Fluorite
| Fluorite | |
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Deep green isolated fluorite crystal showing cubic and octahedral
faces, set upon a micaceous matrix, from Erongo Mountain, Erongo Region,
Namibia (overall size: 50 mm x 27 mm, crystal size: 19 mm wide, 30 g)
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| General | |
| Category | Halide mineral |
| Formula (repeating unit) |
CaF2 |
| Strunz classification | 03.AB.25 |
| Crystal symmetry | Isometric H–M Symbol 4/m 3 2/m |
| Unit cell | a = 5.4626 Å; Z=4 |
| Identification | |
| Color | Colorless, white, purple, blue, green, yellow, orange, red, pink, brown, bluish black; commonly zoned; can be any color of the spectrum. |
| Crystal habit | Occurs as well-formed coarse sized crystals also nodular, botryoidal, rarely columnar or fibrous; granular, massive |
| Crystal system | Isometric, cF12, SpaceGroup Fm3m, No. 225 |
| 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 |
| Refractive index | 1.433–1.448 |
| Fusibility | 3 |
| Other characteristics | sometimes phosphoresces when heated or scratched. Other varieties fluoresce |
| References | [1][2][3][4] |
Fluorite is a colorful mineral, both in visible and ultraviolet light, 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 lenses have low dispersion, so lenses made from it exhibit less chromatic aberration, making them valuable in microscopes and telescopes. Fluorite optics are also usable in the far-ultraviolet range where conventional glasses are too absorbent for use.
Contents |
History and etymology
Fluorite derives from the Latin noun fluo, meaning a stream or flow of water. In verb form this was fluor or fluere, meaning to flow. The mineral is used as a flux in iron smelting to decrease the viscosity of slags. The melting point of calcium fluoride is 1676 K. The term flux comes from the Latin noun fluxus, a wash or current of water. Agricola, a German scientist with expertise in philology, mining, and metallurgy, named fluorspar as a Neo Latinization of the German Flussespar from Flusse (stream, river) and "Spar" (meaning a nonmetallic mineral akin to gypsum, spærstān, spear stone, referring to its crystalline projections).In 1852 fluorite gave its name to the phenomenon of fluorescence, which is prominent in fluorites from certain locations, due to certain impurities in the crystal. Fluorite also gave the name to its constitutive element fluorine.[2]
The mineral fluorite was originally termed fluorospar and was first discussed in print in a 1530 work Bermannus, sive de re metallica dialogus [Bermannus; or a dialogue about the nature of metals], by Georgius Agricola, as a mineral noted for its usefulness as a flux.[5][6] Presently, the word "fluorspar" is most commonly used for fluorite as the industrial and chemical commodity, while "fluorite" is used mineralogically and in most other senses.
Structure
Main article: calcium fluoride
Fluorite crystallises in a cubic motif. Crystal twinning is common and adds complexity to the observed crystal habits.Occurrence and mining
Fluorite is a widely occurring mineral which is found in large deposits in many areas. Notable deposits occur in China, Germany, Austria, Switzerland, England, Norway, Mexico, and both the Province of Ontario and Newfoundland and Labrador 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. Fluorite has been the state mineral of Illinois since 1965. At that time, Illinois was the largest producer of fluorite in the United States, but the last fluorite mine in Illinois was closed in 1995.[7]
The world reserves of fluorite are estimated at 230 million tonnes (Mt) with the largest deposits being in South Africa (about 41 Mt), Mexico (32 Mt) and China (24 Mt). China is leading the world production with about 3 Mt annually (in 2010), followed by Mexico (1.0 Mt), Mongolia (0.45 Mt), Russia (0.22 Mt), South Africa (0.13 Mt), Spain (0.12 Mt) and Namibia (0.11 Mt).[8]
One of the largest deposits of fluorspar in North America is located in the Burin Peninsula, Newfoundland, Canada. The first official recognition of fluorspar in the area was recorded by geologist J.B. Jukes in 1843. He noted an occurrence of "galena" or lead ore and fluorite of lime on the west side of St. Lawrence harbour. It is recorded that interest in the commercial mining of fluorspar began in 1928 with the first ore being extracted in 1933. Eventually at Iron Springs Mine, the shafts reached depths of 970 feet (300 m). In the St. Lawrence area, the veins are persistent for great lengths and several of them have wide lenses. The area with veins of known workable size comprises about 60 square miles (160 km2).[9][10][11]
Cubic crystals up to 20 cm across have been found at Dalnegorsk, Russia.[12] The largest documented single crystal of fluorite was a cube 2.12 m in size and weighing ~16 tonnes.[13]
"Blue John"
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| Small specimen of blue fluorite from China |
Main article: Derbyshire 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 or caves, including the famous Blue John Cavern.
