Fluorite

Cubic fluorite crystals from China

- align=center bgcolor="#9966FF"	Fluorite		- align=center	- align=center bgcolor="#9966FF"	General		-	Category	Mineral	-	Chemical formula	calcium fluoride CaF₂	- align="center" bgcolor="#9966FF"	Identification		-	Color	White or colorless, purple, blue, blue-green, green, yellow, brownish-yellow, pink, or red.	-	Crystal habit	Occurs as well-formed coarse sized crystals also massive - granular.	-	Crystal system	Isometric 4/m bar 3 2/m.	-	Cleavage	Perfect, ^* Perfect.	-	Fracture	Uneven.	-	Mohs Scale hardness	4	-	Luster	Vitreous.	-	Refractive index	1.433-1.435	-	Pleochroism	-	-	Streak	White.	-	Specific gravity	3.18	-	Fusibility	3	-	Solubility	Slightly in water.	-	Other	sometimes phosphoresces when heated or scratched. Other varieties fluoresce beautifully.	- align="center" bgcolor="#9966FF"

Fluorite (also called fluor-spar) 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. Crystal twinning is common and adds complexity to the observed crystal habits.

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. In the United States deposits are found in Missouri, Oklahoma, Illinois, Kentucky, Colorado, New Mexico, Arizona, Ohio, New Hampshire, New York, 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.

Fluorescence

Fluorite gives its name to the property of fluorescence, as many samples fluoresce strongly in ultra-violet light. The fluorescence may be due to impurities such as yttrium or organic matter in the crystal lattice.

Fluorite's fluorescence color is largely dependent on where the original specimen was located. Blue is the most common color but red, purple, yellow, green and white also occur. Fluorite also exhibits the property of thermoluminescence.

Blue John

One of the most famous of the older localities of fluorite is Castleton in Derbyshire, England, where, under the name of Derbyshire Blue John, beautiful purple-blue fluorite was used for ornamental purposes, especially in the 19th century. The name derives from French "bleu et jaune" (blue and yellow) characterising its colour. It is now scarce, and only a few hundred kilograms are mined each year for ornamental and lapidary use. Recent deposits in China have produced fluorite with similar colouring and banding to the classic Blue John stone.

Uses

As well as ornamental uses, fluorite is used in the making of opalescent glass, enamels for cooking utensils, hydrofluoric acid, and as a flux in the manufacture of steel. Fluorite is also used in some high performance telescopes and camera lens elements instead of glass. Exposure tools for the semiconductor industry, make the use of fluorite for the optics for 157 nm wavelength. This wavelength is created by an excimer laser with F₂ gas, and the fluorite is a unique material that has high transparency at this wavelength. It has a very low dispersion so light diffraction is far less than ordinary glass, and in telescopes it allows crisp images of astronomical objects even at high power. The name fluorite is derived from the Latin fluo, meaning "flow", in reference to its use as a flux. Fluorite is slightly soluble in water, and is decomposed by sulfuric acid to form free hydrofluoric acid, which etches glass.

References

Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, pp. 324 - 325, 20th ed., ISBN 0471805807
Mineral Galleries
Webmineral
Mindat.org
Illinois State Geologic Survey
Illinois state mineral
Barber Cup and Crawford Cup, related Roman cups at British Museum.

Gallery

Image:pig.fluorite.750pix.jpg|Pig carved in fluorite, 5 cm (2 inches) long Image:USDA Mineral Flourite 93c3962.jpg|Mineral fluorite Image:Fluorite USA.jpg|Octahedral fluorite crystals from New Mexico, USA image:Fluorite_crystals_270x444.jpg|Cleaved fluorite octahedra.

Calcium minerals | Halide minerals | Derbyshire