Deep green isolated fluorite crystal resembling atruncated 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)
Fluorite (also calledfluorspar) is the mineral form ofcalcium fluoride, CaF2. It belongs to thehalide minerals. It crystallizes inisometriccubic habit, although octahedral and more complex isometric forms are not uncommon.
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 andlapidary uses. Industrially, fluorite is used as aflux for smelting, and in the production of certain glasses and enamels. The purest grades of fluorite are a source of fluoride forhydrofluoric acid manufacture, which is the intermediate source of most fluorine-containingfine chemicals. Optically clear transparent fluorite has anomalous partialdispersion, that is, its refractive index varies with the wavelength of light in a manner that differs from that of commonly used glasses, so fluorite is useful in makingapochromatic lenses, and particularly valuable 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.
The wordfluorite is derived from theLatin verbfluere, meaningto flow. The mineral is used as aflux in ironsmelting to decrease theviscosity ofslag. The termflux comes from the Latin adjectivefluxus, meaningflowing, loose, slack. The mineral fluorite was originally termedfluorspar and was first discussed in print in a 1530 workBermannvs sive de re metallica dialogus [Bermannus; or dialogue about the nature of metals], byGeorgius Agricola, as a mineral noted for its usefulness as a flux.[7][8] Agricola, a German scientist with expertise inphilology,mining, and metallurgy, named fluorspar as aNeo-Latinization of theGermanFlussspat fromFluss (stream,river) andSpat (meaning anonmetallic mineral akin togypsum, spærstān,spear stone, referring to its crystalline projections).[9][10]
In 1852, fluorite gave its name to the phenomenon offluorescence, which is prominent in fluorites from certain locations, due to certain impurities in the crystal. Fluorite also gave the name to its constitutive elementfluorine.[3] Currently, the word "fluorspar" is most commonly used for fluorite as an industrial and chemical commodity, while "fluorite" is used mineralogically and in most other senses.
In archeology, gemmology, classical studies, and Egyptology, the Latin termsmurrina andmyrrhina refer to fluorite.[11] In book 37 of hisNaturalis Historia,Pliny the Elder describes it as a precious stone with purple and white mottling, and noted that the Romans prized objects carved from it. It has been suggested that the Sanskrit mineral namevaikrānta (वैक्रान्तः), known fromSanskrit alchemical texts dating from the early second millennium CE onwards, may refer to fluorite.[12]
Fluorite forms as a late-crystallizing mineral infelsicigneous rocks typically through hydrothermal activity.[15] It is particularly common in granitic pegmatites. It may occur as avein deposit formed throughhydrothermal activity particularly in limestones. In such vein deposits it can be associated withgalena,sphalerite,barite,quartz, andcalcite. Fluorite can also be found as a constituent of sedimentary rocks either as grains or as the cementing material insandstone.[15]
It is a common mineral mainly distributed in South Africa, China, Mexico, Mongolia, the United Kingdom, the United States, Canada, Tanzania, Rwanda and Argentina.
The world reserves of fluorite are estimated at 230 milliontonnes (Mt) with the largest deposits being inSouth 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).[16][needs update]
One of the largest deposits of fluorspar in North America is located on theBurin 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 fluoride 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 widelenses. The area with veins of known workable size comprises about 60 square miles (160 km2).[17][18][19]
In 2018, Canada Fluorspar Inc. commenced mine production again[20] in St. Lawrence; in spring 2019, the company was planned to develop a new shipping port on the west side of Burin Peninsula as a more affordable means of moving their product to markets,[21] and they successfully sent the first shipload of ore from the new port on July 31, 2021. This marks the first time in 30 years that ore has been shipped directly out of St. Lawrence.[22]
Cubic crystals up to 20 cm across have been found atDalnegorsk, Russia.[23] The largest documented single crystal of fluorite was a cube 2.12 meters in size and weighing approximately 16 tonnes.[24]
Fluorite on barite from the Berbes mine, Ribadesella, Asturias (Spain). Fluorite crystal, 2.2 cm.
