different grades of purity. Metallurgical grade fluorite, the lowest of the three grades, has traditionally been used as a flux to lower the melting point of raw materials in steel production to aid the removal of impurities, and later in the production of aluminium. Ceramic (intermediate) grade fluorite is used in the manufacture of opalescent glass, enamels and cooking utensils. The highest grade, acid grade fluorite, is used to make hydrofluoric acid by decomposing the fluorite with sulfuric acid. Hydrofluoric acid is the primary feedstock for the manufacture of virtually all organic and inorganic fluorine-containing compounds, including fluoropolymers and perfluorocarbons, and is also used to etch glass.

Fluorite is used instead of glass in some high performance telescopes and camera lens elements. Exposure tools for the semiconductor industry make use of fluorite optical elements for ultraviolet light at 157 nm wavelength. Fluorite has a uniquely high transparency at this wavelength. Fluorite has a very low dispersion so lenses made from it exhibit less chromatic aberration than those made of ordinary glass. In telescopes it allows crisp images of astronomical objects even at high power. Fluorite also has ornamental and lapidary uses. Canon Inc. produces synthetic fluorite crystals that are used in their more expensive telephoto lenses. Nikon has previously manufactured at least one all-fluorite element camera lens (105 mm f/4.5 UV) for the production of ultraviolet images.

Fluorite objective lenses are manufactured by the larger microscope firms (Nikon, Olympus, Carl Zeiss and Leica). Their transparence to ultraviolet light enables them to be used for fluorescence microscopy. The fluorite also serves to correct optical aberrations in these lenses.

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