Chemistry: CaWO4,
Calcium Tungstate
Crystal System: tetragonal
Class: Sulfates
Subclass: Tungstates
Color: white, yellow, orange
or greenish gray to brown.
Transparency: Crystals are
transparent to translucent.
Cleavage: indistinct in two
directions and good in another
Fracture: conchoidal.
Hardness: 4.5 - 5.
Streak: white.
Luster: adamantine to greasy.
Specific Gravity: apx. 5.9 - 6.1
PHYSICAL CHARACTERISTICS:
Scheelite forms perfect tetragonal dipyramidal crystals that look very
much like octahedrons. The crystals may also be truncated with minor
pyramids, on the top and/or bottom points of the crystal. Which show
Scheelite's true form. Scheelite may also be found in massive and granular
form.
Other minerals that form crystals similar to Scheelite include wardite,
anatase and powellite. 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 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.
Specimens from worldwide localities show little difference in
their color of fluorescence.
Unlike other minerals, Scheelite is a "self-activated" mineral. Its
fluorescence is due to the mineral itself, rather than some chance chemical
impurity. The characteristic blue to
bluish-white fluorescence of this species is a valuable property in
prospecting for Scheelite deposits at night. Old mines have even been
reopened when mine shafts were examined with ultraviolet lamps.
As mentioned above, Scheelite crystals can be mistaken as octahedron
crystals. So fluorite with it's perfect octahedral cleavage and fluorescence.
Can be mistaken for the brownish orange Scheelite. Massive Scheelite
has often been mistaken for massive quartz, but then the fluorescence
of Scheelite is a dead giveaway.
Associated Minerals are quartz, garnets, vesuvianite, epidote, topaz,
schorl, apatite, gold, silver, molybdenite, cassiterite, wolframite
and fluorite.
Since Scheelite is a primary ore of tungsten we will add some
information about tungsten. Tungsten is a very hard, silver-white to
steel-gray metal with a body-centered cubic crystalline structure. In
its chemical properties it resembles molybdenum.
It is sometimes called wolfram, and it's chemical symbol "W"
is taken from this name. When naming compounds of tungsten, use of the
name wolfram as a root is preferred. Tungsten is one of the most dense
metals and has a higher melting point than any other metal. It melts
at about 3,410°C and boils around 5,660°C. Pure tungsten is ductile,
and wires made of it, even those of very small diameter, have a very
high tensile strength.
The element is resistant to ordinary acids and aqua regia but dissolves
in a mixture of hydrofluoric and nitric acids. It forms compounds with
carbon, chlorine, oxygen, sulfur, and some other elements. It forms
tungstic acid (H2WO4), or wolframic acid, which is the basis
of a series of salts called tungstates, or wolframates.
Tungsten metal is used extensively for filaments for light bulbs and
electronic tubes. Carboloy, stellite, and tungsten steels are of importance
in industry because they retain their hardness and strength at high
temperatures. Tungsten is usually added to steel in the form of ferrotungsten,
obtained by the reduction of ferrous tungstate in an electric furnace.
Tungsten carbide is used in place of diamond for dies and as an abrasive.
Sodium wolframate is used in the fireproofing of fabrics, in the weighting
of silk, and as a mordant in dyeing. Tungsten does not occur uncombined
in nature; large deposits of its ores are found in various parts of
the world. The trioxide occurs in nature as the mineral wolfram ochre;
Scheelite and wolframite are the chief wolframate minerals. Tungsten
is usually prepared from the trioxide by reduction with hydrogen or
carbon.
LOCALITIES:
Hollinger Mine, Ontario, Canada; Saxony, Germany; Tong Wha, Korea; Brazil;
Sonora, Mexico; Cornwall, England; New South Wales and Queensland, Australia:
Mill City, Nevada, Atolia, San Bernardino Co., California; Kharmang
Valley, Northeast of Skardu, Pakistan; Black
Morel Mine, Greenhorn Summit District, California; Cochise Co., Arizona;
Utah and Colorado. Many of these are older locations and all that may
be left from some of them. Are the coveted mineral specimens collected
from them. At present an area in China's
Sichuan Provence is producing fine mineral specimens which we are offering.
USES:
Scheelite is an important source of tungsten,which is a strategically
important metal. Scheelite is rarely cut as gemstones. But it is
considered as a rare mineral specimen by collectors and good specimens
can command very high prices.
HISTORY & FACTS:
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. The
word "Tungsten" was probably first used by A. F. Cronstedt in 1755,
who applied it to the mineral subsequently known as "Scheelite," which
is the natural form of calcium tungstate.
C. C. Leonhard named this mineral Scheelite in 1821 in
recognition of the discovery made by K. W. Scheele, in 1781. He believed
that the mineral was a compound of lime and a previously unknown acid,
which he called "Tungstic Acid. Tungsten was first isolated from tungstic
acid
in 1783 by the de Elhuyar brothers. Before Scheele made his discovery,
the mineral was generally regarded as containing tin.
The word tungsten denotes a substance of high density and is derived
from the Swedish language, "tung," meaning heavy, and "sten," meaning
stone. The metal is known as tungsten in some countries and as wolfram
in others, including Sweden, the country of origin of the name tungsten.
The chemical symbol W, which is universally used to denote tungsten,
suggests that wolfram was formerly the more generally accepted name
for the element. In Britain the mineral wolframite is also known as
wolfram.
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