InAsturias (Spain) there are several fluorite deposits known internationally for the quality of the specimens they have yielded. In the area ofBerbes,Ribadesella, fluorite appears as cubic crystals, sometimes with dodecahedron modifications, which can reach a size of up to 10 cm of edge, with internal colour zoning, almost always violet in colour. It is associated with quartz and leafy aggregates of baryte. In theEmilio mine, in Loroñe,Colunga, the fluorite crystals, cubes with small modifications of other figures, are colourless and transparent. They can reach 10 cm of edge. In theMoscona mine, in Villabona, the fluorite crystals, cubic without modifications of other shapes, are yellow, up to 3 cm of edge. They are associated with large crystals of calcite and barite.[25]
One of the most famous of the older-known localities of fluorite isCastleton inDerbyshire,England, where, under the name of "Derbyshire Blue John", purple-blue fluorite was extracted from several mines or caves. 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 andlapidary use. Mining still takes place inBlue John Cavern andTreak Cliff Cavern.[26]
Recently discovered deposits in China have produced fluorite with coloring and banding similar to the classic Blue John stone.[27]
Many samples of fluorite exhibitfluorescence underultraviolet light, a property that takes its name from fluorite.[28] 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 asyttrium andytterbium, or organic matter, such as volatilehydrocarbons in the crystal lattice. In particular, the blue fluorescence seen in fluorites from certain parts ofGreat Britain responsible for the naming of the phenomenon of fluorescence itself, has been attributed to the presence of inclusions of divalenteuropium in the crystal.[30] Natural samples containing rare earth impurities such aserbium have also been observed to displayupconversion fluorescence, in which infrared light stimulates emission of visible light, a phenomenon usually only reported in synthetic materials.[31]
One fluorescent variety of fluorite ischlorophane, which is reddish or purple in color and fluoresces brightly in emerald green when heated (thermoluminescence), or when illuminated with ultraviolet light.
The color of visible light emitted when a sample of fluorite is fluorescing depends 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 fromNorthumberland,County Durham, and easternCumbria are the most consistently fluorescent, whereas fluorite fromYorkshire,Derbyshire, andCornwall, if they fluoresce at all, are generally only feebly fluorescent.
Fluorite is allochromatic, meaning that it can be tinted with elemental impurities. Fluorite comes in a wide range of colors and has consequently been dubbed "the most colorful mineral in the world". Every color of the rainbow in various shades is represented by fluorite samples, along with white, black, and clear crystals. The most common colors are purple, blue, green, yellow, or colorless. Less common are pink, red, white, brown, and black. Color zoning or banding is commonly present. The color of the fluorite is determined by factors including impurities, exposure to radiation, and the absence of voids of thecolor centers.
Pastel green fluorite crystal on galena
A golden yellow with hints of purple fluorite
Freestanding purple fluorite cluster between two quartzes
Light to dark burgundy color fluorite
Transparent teal color fluorite with purple highlights
Grass-green fluorite octahedrons clustered on a quartz-rich matrix
Fluorite is a major source ofhydrogen fluoride, a commodity chemical used to produce a wide range of materials. Hydrogen fluoride is liberated from the mineral by the action of concentratedsulfuric acid:
The resulting HF is converted into fluorine,fluorocarbons, and diverse fluoride materials. As of the late 1990s, five billion kilograms were mined annually.[33]
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 aflux to lower the melting point of raw materials insteel production to aid the removal of impurities, and later in the production ofaluminium. Ceramic grade fluorite (85–95% CaF2) is used in the manufacture ofopalescentglass,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 makehydrogen fluoride andhydrofluoric acid by reacting the fluorite withsulfuric acid.[34]
Internationally, acid-grade fluorite is also used in the production ofAlF3 andcryolite (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 react with excess sodium from the alumina to form Na3AlF6.[34]
Natural fluorite mineral has ornamental andlapidary 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. It is also used for ornamental carvings, with expert carvings taking advantage of the stone's zonation.
In the laboratory, calcium fluoride is commonly used as a window material for bothinfrared andultraviolet wavelengths, since it is transparent in these regions (about 150 to 9000 nm) and exhibits an extremely low change inrefractive index with wavelength. Furthermore, the material is attacked by few reagents. At wavelengths as short as 157 nm, a common wavelength used forsemiconductor stepper manufacture forintegrated circuitlithography, 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 aidapochromatic design, and to reducelight 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 theEastman Kodak trademarked name "Irtran-3", although this designation is obsolete.
Fluorite should not be confused with fluoro-crown (or fluorine crown) glass, a type oflow-dispersion glass that has special optical properties approaching fluorite. True fluorite is not a glass but a crystalline material. Lenses oroptical groups made using this low dispersion glass as one or more elements exhibit lesschromatic aberration than those utilizing conventional, less expensivecrown glass andflint glass elements to make anachromatic lens. Optical groups employ a combination of different types of glass; each type of glassrefracts light in a different way. By using combinations of different types of glass, lens manufacturers are able to cancel out or significantly reduce unwanted characteristics; chromatic aberration being the most important. The best of such lens designs are often called apochromatic (see above). Fluoro-crown glass (such as Schott FK51) usually in combination with an appropriate"flint" glass (such as Schott KzFSN 2) can give very high performance in telescope objective lenses, as well as microscope objectives, and camera telephoto lenses. Fluorite elements are similarly paired with complementary "flint" elements (such as Schott LaK 10).[36] The refractive qualities of fluorite and of certain flint elements provide a lower and more uniform dispersion across the spectrum of visible light, thereby keeping colors focused more closely together. Lenses made with fluorite are superior to fluoro-crown based lenses, at least for doublet telescope objectives; but are more difficult to produce and more costly.[37]
The use of fluorite for prisms and lenses was studied and promoted byVictor Schumann near the end of the 19th century.[38] Naturally occurring fluorite crystals without optical defects were only large enough to produce microscope objectives.
