Fluorite

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.

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 glasses, so fluorite is useful in making apochromatic 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.

Fluorite

General

Category Halide mineral

Formula CaF2

IMA symbol Flr

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

History and etymology

The word fluorite is derived from the Latin verb fluere, meaning to flow. The mineral is used as a flux in iron smelting to decrease the viscosity of slag. The term flux comes from the Latin adjective fluxus, meaning flowing, loose, slack. The mineral fluorite was originally termed fluorspar and was first discussed in print in a 1530 work Bermannvs sive de re metallica dialogus [Bermannus; or dialogue about the nature of metals], by Georgius Agricola, as a mineral noted for its usefulness as a flux. Agricola, a German scientist with expertise in philology, mining, and metallurgy, named fluorspar as a Neo-Latinization of the German Flussspat from Fluss (stream, river) and Spat (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. 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 terms murrina and myrrhina refer to fluorite. In book 37 of his Naturalis 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 name vaikrānta (वैक्रान्तः), known from Sanskrit alchemical texts dating from the early second millennium CE onwards, may refer to fluorite.

Classification

In the outdated, but still partly used 8th edition of the mineral systematics according to Strunz, fluorite belonged to the general division of the "simple halides", where it formed a separate group together with coccinite, frankdicksonite, gagarinite-(Y), laurelite, tveitite-(Y) and gagarinite-(Ce) (formerly zajacite-(Ce)).

The 9th edition of Strunz's mineral classification, valid since 2001 and used by the International Mineralogical Association (IMA), classifies fluorite in the new and more precise division of "simple halides without H 2 O". This division is further subdivided according to the molar ratio of cations (M) to anions (X), so that the mineral can be found in the subdivision "M: X = 1: 2" according to its composition, where it gives its name to the "fluorite group" with the system number 3.AB.25 and the other members fluorocronite (IMA 2010-023), frankdicksonite, and strontiofluorite (IMA 2009-014).

The Dana classification of minerals commonly used in English-speaking countries classifies fluorite in the class of "halides (and related)" and within the division of "halides." Here, it is the eponymous mineral of the "fluorite group" with the system number 09.02.01, along with the other members frankdicksonite, tveitite-(Y), and strontiofluorite within the subdivision of " anhydrous and hydrous halides with the formula AX 2 ".

Crystal structure

Fluorite crystallizes in the cubic crystal system in the highly symmetric crystal class 4/ m 3 2/ m (cubic-hexakisoctahedral) or the space group Fm 3 m (space group no. 225) with the lattice parameter a = 5.463 Å and 4 formula units per unit cell. 

Fluorite crystallizes in a cubic motif. Crystal twinning is common and adds complexity to the observed crystal habits. Fluorite has four perfect cleavage planes that help produce octahedral fragments. The structural motif adopted by fluorite is so common that the motif is called the fluorite structure. Element substitution for the calcium cation often includes strontium and certain rare-earth elements (REE), such as yttrium and cerium.

The crystal structure of fluorite was elucidated in 1914 by William Henry Bragg and his son William Lawrence Bragg using X-ray diffraction experiments. Ca 2+ ions form a cubic closest packing, which corresponds to a face-centered cubic lattice (fcc). The face-centered position of the unit cell can also be read from the space group symbol ("F"). The fluoride ions (F −) occupy all tetrahedral holes in the closest packing of calcium ions. Since there are always twice as many tetrahedral holes as packing particles in a closest packing, the structure has a calcium to fluorine ratio of 1:2, which is also reflected in the chemical formula of fluorite, CaF 2. The fluoride ions therefore form a tetrahedron of four calcium ions, which are surrounded by eight fluorine atoms in the shape of a cube. The cation and anion sub-lattices are non-commutative, i.e., they are interchangeable. The so-called fluorite structure is found in a number of other salts, such as the fluorides SrF 2, BaF 2, CdF 2, HgF 2, and PbF 2. The fluorite structure also occurs, for example, in Li 2 O, Li 2 S, Na 2 O, Na 2 S, K 2 O, K 2 S, Rb 2 O, and Rb 2 S. Its crystal structure is isotypic with uraninite

Characteristics

Morphology

Fluorite frequently forms well-formed, cubic and octahedral crystals. In combination with these main forms, fluorite crystals often exhibit faces of other shapes. Common are the faces of the rhombic dodecahedron {110}, the tetrakis hexahedron {210} (additional faces parallel to the cube edges), the icositetrahedron {211} or {311}, and the hexakis octahedron {421}.

