Tungsten occurs in nature only in the form of chemical compounds. Although more than thirty tungsten bearing minerals are known, only two of them are important for industrial use, namely wolframite and scheelite.
Pure scheelite has blue-white fluorescence in ultraviolet light, a property which is utilised in prospecting. Wolframite is a general term for iron and manganese tungstates where the iron/manganese ratio can vary. A mineral with more than 80% FeWO4 is called ferberite and a mineral with more than 80% MnWO4 is called hübnerite.
All tungsten deposits are of magmatic or hydrothermal origin. During cooling of the magma, differential crystallisation occurs, and scheelite and wolframite are often found in veins where the magma has penetrated cracks in the earth's crust. Most of the tungsten deposits are in younger mountain belts, ie the Alps, the Himalayas and the circum-Pacific belt.
The concentration of workable ores is usually between 0.3 and 1.0% WO3.
For the full story, click to read the article in Newsletter June 2006.