With the advent of synthetically grown fluorite crystals in the 1950s - 60s, it could be used instead of glass in some high-performanceoptical telescope andcamera lens elements. In telescopes, fluorite elements allow high-resolution images of astronomical objects at highmagnifications.Canon Inc. produces synthetic fluorite crystals that are used in their bettertelephoto lenses. The use of fluorite for telescope lenses has declined since the 1990s, as newer designs using fluoro-crown glass, including triplets, have offered comparable performance at lower prices. Fluorite and various combinations of fluoride compounds can be made into synthetic crystals which have applications in lasers and special optics for UV and infrared.[39]
Exposure tools for thesemiconductor industry make use of fluorite optical elements forultraviolet light atwavelengths of about 157nanometers. Fluorite has a uniquely high transparency at this wavelength. Fluoriteobjective lenses are manufactured by the larger microscope firms (Nikon,Olympus,Carl Zeiss and Leica). Their transparence to ultraviolet light enables them to be used forfluorescence microscopy.[40] The fluorite also serves to correctoptical aberrations in these lenses.Nikon has previously manufactured at least one fluorite and synthetic quartz element camera lens (105 mm f/4.5 UV) for the production ofultraviolet images.[41]Konica produced a fluorite lens for their SLR cameras – the Hexanon 300 mm f/6.3.
In 2012, the first source of naturally occurring fluorine gas was found in fluorite mines in Bavaria, Germany. It was previously thought thatfluorine gas did not occur naturally because it is so reactive, and would rapidly react with other chemicals.[42] Fluorite is normally colorless, but some varied forms found nearby look black, and are known as 'fetid fluorite' orantozonite. The minerals, containing small amounts ofuranium 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 color of fetid fluorite is predominantly due to thecalcium atoms remaining. Solid-state fluorine-19NMR carried out on the gas contained in the antozonite, revealed a peak at 425 ppm, which is consistent with F2.[43]
^Anthony, John W.; Bideaux, Richard A.; Bladh, Kenneth W.; Nichols, Monte C., eds. (1990). "Fluorite".Handbook of Mineralogy(PDF). Vol. III (Halides, Hydroxides, Oxides). Chantilly, VA, US: Mineralogical Society of America.ISBN0962209724.Archived(PDF) from the original on 2006-09-06. RetrievedDecember 5, 2011.
^Strong, D. F.; Fryer, B. J.; Kerrich, R. (1984). "Genesis of the St. Lawrence fluorspar deposits as indicated by fluid inclusion, rare earth element, and isotopic data".Economic Geology.79 (5): 1142.Bibcode:1984EcGeo..79.1142S.doi:10.2113/gsecongeo.79.5.1142.
^Korbel, P. and Novak, M. (2002)The Complete Encyclopedia of Minerals, Book Sales,ISBN0785815201.
^Rickwood, P. C. (1981)."The largest crystals"(PDF).American Mineralogist.66:885–907.Archived(PDF) from the original on 2009-06-20.
^Calvo Sevillano, Guiomar; Calvo Rebollar, Miguel (2006). "Fluorite from Spain. Every color under the Sun".Fluorite. The Collector's Choice. Connecticut, USA: Lithographie LLC. Connecticut, USA. pp. 38–42.
^Hill, Graham; Holman, John (2000).Chemistry in context. Nelson Thornes.ISBN0174482760.
^Stokes, G. G. (1853). "On the Change of Refrangibility of Light. No. II".Philosophical Transactions of the Royal Society of London.143:385–396, at p. 387.doi:10.1098/rstl.1853.0016.JSTOR108570.S2CID186207789.
^Aigueperse, Jean; Paul Mollard; Didier Devilliers; Marius Chemla; Robert Faron; Renée Romano; Jean Pierre Cuer (2005). "Fluorine Compounds, Inorganic".Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH.doi:10.1002/14356007.a11_307.ISBN3527306730.
^abMiller, M. Michael.Fluorspar, USGS 2009 Minerals Yearbook
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