viernes, 12 de marzo de 2010


CategorySulfide mineral
Chemical formula(Zn,Fe)S
Strunz classification02.CB.05a
Dana classification02.08.02.01
ColorBrown, yellow, red, green, black.
Crystal habitEuhedral Crystals - Occurs as well-formed crystals showing good external form. Granular - Generally occurs as anhedral to subhedral crystals in matrix. Colloform
Crystal systemIsometric hextetrahedral (4 3m)
TwinningSimple contact twins or complex lamellar forms, twin axis [111]
Cleavage[110] Perfect
FractureUneven to conchoidal
Mohs scalehardness3.5-4
LusterAdamantine, resinous, greasy
Streakbrownish white, pale yellow
DiaphaneityTransparent to translucent, opaque when iron-rich
Specific gravity3.9 - 4.2
Optical propertiesIsotropic
Refractive indexnα = 2.369 ZnS
Other characteristicsnon-radioactive, non-magnetic, Fluorescent and triboluminescent.
Sphalerite ((Zn,Fe)S) is a mineral that is the chief ore of zinc. It consists largely of zinc sulfidein crystallineform but almost always contains variable iron. When iron content is high it is an opaque black variety, marmatite. It is usually found in association withgalenapyrite, and othersulfides along with calcitedolomite, and fluorite. Miners have also been known to refer to sphalerite aszinc blendemock leadfalse galena andblack-jack.

The crystal structure of sphalerite
The mineral crystallizes in thecubic crystal system. In the crystal structure, zinc and sulfur atoms are tetrahedrally coordinated. The structure is closely related to the structure of diamond. Thehexagonalanalog is known as the wurtzitestructure. 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 brownstreak, 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 alsofluorescent in ultravioletlight. The refractive index of sphalerite (as measured viasodium 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 dispersionof 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 onecarat (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,SonoraMexico; and the Picos de EuropaCordillera Cantabrica, near Santander on Spain's northern coast.

Jose Galviz  EES

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