Scheelite

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.

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Properties

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, 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.(Bulletin, p81). 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 presence of molybdenum trace impurities occasionally results in a green glow.

Composition

The scheelite structure consists of isolated tetrahedra. The tetrahedra form (non touching) 1D chains. There are two directions that the "chains" line up (Figure 3). Tungsten deposits only occur where mineralization has taken place at high temperatures and pressures. Research shows these were deposited mostly between 200 to 500 degrees Celsius, and from 200 to 1,500 bars. (Ore deposits of the western states, p518, 535).

Special Characteristics

Scheelite is often found to have a grayish white color; yellowish, brownish or translucent (Figure 1). Its streak is white. Scheelite has a greasy luster which helps distinguish it. Moreover, when looking for scheelite, miners use ultraviolet light which causes it to fluoresce with a bright blue color. Many prospectors for scheelite have made good use of scheelite's typically bright blue fluorescence by searching for scheelite deposits by night with ultraviolet lamps (Figure 2). Many old mines have even been reopened after examination of the mine shafts with ultraviolet lamps have proven that the ore is not quite yet exhausted. Tungsten can be combined with carbon, and when it does, it forms tungsten carbide. This substance is one of the hardest known other than diamond. This substance is used in abrasive wheels and cutting tools, which the demand for is steadily increasing. (Bulletin, p81).

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 earth elements, but the spectrum produced is unlike that of natural stones.

History

Named in 1821 after Carl Wilhelm Scheele (1742-1786).(The Manual of Mineral Science, p426). The Swedish chemist and apothecary, proved the existence of tungstic oxide in the mineral in 1781. Born in Stralsund, Pomerania, he grew up studying chemistry. He then opened a pharmacy where he continued his research and soon made many original discoveries. Some of the papers he wrote were related to many important minerals today such as quartz, alum and clay. He also made many important discoveries not related to minerals such as lactic acid being the source of the acidity of sour milk. His discovery in 1781 was probably his call to fame, where a mineral was named after him. This discovery was about the composition of the mineral Tungsten, later called Scheelite (Calcium Tungstate). From this he obtained tungstic acid, which he is also famous for discovering.

References

• Anderson, B. W., Jobbins, E. A. (Ed.) (1990). Gem testing. Butterworth & Co Ltd, Great Britain. ISBN 0-408-02320-1
• Lindgren, W. (1933) Ore deposits of the western states lindgren, 518, 535, 555.
• Klein, C. (2002) The Manual of Mineral Science 23, 426.
• University of Arizona State Bureau of Mines. (1975) Bulletin 182, 80-81.
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