Powellite, CaMoO₄, is isostructural with scheelite which is why it forms similar crystals. The two minerals form a series in which the tungsten of scheelite is substituted for by the molybdenum of powellite. Powellite fluoresces a yellow color while scheelite fluoresces a bright blue under short wave ultraviolet light. Of course since molybdenum can substitute for tungsten, some scheelite specimens will show a yellow fluorescence.

The crystals of scheelite can look like fluorite octahedrons which can also fluoresce. However, fluorite has perfect octahedral cleavage and a lower luster. Massive scheelite has often been mistaken for massive quartz, but then the fluorescence of scheelite is a dead giveaway.

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. 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.

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When tungsten carbide cutting tools are employed in machining operation, the entire process of chip formation comprises primary shear, secondary shear and fracture process.

The primary shear of chip formation process is also referred to the workpiece material’s plastic deformation, which is severe and could be found in front of the cutting edge where the chip body is  delineated from a large amount of workpiece material. For the secondary shear, the chip slides over the rack face of the tungsten carbide tool. In the final separation stage, a fracture process takes over.

It is shown that the relative ease of the chip formation process is determined by the workpiece material’s flow properties. In addition, besides the existing conditions, the varied chips’ size and shapes are also under the control of factors like workpiece ductility, inclusion characteristics and etc..


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Chemistry: CaWO4, Calcium Tungstate

Class: Sulfates

Subclass: Tungstates

Uses: An important source of tungsten, rarely cut as gemstones and as mineral specimens.

Scheelite is an important ore of tungsten which is a strategically important metal. Scheelite is named for the discoverer of tungsten, K. W. Scheele. Although most of the world wide production of tungsten comes from the mineral wolframite, scheelite is especially abundant in the US and provides the United States with most of its supply.

Scheelite is a popular mineral for collectors. It forms perfect tetragonal dipyramidal crystals that look very much like octahedrons. These pseudo-octahedral crystals are sometimes truncated with minor pyramids, but only on the top and/or bottom points of the crystal; giving evidence of their true symmetry. Other minerals that form pseudo-octahedral crystals similar to scheelite include wardite, anatase and powellite.

 

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Chipping

It is known that during the machining operation, when sudden loads or thermal shocks occur, there is a potential for the tungsten carbide inserts to suffer from severe damage due to cracking or chipping.

In some comprehensive model, the researchers have described in details, the toughness of tungsten carbide is strongly influenced by the binder composition. It has been pointed out that the toughness of WC-Co is superior to that of TiC-Ni, in addition, the problem of tungsten carbide inerts failure due to chipping is particularly severe for alumina based tools, but with zirconia additions, the toughness of  tungsten carbide tool could be markedly improved.


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