The crystal structure of fluorite crystals is temperature-dependent. Thus, at high formation temperatures, octahedra {111} are predominantly formed, at medium temperatures, rhombic dodecahedra {110} are more likely, and at low temperatures, cubes {100} are the dominant structures. 

The cube faces are usually smooth and shiny. Octahedral and rhombic dodecahedral faces, on the other hand, often appear rough and matte and are usually composed of tiny cube faces. The loose, octahedral fluorite crystals with smooth, shiny faces commonly found in the trade are almost never crystals grown in this shape, but rather cleavage octahedra.

Fluorite also forms spherical and grape-shaped aggregates, crusts, and even stalactites. Scalenohedral fluorites are a special case, as evidenced by finds from the Cäcilia fluorspar mine near Freiung (Stulln municipality in Upper Palatinate). However, closer examination revealed that these also belong to the cubic system and were formed solely through etching. 

Physical properties

By incorporating lanthanides, for example Eu 2+, fluorite can exhibit strong fluorescence when excited by UV light and also phosphorescence when heated and triboluminescence when subjected to strong mechanical stress.

Fluorite is an electrical non-conductor. Its melting point is 1392 °C. 

In thin sections under the microscope, fluorite is striking in linearly polarized light because, due to its relatively low refractive index, it exhibits a strongly negative relief compared to almost all accompanying minerals. Under crossed polarizing filters, it exhibits isotropy as a cubically crystallized mineral, meaning it remains dark. 

Chemical properties

When in contact with strong acids such as sulfuric acid, fluorite releases highly toxic hydrogen fluoride.

Fluorescence

George Gabriel Stokes named the phenomenon of fluorescence from fluorite, in 1852.

Many samples of fluorite exhibit fluorescence under ultraviolet 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 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 and ytterbium, or organic matter, such as volatile hydrocarbons 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. Natural samples containing rare earth impurities such as erbium have also been observed to display upconversion fluorescence, in which infrared light stimulates emission of visible light, a phenomenon usually only reported in synthetic materials.

One fluorescent variety of fluorite is chlorophane, 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 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.

Color

Although pure CaF2 is colorless, fluorite is one of the minerals with the most color variation. The dark color of many fluorites is caused by embedded rare earth elements or radioactive irradiation of the fluorspar (smelly spar), although intergrown uranium minerals can also enhance the color.

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 the color centers.

The causes of color are diverse and not always fully understood. The coloring is usually caused by trace amounts of rare earth elements, which are often only ionized into coloring ions through radioactive irradiation. Which rare earth elements are ionized can depend on the type of irradiation. For example, fluorites with the same trace element contents can develop different colors in the vicinity of thorium-containing minerals than in the vicinity of uranium-containing minerals. Furthermore, the temperature history can influence the color, as can the incorporation of oxygen ions and OH − or other coloring ions. Trace amounts of non-coloring ions such as Na + or Fe 3+ stabilize coloring lattice defects and thus also influence the color. 

Yellow: The color of yellow fluorites is due to the incorporation of O 3 − and O 2 − in place of two adjacent F − ions. Charge balancing occurs through the replacement of Ca 2+ by Na +. 

Light green: The light green color of many fluorites is due to trace amounts of Sm 2+. Samarium (Sm) is incorporated as Sm 3+ instead of Ca 2+. Reduction to Sm 2+ occurs by the absorption of an electron released during the oxidation of other cations by ionizing radiation. 

