wolframite

A mineral with composition (Fe,Mn)WO₄, intermediate between ferberite and huebnerite, which form a complete solid solution series. Wolframite occurs commonly in short, brownish-black, monoclinic, prismatic, bladed crystals. It is probably the most important tungsten mineral. China is the major producer of wolframite. Tungsten minerals of the wolframite series occur in many areas of the western United States; the major producing district is Boulder and northern Gilpin counties in Colorado. See also Huebnerite.

Environment

High-temperature and medium-temperature quartz veins, occasionally in granitic pegmatites.

Wolframite usually occurs in fair-sized 1-6-in. (3-15 cm) black blades commonly embedded in or surrounded by quartz crystals, the edges revealing the perfect cleavage on the fracture face. More often than not the crystals are twinned on the front face so the termination has a V-shaped notch. Ferberite can also form crusts of small stubby black blades on rock surfaces in open cavities (Colorado); also massive granular. Ferberite is dead black and completely opaque; huebnerite tends toward reddish brown, usually showing color through clinging translucent cleavage flakes over inner cracks.

Black to red-brown. Luster submetallic; hardness 4-4Ɖ; specific gravity 7.1-7.5; streak black to red-brown; fracture uneven; cleavage perfect side pinacoid. Extremely brittle; ferberite is opaque, huebnerite translucent in thin flakes.

Tungstates of iron and manganese, the two elements form a continuous series with end members known as ferberite (with 23.7% FeO and 76.3% WO ₃ ) and huebnerite (with 23.4% MnO and 76.6% WO ₃ ).

Decrepitates (flies apart), then fuses to a globule that assumes, on solidifying, a slightly faceted (crystalline) surface (that from ferberite is magnetic). The best tungsten test is to fuse the material on the charcoal under sodium carbonate, dissolve the slaggy mixture in strong hydrochloric acid, and add pure tin, which imparts a blue color to the solution. It's best to try out this test several times with known tungsten-bearing ores, to acquire the technique. Exaggerated heat sensitivity makes crystals vulnerable to damage if too intensely illuminated, as in the mineral-show display cases.

The tungsten test is the best guide and may be essential. Manganese-rich huebnerite can resemble goethite, which is magnetic (huebnerite is not magnetic after fusion) and will give no tungsten test. It also resembles columbite, tantalite, and manganotantalite, and again the tungsten test is the one to apply, although tungsten sometimes can be found in these minerals; then the problem may become complicated.

Wolframite is a widely distributed mineral most often found in deep-seated quartz veins. Frequently shows embedded blades extending in from the wall, so perfect that they could serve as textbook vein illustrations. The perfect cleavage is very pronounced and helps to distinguish it from the interrupted surface of columbite. However, though pegmatitic occurrences of wolframite are rare, they are the customary environment of columbite-tantalite, so with knowledge of the formation on which you are hammering, laboratory distinction is largely only of academic interest. Ferberite, on the other hand, can also form drusy crusts of small, free-growing, brilliant black crystals over rock surfaces in open low-pressure but high-temperature veins, like those of Nederland, Boulder Co., Colorado, and Llallagua, Bolivia. The finest large crystals come from Panasqueira, Portugal, from n. Peru, and from Tong Wha, South Korea.

Specimens have been found in quartz veins at Frederick, Missouri; Ouray Co., Colorado; Lincoln Co., New Mexico; Nye Co., Nevada; and Townsville, North Carolina. Ferberite is best developed in Colorado with similar specimens in Bolivia. Colorado huebnerite veins form brown quartz-coated sprays. Daredevil dealers dissolve the quartz with hydrofluoric acid -- a risky, not recommended procedure -- to obtain fragile specimens of the crystals.

An important ore of tungsten. Its name refers to the early name for tungsten, which, in an ore mixture with tin ore, reduced the recovery of that metal by "wolfing" the ore.