Sphalerite

Sphalerite
General
Category	Sulfide mineral
Chemical formula	(Zn,Fe)S
Strunz classification	02.CB.05a
Dana classification	02.08.02.01
Identification
Color	Brown, yellow, red, green, black.
Crystal habit	Euhedral Crystals - Occurs as well-formed crystals showing good external form. Granular - Generally occurs as anhedral to subhedral crystals in matrix. Colloform
Crystal system	Isometric hextetrahedral (4 3m)
Twinning	Simple contact twins or complex lamellar forms, twin axis [111]
Cleavage	[110] Perfect
Fracture	Uneven to conchoidal
Mohs scalehardness	3.5-4
Luster	Adamantine, resinous, greasy
Streak	brownish white, pale yellow
Diaphaneity	Transparent to translucent, opaque when iron-rich
Specific gravity	3.9 - 4.2
Optical properties	Isotropic
Refractive index	nα = 2.369 ZnS
Other characteristics	non-radioactive, non-magnetic, Fluorescent and triboluminescent.
References	[1][2][3]

Sphalerite ((Zn,Fe)S) is a mineral that is the chief ore of zinc. It consists largely of zinc sulfidein crystalline form but almost always contains variable iron. When iron content is high it is an opaque black variety, marmatite. It is usually found in association with galena, pyrite, and othersulfides 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 crystal structure of sphalerite

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 hexagonalanalog 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 adamantine, resinous to submetallic for high iron varieties. It has a yellow or light brown streak, ahardness of 3.5 - 4, and a specific gravity of 3.9-4.1. Some specimens have a red iridescencewithin 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.

Another sphalerite sample

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.