The stability of the green color depends on which cations are oxidized. Fluorites formed under reducing conditions contain traces of Fe 2+, which can be oxidized to Fe 3+ and, together with samarium, produces a temperature-stable green color. In fluorites formed under more oxidizing conditions, the generation and stabilization of the green-coloring Sm 2+ ions occurs by oxidation of Ce(Pr, Tb) 3+ to Ce(Pr, Tb) 4+. Fluorites colored by this mechanism bleach in daylight or upon heating. 

Yellow-green: Fluorites from some localities (e.g., Redruth in England) exhibit a green to yellow-green coloration, which is due to the co-occurrence of traces of yttrium (Y 3+) and cerium (Ce 3+), each with a color center (vacancy at an F − position containing two electrons) in close proximity. 

Light blue: Fluorites containing only Y 3+, together with a vacancy at an F − position containing two electrons, are colored light blue. 

Dark blue: Synthetic fluorites can be colored intensely blue to violet by the formation of colloidal calcium (metallic). The blue-violet color of the naturally occurring "Blue John Fluorite" from Castelton near Derbyshire in England is also attributed to this. 

Violet: The cause of the widespread violet coloration of natural fluorite is not fully understood. Currently, electron defects in the crystal lattice are considered the most likely cause of the violet color of fluorite. 

Pink, red: The pink to red color of fluorites is caused by the incorporation of O 2 3− molecules, which are stabilized by neighboring Y 3+ ions. 

Stinkspar (antozonite)

Stink spar is a dark purple to black variety of fluorite that develops a pungent odor when crushed. Stink spar often (but not always) occurs together with uranium minerals, some of which may be enclosed in the spar as very fine particles. The type locality and best-known German site is Wölsendorf in the Upper Palatinate.

Rubbing or striking the crystal releases gaseous toxic fluorine (F 2), which causes the odor.

The dark purple to black color has several causes. Colloidal metallic calcium plays a major role, resulting in a dark blue to black color. In addition, there are free electrons on empty fluorine positions (F centers), which are typical of violet fluorite. 

All of these properties of fluorite are due to radioactive irradiation of the fluorite. Fluorite typically occurs together with uranium-bearing minerals. The uranium and thorium they contain decay, emitting gamma radiation. This radiation releases electrons from the F− ions, forming an H center: a neutral fluorine atom in an otherwise empty lattice position that forms an atomic bond with a neighboring F− ion. The released electrons are captured by lattice vacancies, empty fluorine positions, where they form F centers: single electrons in an F− position surrounded by four Ca2 + ions. These F centers are not spatially stable. They diffuse through the crystal lattice and combine with other F centers to form fluorine-free Ca nanoparticles with a diameter of 5 to 30 nm. These clusters are also called “colloidal Ca” and contribute to the blue-black color of the smelt. 

Varieties

To date, several varieties of fluorite are known in which small amounts of calcium are replaced by rare earths such as cerium and yttrium:

Yttriumfluorite (1911) and cerium fluorite were first described by Thorolf Vogt as new mineral species from northern Norway. After further melt-flow analyses in 1913 by Gustav Tammann and Vogt, it was determined that fluorite can contain up to 50% by weight yttrium fluoride (YF 3, yttrofluorite) and up to 55.8% cerium(III) fluoride (CeF 3, cerium fluorite). Therefore, a three-component solid solution system theoretically exists, although the idealized compositions YF 3 and CeF 3 have so far only been known synthetically and have only been investigated up to the above-mentioned percentages of fluorite. 

Neither cerium fluorite nor yttrium fluorite have so far been discovered in nature in a material purity high enough to be recognized by the International Mineralogical Association (IMA). Yttrofluorite was officially discredited by the IMA in 2006, while cerium fluorite remained listed as a hypothetical mineral in the IMA mineral list until 2009. 

Yttrocerite was described as a mineral in 1815 by Johan Gottlieb Gahn and Jöns Jakob Berzelius; However, in 1913, Vogt stated in his analytical results that it was a mixture of yttrofluorite and cerium fluorite. 

