Everything about Zincblende totally explained
Sphalerite ((
Zn,
Fe)
S) is a
mineral that's the chief
ore of
zinc. It consists largely of
zinc sulfide in
crystalline form but almost always contains variable
iron. When iron content is high it's an opaque black variety,
marmatite. It is usually found in association with
galena,
pyrite, and other
sulfides along with
calcite,
dolomite, and
fluorite. Miners have also been known to refer to sphalerite as
zinc blende,
mock lead,
false galena and
black-jack.
The mineral crystallizes in the
cubic crystal system. In the crystal structure, zinc and sulfur atoms are tetrahedrally coordinated. The structure is closely related to the structure of
diamond. The
hexagonal analog is known as the
wurtzite structure. The lattice constant for zinc sulfide in the zincblende crystal structure is 0.596
nm, calculated from geometry and ionic radii of 0.074 nm (zinc) and 0.184 nm (sulfide). It forms ABCABC layers.
Its color is usually yellow, brown, or gray to gray-black, and it may be shiny or dull. Its
luster is resinous. It has a yellow or light brown
streak, a
hardness of 3.5 - 4, and a
specific gravity of 3.9-4.1. Some specimens have a red
iridescence within the gray-black crystals; these are called "ruby sphalerite." The pale yellow and red varieties have very little iron and are translucent. The darker more opaque varieties contain more iron. Some specimens are also
fluorescent in
ultraviolet light. The
refractive index of sphalerite (as measured via
sodium light, 589.3 nm) is 2.37. Sphalerite crystallizes in the
isometric crystal system and possesses perfect dodecahedral
cleavage. Gemmy, pale specimens from
Franklin, New Jersey (see
Franklin Furnace) are highly fluorescent orange and/or blue under longwave ultraviolet light and are known as
cleiophane, an almost pure ZnS variety.
Crystals of suitable size and transparency have been fashioned into
gemstones, usually featuring the
brilliant cut to best display sphalerite's high
dispersion of 0.156 (B-G interval)—over three times that of
diamond. Freshly cut gems are lively with an adamantine luster and could conceivably be mistaken for a fancy-colored diamond in passing, but due to sphalerite's softness and fragility the gems are best left unset as collector's or
museum pieces (although some have been set into pendants). Collectors may pay a premium for stones over one
carat (200 mg), as clean crystals are usually quite small. Gem-quality material is usually a yellowish to honey brown, red to orange, or green; the two most important sources are the Chivera mine,
Cananea,
Sonora,
Mexico; and the
Picos de Europa,
Cordillera Cantabrica, near
Santander on
Spain's northern coast.
Further Information
Get more info on 'Zincblende'.
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