Scheelite

Scheelite
General
Category	Tungstate mineral
Formula (repeating unit)	CaWO4
IMA symbol	Sch[1]
Strunz classification	7.GA.05
Crystal system	Tetragonal
Crystal class	Dipyramidal (4/m) H-M symbol: (4/m)
Space group	I41/a
Unit cell	a = 5.2429(3), Å c = 11.3737(6) Å; Z = 4
Identification
Color	Colorless, white, gray, dark brown, brown, tan, pale yellow, yellow-orange, golden yellow, pale shades of orange, red, green, etc.; colorless in transmitted light and may be compositionally color zoned
Crystal habit	Pseudo-octahedra, massive, columnar, granular
Twinning	Common, penetration and contact twins, composition plane {110} or {001}
Cleavage	On {101}, distinct; on {112}, interrupted; on {001}, indistinct
Fracture	Subconchoidal to uneven
Tenacity	Brittle
Mohs scale hardness	4.5–5
Luster	Vitreous to adamantine
Streak	White
Diaphaneity	Transparent to opaque
Specific gravity	5.9–6.1
Optical properties	Uniaxial (+)
Refractive index	nω = 1.918–1.921, nε = 1.935–1.938
Birefringence	δ = 0.017
Pleochroism	Definite dichoric in yellow (yellow to orange-brown)
Fusibility	With difficulty
Solubility	Soluble in alkalis. Insoluble in acids
Other characteristics	Fluorescence under short-wave UV is bright blue, bluish white to yellow. Specimens with more molybdenum tend to fluoresce white to yellow, similar to powellite. Occasionally fluoresces red under mid-wave UV.
References	[2][3][4][5]

Scheelite is a calcium tungstate mineral with the chemical formula CaWO₄. It is an important ore of tungsten (wolfram). Scheelite is originally named after Swedish chemist Carl Wilhelm Scheele (1742–1786). Well-formed crystals are sought by collectors and are occasionally fashioned into gemstones when suitably free of flaws. Scheelite has been synthesized using the Czochralski process; the material produced may be used to imitate diamond, as a scintillator, or as a solid-state lasing medium. It was also used in radium paint in the same fashion as was zinc sulphide, and Thomas Edison invented a fluoroscope with a calcium tungstate-coated screen, making the images six times brighter than those with barium platinocyanide; the latter chemical allowed Röntgen to discover X-rays in early November 1895. Note, the semi-precious stone marketed as 'blue scheelite' is actually a rock type consisting mostly of calcite and dolomite, with occasional traces of yellow-orange scheelite.