During the 19th century, this attractive fluorite was mined for its
ornamental value. The mineral 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 both the Blue John Cavern and the nearby Treak Cliff Cavern.[14]Recently discovered deposits in China have produced fluorite with coloring and banding similar to the classic Blue John stone.[15]
Fluorescence
Many samples of fluorite exhibit fluorescence under ultraviolet light, a property that takes its name from fluorite.[18] Many minerals, as well as other substances, fluoresce. Fluorescence involves the elevation of electron energy levels by quanta of ultraviolet 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 mineral impurities such as yttrium, ytterbium, or organic matter in the crystal lattice. In particular, the blue fluorescence seen in fluorites from certain parts of Great Britain responsible for the naming of the phenomenon of fluorescence itself, has been attributed to the presence of inclusions of divalent europium in the crystal.[19]
The color of visible light emitted when a sample of fluorite is fluorescing is dependent on where the original specimen was collected; different impurities having been included in the crystal lattice in different places. Neither does all fluorite fluoresce equally brightly, even from the same locality. Therefore, ultraviolet 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 fluorite from Yorkshire, Derbyshire, and Cornwall, if they fluoresce at all, are generally only feebly fluorescent.
Fluorite also exhibits the property of thermoluminescence.[20]
Color
Uses
Source of fluorine and fluoride
Fluorite is a major source of hydrogen fluoride, a commodity chemical used to produce a wide range of materials. Hydrogen fluoride is liberated from the mineral by the action of concentrated sulfuric acid:The resulting HF is converted into fluorine, fluorocarbons, and diverse fluoride materials. As of the late 1990s, five billion kilograms were mined annually.[21]
There are three principal types of industrial use for natural fluorite, commonly referred to as "fluorspar" in these industries, corresponding to different grades of purity. Metallurgical grade fluorite (60–85% CaF2), 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 grade fluorite (85–95% CaF2) is used in the manufacture of opalescent glass, enamels and cooking utensils. The highest grade, "acid grade fluorite" (97% or more CaF2), accounts for about 95% of fluorite consumption in the US where it is used to make hydrogen fluoride and hydrofluoric acid by reacting the fluorite with sulfuric acid.[22]
Internationally, acid-grade fluorite is also used in the production of AlF3 and cryolite (Na3AlF6), which are the main fluorine compounds used in aluminium smelting. Alumina is dissolved in a bath that consists primarily of molten Na3AlF6, AlF3, and fluorite (CaF2) to allow electrolytic recovery of aluminium. Fluorine losses are replaced entirely by the addition of AlF3, the majority of which will react with excess sodium from the alumina to form Na3AlF6.[22]
Niche uses
Lapidary uses
Natural fluorite mineral has ornamental and lapidary uses. Fluorite may be drilled into beads and used in jewelry, although due to its relative softness it is not widely used as a semiprecious stone.Laboratory applications
In the laboratory, calcium fluoride is commonly used as a window material for both infrared and ultraviolet wavelengths, since it is transparent in these regions (about 0.15 µm to 9 µm) and exhibits extremely low change in refractive index with wavelength. Furthermore the material is attacked by few reagents. At wavelengths as short as 157 nm, a common wavelength used for semiconductor stepper manufacture for integrated circuit lithography, the refractive index of calcium fluoride shows some non-linearity at high power densities which has inhibited its use for this purpose. In the early years of the 21st century the stepper market for calcium fluoride collapsed and many large manufacturing facilities have been closed. Canon and other manufacturers have used synthetically grown crystals of calcium fluoride components in lenses to aid apochromatic design, and to reduce light dispersion. This use has largely been superseded by newer glasses and computer aided design. As an infrared optical material, calcium fluoride is widely available and was sometimes known by the Eastman Kodak trademarked name "Irtran-3," although this designation is obsolete.Optics
Fluorite has a very low dispersion, so lenses made from it exhibit less chromatic aberration than those made of ordinary glass.[23] However, naturally-occurring fluorite crystals without optical defects were only large enough to produce microscope elements.With the advent of synthetically-grown fluorite (calcium fluoride crystal), it could be used instead of glass in some high-performance telescopes and camera lens elements. Its use for prisms and lenses was studied and promoted by Victor Schumann near the end of the 19th century.[24]
In telescopes, fluorite elements allow high-resolution images of astronomical objects at high magnifications. Canon Inc. produces synthetic fluorite crystals that are used in their more expensive telephoto lenses.
Exposure tools for the semiconductor industry make use of fluorite optical elements for ultraviolet light at wavelengths of about 157 nanometers. Fluorite has a uniquely high transparency at this wavelength. 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.[25] The fluorite also serves to correct optical aberrations in these 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.[26] Konica Produced a flourite lens for their SLR cameras - the Hexanon 300mm f6.3.
Source of fluorine gas in nature
In 2012, the first source of naturally occurring fluorine gas was found in fluorite mines in Bavaria, Germany. It was previously thought that fluorine gas did not occur naturally because it is so reactive and would rapidly react with other chemicals.[27] Fluorite is normally colourless but some varied forms found nearby look black and are known as 'fetid fluorite' or antozonite. The minerals, containing small amounts of uranium and its daughter products, release radiation sufficiently energetic to induce oxidation of fluoride anions within the structure to fluorine that becomes trapped inside the mineral. The colour of fetid fluorite is predominantly due to the calcium atoms remaining. Solid state fluorine-19 NMR was carried out on the gas escaping the antozonite revealed a peak at 425 ppm, which is consistent with F2.[28]See also
- List of minerals
- Magnesium fluoride – also used in UV optics
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