Editing Fluorite (section)

==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]]:
:CaF<sub>2</sub>([[solid|s]]) + H<sub>2</sub>SO<sub>4</sub> → [[calcium sulfate|CaSO<sub>4</sub>]](s) + 2 HF([[gas|g]])

The resulting HF is converted into fluorine, [[fluorocarbon]]s, and diverse fluoride materials. As of the late 1990s, five billion kilograms were mined annually.<ref name=Aigueperse>{{Cite book |first= Jean |last= Aigueperse |author2=Paul Mollard |author3=Didier Devilliers |author4=Marius Chemla |author5=Robert Faron |author6=Renée Romano |author7=Jean Pierre Cuer |contribution= Fluorine Compounds, Inorganic |title= Ullmann's Encyclopedia of Industrial Chemistry |year= 2005 |publisher= Wiley-VCH |place= Weinheim|doi= 10.1002/14356007.a11_307 |isbn= 3527306730}}</ref>

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% CaF<sub>2</sub>), the lowest of the three grades, has traditionally been used as a [[flux (metallurgy)|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% CaF<sub>2</sub>) is used in the manufacture of [[opalescence|opalescent]] [[glass]], [[vitreous enamel|enamels]], and cooking utensils. The highest grade, "acid grade fluorite" (97% or more CaF<sub>2</sub>), 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]].<ref name=usgs/>

Internationally, acid-grade fluorite is also used in the production of [[Aluminium fluoride|AlF<sub>3</sub>]] and [[cryolite]] (Na<sub>3</sub>AlF<sub>6</sub>), which are the main fluorine compounds used in aluminium smelting. [[Alumina]] is dissolved in a bath that consists primarily of molten Na<sub>3</sub>AlF<sub>6</sub>, AlF<sub>3</sub>, and fluorite (CaF<sub>2</sub>) to allow electrolytic recovery of aluminium. Fluorine losses are replaced entirely by the addition of AlF<sub>3</sub>, the majority of which react with excess sodium from the alumina to form Na<sub>3</sub>AlF<sub>6</sub>.<ref name=usgs>Miller, M. Michael. [http://minerals.usgs.gov/minerals/pubs/commodity/fluorspar/myb1-2009-fluor.pdf Fluorspar], USGS 2009 Minerals Yearbook</ref>

===Niche uses===
[[File:Fluorite Crawford Cup AD 50 100.jpg|thumb|Crawford Cup (Roman, 50-100 CE) in the collection of the [[British Museum]].<ref>{{cite web|title=The Crawford Cup|url=https://www.britishmuseum.org/explore/highlights/highlight_objects/gr/t/the_crawford_cup.aspx|publisher=[[British Museum]]|access-date=20 December 2014}}</ref> Made of fluorite.]]

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

====Optics====
{{See also|Fluoride glass}}
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&nbsp;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&nbsp;nm, a common wavelength used for [[semiconductor]] stepper manufacture for [[integrated circuit]] [[Photolithography|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 Inc.|Canon]] and other manufacturers have used synthetically grown crystals of calcium fluoride components in lenses to aid [[apochromatic]] design, and to reduce [[Dispersion (optics)|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 [[List of lens designs|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 (optics)|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 [[Refraction|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 (see above). Fluoro-crown glass (such as Schott FK51) usually in combination with an appropriate [[Flint glass|"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).<ref>{{cite web|url=https://www.schott.com/advanced_optics/english/knowledge-center/technical-articles-and-tools/abbe-diagramm.html|title=Interactive Abbe Diagram|publisher=SCHOTT AG|year=2019|access-date=February 20, 2018}}</ref> 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.<ref>Rutten, Harrie; van Venrooij, Martin (1988). Telescope Optics Evaluation and Design.  Willmann-Bell, Inc.</ref>

The use of fluorite for prisms and lenses was studied and promoted by [[Victor Schumann]] near the end of the 19th century.<ref name="Lyman">{{Cite journal|last=Lyman|first=T.|title=Victor Schumann|journal=Astrophysical Journal|volume=38|pages=1–4|year=1914|doi=10.1086/142050|bibcode=1914ApJ....39....1L|doi-access=free}}</ref> 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 [[magnification]]s. [[Canon Inc.]] produces synthetic fluorite crystals that are used in their better [[telephoto lens]]es. 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.<ref>{{cite book|url=https://books.google.com/books?id=ZqTkCF6Ra9kC&pg=PA339|page=339|title=Bulk crystal growth of electronic, optical & optoelectronic materials|author=Capper, Peter |publisher=John Wiley and Sons|year= 2005|isbn=0-470-85142-2}}</ref>

Exposure tools for the [[semiconductor]] industry make use of fluorite optical elements for [[ultraviolet light]] at [[wavelength]]s of about 157 [[nanometer]]s. Fluorite has a uniquely high transparency at this wavelength. Fluorite [[Objective (optics)|objective lenses]] are manufactured by the larger microscope firms (Nikon, [[Olympus Corporation|Olympus]], [[Carl Zeiss AG|Carl Zeiss]] and Leica). Their transparence to ultraviolet light enables them to be used for [[Fluorescence microscope|fluorescence microscopy]].<ref>{{cite book|url=https://books.google.com/books?id=IaQOh28E0vgC&pg=PA157|page=157|title=Photography with a microscope|author1=Rost, F. W. D.  |author2=Oldfield, Ronald Jowett |publisher=Cambridge University Press|year=2000|isbn=0-521-77096-3}}</ref> The fluorite also serves to correct [[optical aberration]]s in these lenses. [[Nikon]] has previously manufactured at least one fluorite and synthetic quartz element camera lens (105&nbsp;mm f/4.5 UV) for the production of [[ultraviolet photography|ultraviolet images]].<ref>{{cite book|url=https://books.google.com/books?id=AEFPNfghI3QC&pg=PA388|pages=387–388|title=Scientific photography and applied imaging|author=Ray, Sidney F. |publisher=Focal Press|year=1999|isbn=0-240-51323-1}}</ref> [[Konica]] produced a fluorite lens for their SLR cameras – the Hexanon 300&nbsp;mm f/6.3.