Scheelite

Scheelite is a lasing medium.

Properties

Its crystals are in the streaks white and is brittle.

Gems cut from transparent material are fragile yet attractive: Scheelite's diamond. Owing to low hardness, however, cut scheelites are best enjoyed unset as valuable collector's pieces.

Rockhounds treasure scheelite for its 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 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 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.
• Mineral Galleries
• Webmineral
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