Yellowish to brownish but sometimes almost orange-yellow in color, the calcium tungstate mineral scheelite is an important ore of tungsten. Although most of the world wide production of tungsten comes from the mineral wolframite, scheelite is abundant in the United States and provides the country with most of its supply. Scheelite was named for the discoverer of tungsten, K. W. Scheele, the Swedish chemist who proved the existence of tungsten in the mineral.
Scheelite is a member of the tetragonal system and crystals often show dipyramidal habits 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. The crystals of scheelite can look like fluorite octahedrons which can also fluoresce but fluorite has perfect octahedral cleavage and a much lower luster. Massive sheelite has also been misidentified as massive quartz but its strong fluorescence provides conclusive proof of its true identity.
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. Since molybdenum can substitute for tungsten, some scheelite specimens will also show a yellow fluorescence.
Many prospectors for scheelite have made good use of scheelite's typically bright blue fluorescence by searching for scheelite deposits at night with ultraviolet lamps. Many old mines have even been reopened after examinations of the mine shafts with ultraviolet lamps which proved that the ore was not yet exhausted.
Associated Minerals include quartz, garnets, vesuvianite, epidote, topaz, schorl, apatite, gold,
silver, molybdenite, cassiterite, wolframite and fluorite. The best field indicators are crystal habit, color, density, luster and especially SW fluorescence.
