Scheelite
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| Scheelite | |
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| General | |
| Category | Mineral |
| Formula | Calcium tungstate - CaWO4 |
| Identification | |
| Colour | Golden yellow, brownish green, brown, pinkish to reddish gray, colourless |
| Habit | Pseudo-octahedra, massive, columnar, granular |
| System | Tetragonal |
| Cleavage | Distinct, two directions |
| Fracture | Subconchoidal to uneven - brittle |
| Hardness | 4.5-5 |
| Lustre | Vitreous to adamantine |
| RI | 1.918–1.937 (DR +0.016) |
| Pleochroism | Definite dichoric in yellow (yellow to orange-brown) |
| Streak | White |
| SG | 5.9–6.1 |
| Fusibility | With difficulty |
| Solubility | Soluble in acids |
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 fastened 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.
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Properties
Its crystals are in the tetragonal crystal system, appearing as dipyramidal pseudo-octahedra. Colours include golden yellow, brownish green to dark brown, pinkish to reddish gray, and colourless. 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. Twinning is also commonly observed and crystal faces may be striated. Scheelite streaks white and is brittle.
Gems cut from transparent material are fragile yet attractive: Scheelite's refractive index (1.918–1.937 uniaxial positive, with a maximum birefringence of 0.016) and dispersion (0.026) are both moderately high. These factors combine to result in scheelite's high lustre and perceptible "fire", approaching that of diamond. Owing to low hardness, however, cut scheelites are best enjoyed unset as valuable collector's pieces.
Rockhounds treasure scheelite for its fluorescent properties: under shortwave ultraviolet light, the mineral glows a bright sky-blue. The prescence of molybdenum trace impurities occasionally results in a green glow.
Occurrence
Scheelite is associated with wolframite in placer deposits and forms in pegmatites, hydrothermal veins, and areas of contact metamorphism. Other associated minerals include fluorite, muscovite, dolomite and molybdenite. Its type locality is the Bisperg iron mine in Säter, Dalecarlia, Sweden. Officially recognized in 1821, scheelite was named after Swedish chemist Carl Wilhelm Scheele, the discoverer of tungsten.
Crystals exceeding 0.5 kilograms (1 pound) have been found in Brazil: Other notable localities include Australia, Austria, Bolivia, Burma, England, Finland, France, Italy, Japan, Sri Lanka, Switzerland and the United States. The Sichuan Province of China has emerged as a newly important source with many "gemmy" specimens recovered. Fine crystals are also found in Tong Wha, Korea.
Synthetics
Although it is now uncommon as a diamond imitation—much more convincing products, like cubic zirconia and moissanite have long since superseded it—synthetic scheelite is occasionally offered as natural scheelite, and collectors may thus be fooled into paying high prices for them. Gemmologists distinguish natural scheelite from synthetic material mainly by microscopic examination: Natural material is very seldom without internal growth features and inclusions (imperfections), while synthetic material is usually very clean. Distinctly artificial curved striae and clouds of minute gas bubbles may also be obvserved in synthetic scheelite.
The visible absorption spectrum of scheelite, as seen by a hand-held (direct vision) spectroscope, may also be of use: Most natural stones show a number of faint absorption lines in the yellow region of the spectrum (~585 nm) due to praseodymium and neodymium trace impurities. Conversely, synthetic scheelite is often without such a spectrum. Some synthetics may however be doped with neodymium or other rare earths, but the spectrum produced is unlike that of natural stones.
References
- Anderson, B. W., Jobbins, E. A. (Ed.) (1990). Gem testing. Butterworth & Co Ltd, Great Britain. ISBN 0408023201
- Mineral Galleries (http://mineral.galleries.com/minerals/sulfates/scheelit/scheelit.htm)
- Webmineral (http://webmineral.com/data/Scheelite.shtml)pl:Szelit