Occurrence and mining

Fluorite forms as a late-crystallizing mineral in felsic igneous rocks typically through hydrothermal activity. It is particularly common in granitic pegmatites. It may occur as a vein deposit formed through hydrothermal activity particularly in limestones. In such vein deposits it can be associated with galena, sphalerite, barite, quartz, and calcite. Fluorite can also be found as a constituent of sedimentary rocks either as grains or as the cementing material in sandstone.

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 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).

One of the largest deposits of fluorspar in North America is located on 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 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 wide lenses. The area with veins of known workable size comprises about 60 square miles (160 km2).

In 2018, Canada Fluorspar Inc. commenced mine production again 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, 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.

Cubic crystals up to 20 cm across have been found at Dalnegorsk, Russia. The largest documented single crystal of fluorite was a cube 2.12 meters in size and weighing approximately 16 tonnes.

In Asturias (Spain) there are several fluorite deposits known internationally for the quality of the specimens they have yielded. In the area of Berbes, 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 the Emilio 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 the Moscona 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.

"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. 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 Blue John Cavern and Treak Cliff Cavern.

Recently discovered deposits in China have produced fluorite with coloring and banding similar to the classic Blue John stone.

Uses

As a raw material

Fluorite is mainly used industrially

as metallurgical spar in the metal industry as a flux for slag in the iron and steel process, especially as an additive in the Siemens-Martin furnace and in the electric arc furnace, and for the production of artificial cryolite for the production of aluminium,

as acid spar for the production of fluorine and hydrofluoric acid as well as various fluorides and derivatives such as fluorocarbons and polymeric fluorine compounds (e.g. polytetrafluoroethylene),

as ceramic spar in the glass industry as a flux and opacifier for e.g. milk glass, frosted glass and opalescent glasses, for ceramic materials and as a basic material for optical lenses (CaF2 single crystals, fluoride glasses based on beryllium fluoride, fluorite and sodium fluoride). Due to the property of refracting the light spectrum evenly, the chromatic aberration of lenses can be compensated. The problem here is that particularly large crystals are needed for high-performance lenses, and these are grown artificially. Crystals of this size have the property of warping due to heat (from sunlight) to such an extent that they significantly change the optical calculations.

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:

CaF2(s) + H2SO4 → CaSO4(s) + 2 HF(g)

The resulting HF is converted into fluorine, fluorocarbons, and diverse fluoride materials. As of the late 1990s, five billion kilograms were mined annually.

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.

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 react with excess sodium from the alumina to form Na3AlF6.

Uses as a gemstone

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. It is also used for ornamental carvings, with expert carvings taking advantage of the stone's zonation.

Due to its relatively low hardness and perfect cleavage, fluorite is of little interest as a gemstone for the commercial jewelry industry. Occasionally, it is processed by glyptics and hobby cutters into small, handcrafted objects or faceted gemstones. However, because of its wide range of colors, it can be confused with many gemstone minerals, and is often used as a basis for imitations. To change the colors, fluorite is either fired or irradiated. To protect against damage or to cover cracks, fluorite gemstones are often stabilized with synthetic resin.

Optics

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 150 to 9000 nm) and exhibits an 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.

Fluorite should not be confused with fluoro-crown (or fluorine crown) glass, a type of low-dispersion glass that has special optical properties approaching fluorite. True fluorite is not a glass but a crystalline material. Lenses or optical groups made using this low dispersion glass as one or more elements exhibit less chromatic aberration than those utilizing conventional, less expensive crown glass and flint glass elements to make an achromatic lens. Optical groups employ a combination of different types of glass; each type of glass refracts 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. 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). 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.

The use of fluorite for prisms and lenses was studied and promoted by Victor Schumann near the end of the 19th century. 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-performance optical telescope and camera lens elements. 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 better telephoto 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.

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. The fluorite also serves to correct optical 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 of ultraviolet images. Konica produced a fluorite lens for their SLR cameras – the Hexanon 300 mm f/6.3.

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. Fluorite is normally colorless, 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 color of fetid fluorite is predominantly due to the calcium atoms remaining. Solid-state fluorine-19 NMR carried out on the gas contained in the antozonite, revealed a peak at 425 ppm, which is consistent with F2.

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