Sphalerite

Sphalerite
Sphalerite - Creede, Mineral County, Colorado, USA.jpg
Black crystals of sphalerite with minor chalcopyrite and calcite
General
CategorySulfide mineral
Formula
(repeating unit)
(Zn,Fe)S
IMA symbolSp
Strunz classification2.CB.05a
Dana classification02.08.02.01
Crystal systemCubic
Crystal classHextetrahedral (43m)
H-M symbol: (4 3m)
Space groupF43m (No. 216)
Unit cella = 5.406 Å; Z = 4
Structure
Jmol (3D)Interactive image
Identification
ColorLight to dark brown, red-brown, yellow, red, green, light blue, black and colourless.
Crystal habitEuhedral crystals – occurs as well-formed crystals showing good external form. Granular – generally occurs as anhedral to subhedral crystals in matrix.
TwinningSimple contact twins or complex lamellar forms, twin axis [111]
Cleavageperfect dodecahedral on [011]
FractureUneven to conchoidal
Mohs scale hardness3.5–4
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
Other characteristicsnon-radioactive, non-magnetic, fluorescent and triboluminescent.
References

Sphalerite (sometimes spelled sphaelerite) is a sulfide mineral with the chemical formula (Zn,Fe)S. It is the most important ore of zinc. Sphalerite is found in a variety of deposit types, but it is primarily in sedimentary exhalative, Mississippi-Valley type, and volcanogenic massive sulfide deposits. It is found in association with galena, chalcopyrite, pyrite (and other sulfides), calcite, dolomite, quartz, rhodochrosite, and fluorite.

German geologist Ernst Friedrich Glocker discovered sphalerite in 1847, naming it based on the Greek word sphaleros, meaning "deceiving", due to the difficulty of identifying the mineral.

In addition to zinc, sphalerite is an ore of cadmium, gallium, germanium, and indium. Miners have been known to refer to sphalerite as zinc blende, black-jack, and ruby blende. Marmatite is an opaque black variety with a high iron content.

Crystal habit and structure

The crystal structure of sphalerite

Sphalerite crystalizes in the face-centered cubic zincblende crystal structure, which named after the mineral. This structure is a member of the hextetrahedral crystal class (space group F43m). In the crystal structure, both the sulfur and the zinc or iron ions occupy the points of a face-centered cubic lattice, with the two lattices displaced from each other such that the zinc and iron are tetrahedrally coordinated to the sulfur ions, and vice versa. Minerals similar to sphalerite include those in the sphalerite group, consisting of sphalerite, colaradoite, hawleyite, metacinnabar, stilleite and tiemannite. The structure is closely related to the structure of diamond. The hexagonal polymorph of sphalerite is wurtzite, and the trigonal polymorph is matraite. Wurtzite is the higher temperature polymorph, stable at temperatures above 1,020 °C (1,870 °F). The lattice constant for zinc sulfide in the zinc blende crystal structure is 0.541 nm. Sphalerite has been found as a pseudomorph, taking the crystal structure of galena, tetrahedrite, barite and calcite. Sphalerite can have Spinel Law twins, where the twin axis is [111].

The chemical formula of sphalerite is (Zn,Fe)S; the iron content generally increases with increasing formation temperature and can reach up to 40%. The material can be considered a ternary compound between the binary endpoints ZnS and FeS with composition ZnxFe(1-x)S, where x can range from 1 (pure ZnS) to 0.6.

All natural sphalerite contains concentrations of various impurities, which generally substitute for zinc in the cation position in the lattice; the most common cation impurities are cadmium, mercury and manganese, but gallium, germanium and indium may also be present in relatively high concentrations (hundreds to thousands of ppm). Cadmium can replace up to 1% of zinc and manganese is generally found in sphalerite with high iron abundances. Sulfur in the anion position can be substituted for by selenium and tellurium. The abundances of these impurities are controlled by the conditions under which the sphalerite formed; formation temperature, pressure, element availability and fluid composition are important controls.

Properties

Physical properties

Sphalerite possesses perfect dodecahedral cleavage, having six cleavage planes. In pure form, it is a semiconductor, but transitions to a conductor as the iron content increases. It has a hardness of 3.5 to 4 on the Mohs scale of mineral hardness.

It can be distinguished from similar minerals by its perfect cleavage, its distinctive resinous luster, and the reddish-brown streak of the darker varieties.

Optical properties

Sphalerite fluorescing under ultraviolet light. (Sternberg Museum of Natural History, Kansas, USA)

Pure zinc sulfide is a wide-bandgap semiconductor, with bandgap of about 3.54 electron volts, which makes the pure material transparent in the visible spectrum. Increasing iron content will make the material opaque, while various impurities can give the crystal a variety of colors. In thin section, sphalerite exhibits very high positive relief and appears colorless to pale yellow or brown, with no pleochroism.

The refractive index of sphalerite (as measured via sodium light, average wavelength 589.3 nm) ranges from 2.37 when it is pure ZnS to 2.50 when there is 40% iron content. Sphalerite is isotropic under cross-polarized light, however sphalerite can experience birefringence if intergrown with its polymorph wurtzite; the birefringence can increase from 0 (0% wurtzite) up to 0.022 (100% wurtzite).

Depending on the impurities, sphalerite will fluoresce under ultraviolet light. Sphalerite can be triboluminescent. Sphalerite has a characteristic triboluminescence of yellow-orange. Typically, specimens cut into end-slabs are ideal for displaying this property.

