Calcium tungstate
| Scheelite | |
|---|---|
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|
| General | |
| Category | Tungstate mineral |
|
Formula (repeating unit) |
CaWO4 |
| 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) c = 11.3737(6) [Å]; Z = 4 |
| Identification | |
| Color | Colorless, white, gray, brown, pale yellow, yellow-orange, pale shades of orange, red, green; 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 | |
Scheelite is a calcium tungstate mineral with the chemical formula CaWO4. It is an important ore of tungsten. Well-formed crystals are sought by collectors and are occasionally fashioned into gemstones when suitably free of flaws. Scheelite has been synthesized via 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 that those with barium platinocyanide; the latter chemical allowed Röntgen to discover X-rays in early November 1895.
Its crystals are in the tetragonal crystal system, appearing as dipyramidal pseudo-octahedra. Colors include golden yellow, brownish green to dark brown, pinkish to reddish gray, orange and colorless. Transparency ranges from translucent to transparent and crystal faces are highly lustrous (vitreous to adamantine). Scheelite possesses distinct cleavage and its fracture may be subconchoidal to uneven. Its specific gravity is high at 5.9–6.1 and its hardness is low at 4.5–5. Aside from pseudo-octahedra, scheelite may be columnar, granular, tabular or massive in habit. Druzes are quite rare and occur almost exclusively at Zinnwald, Czech Republic. Twinning is also commonly observed and crystal faces may be striated. Scheelite streaks white and is brittle.
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