Varieties

Gemmy, colorless to pale green sphalerite 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. Cleiophane contains less than 0.1% of iron in the sphalerite crystal structure. Marmatite or christophite is an opaque black variety of sphalerite and its coloring is due to high quantities of iron, which can reach up to 25%; marmatite is named after Marmato mining district in Colombia and christophite is named for the St. Christoph mine in Breitenbrunn, Saxony. Both marmatite and cleiophane are not recognized by the International Mineralogical Association (IMA). Red, orange or brownish-red sphalerite is termed ruby blende or ruby zinc, whereas dark colored sphalerite is termed black-jack.

Deposit types

Sphalerite is amongst the most common sulfide minerals, and it is found worldwide and in a variety of deposit types. The reason for the wide distribution of sphalerite is that it appears in many types of deposits; it is found in skarns, hydrothermal deposits, sedimentary beds, volcanogenic massive sulfide deposits (VMS), Mississippi-valley type deposits (MVT), granite and coal.

Sedimentary exhalitive

Approximately 50% of zinc (from sphalerite) and lead comes from Sedimentary exhalative (SEDEX) deposits, which are stratiform Pb-Zn sulfides that form at seafloor vents.. The metals precipitate from hydrothermal fluids and are hosted by shales, carbonates and organic-rich siltstones in back-arc basins and failed continental rifts. The main ore minerals in SEDEX deposits are sphalerite, galena, pyrite, pyrrhotite and marcasite, with minor sulfosalts such as tetrahedrite-freibergite and boulangerite; the Zn + Pb grade typically ranges between 10-20%. Important SEDEX mines are Red Dog in Alaska, Sullivan in British Columbia, Mount Isa and Broken Hill in Australia and Mehdiabad in Iran.

Mississippi-Valley type

Similar to SEDEX, Mississippi-Valley type (MVT) deposits are also a Pb-Zn deposit which contains sphalerite. However, they only account for 15–20% of zinc and lead, are 25% smaller in tonnage than SEDEX deposits and have lower grades of 5–10% Pb + Zn. MVT deposits form from the replacement of carbonate host rocks such as dolostone and limestone by ore minerals; they are located in platforms and foreland thrust belts. Furthermore, they are stratabound, typically Phanerozoic in age and epigenetic (form after the lithification of the carbonate host rocks). The ore minerals are the same as SEDEX deposits: sphalerite, galena, pyrite, pyrrhotite and marcasite, with minor sulfosalts. Mines that contain MVT deposits include Polaris in the Canadian arctic, Mississippi River in the United States, Pine Point in Northwest Territories, and Admiral Bay in Australia.

Volcanogenic massive sulfide

Volcanogenic massive sulfide (VMS) deposits can be Cu-Zn- or Zn-Pb-Cu-rich, and accounts for 25% of Zn in reserves. There are various types of VMS deposits with a range of regional contexts and host rock compositions; a common characteristic is that they are all hosted by submarine volcanic rocks. They form from metals such as copper and zinc being transferred by hydrothermal fluids (modified seawater) which leach them from volcanic rocks in the oceanic crust; the metal-saturated fluid rises through fractures and faults to the surface, where it cools and deposits the metals as a VMS deposit. The most abundant ore minerals are pyrite, chalcopyrite, sphalerite and pyrrhotite. Mines that contain VMS deposits include Kidd Creek in Ontario, Urals in Russia, Troodos in Cyprus and Besshi in Japan.

Localities

The top producers of sphalerite include the United States, Russia, Mexico, Germany, Australia, Canada, China, Ireland, Peru, Kazakhstan and England.

Sources of high quality crystals include:

Place Country
Freiberg, Saxony,
Neudorf, Harz Mountains
Germany
Lengenbach Quarry, Binntal, Valais Switzerland
Horni Slavkov and Příbram Czech Republic
Rodna Romania
Madan, Smolyan Province, Rhodope Mountains Bulgaria
Aliva mine, Picos de Europa Mountains, Cantabria [Santander] Province Spain
Alston Moor, Cumbria England
Dalnegorsk, Primorskiy Kray Russia
Watson Lake, Yukon Territory Canada
Flin Flon, Manitoba Canada
Tri-State district including deposits near
Baxter Springs, Cherokee County, Kansas;
Joplin, Jasper County, Missouri
and Picher, Ottawa County, Oklahoma
USA
Elmwood mine, near Carthage, Smith County, Tennessee USA
Eagle mine, Gilman district, Eagle County, Colorado USA
Santa Eulalia, Chihuahua Mexico
Naica, Chihuahua Mexico
Cananea, Sonora Mexico
Huaron Peru
Casapalca Peru
Huancavelica Peru
Zinkgruvan Sweden

Uses

Metal ore

Sphalerite is an important ore of zinc; around 95% of all primary zinc is extracted from sphalerite ore. However, due to its variable trace element content, sphalerite is also an important source of several other metals such as cadmium, gallium, germanium, and indium which replace zinc. The ore was originally called blende by miners (from German blind or deceiving) because it resembles galena but yields no lead.

Brass and bronze

The zinc in sphalerite is used to produce brass, an alloy of copper with 3–45% zinc. Major element alloy compositions of brass objects provide evidence that sphalerite was being used to produce brass by the Islamic as far back as the medieval ages between the 7th and 16th century CE. Sphalerite may have also been used during the cementation process of brass in Northern China during the 12th–13th century CE (Jin Dynasty). Similarly to brass, the zinc in sphalerite can also be used to produce certain types of bronze; bronze is dominantly copper which is alloyed with other metals such as tin, zinc, lead, nickel, iron and arsenic.

Other

Gallery

See also


This page was last updated at 2022-10-11 03:00 UTC. Update now. View original page.

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