cassiterite

[French cassitérite, from Greek kassiteros, tin.]

A mineral having the composition SnO₂. It is the principal ore of tin. Cassiterite is usually massive granular, but may be in radiating fibrous aggregates with reniform shapes (wood tin). The hardness is 6–7 (Mohs scale), and the specific gravity is 6.8–7.1 (unusually high for a nonmetallic mineral). The luster is adamantine to submetallic. Pure tin oxide is white, but cassiterite is usually yellow, brown, or black because of the presence of iron.

Cassiterite is most abundantly found as stream tin (rolled pebbles in placer deposits). The world's supply comes mostly from placer or residual deposits in the Malay Peninsula, Indonesia, Zaire, and Nigeria. It is also mined in Bolivia. See also Tin.

Environment

Pegmatites and high-temperature veins.

Commonly in well-formed crystals, sometimes prismatic (Cornwall), even needlelike (Cornwall and Bolivia), but more often bipyramidal. Frequently twinned, showing the characteristic re-entrant angle of a twin crystal. Also in red-brown, fibrous, banded crusts ("wood-tin"), in waterworn gray pebbles with a greasy luster ("stream-tin"), and in granular masses.

Light yellowish to red-brown to black, usually banded within a single crystal. Luster adamantine to greasy; hardness 6-7; specific gravity 6.8-7.1; streak nearly white; fracture subconchoidal to uneven; cleavage poor prismatic. Brittle; transparent to translucent.

Tin oxide (78.6% Sn, 21.4% O); usually with a fair amount of iron and sometimes some tantalum, which substitutes for the tin.

Infusible. Grain slowly becomes coated with gray or silvery film of tin metal on standing in cool dilute hydrochloric acid with a strip of zinc (cut from casing of dry-cell battery).

The light streak, high gravity, and high hardness rule out most similar minerals. The hydrochloric test for tin then eliminates the rest. Might be confused with black tourmaline (which is much lighter), with rutile or columbite-tantalite (make the tin test), and with magnetite (try for magnetism). Some cassiterite is very black, so tests are sometimes essential. Dark bands across broken crystals and the light true color (despite a black exterior appearance) are very typical.

The only important ore of tin. Occurrence is limited to pegmatites and high-temperature veins, often associated with tungsten ores and with silicate gangue minerals; in pegmatites it is an important dark accessory. Often found in stream placers as waterworn pebbles; in Cornwall these were mined before the veins were. In fumarolic deposits (Durango, Mexico) it has formed with hematite on seams in rhyolite flows.

Good crystals come from Cornwall, where it has been mined since Roman days. Bohemia and Saxony have important high-temperature vein occurrences. Alluvial deposits--which are still worked in China, in West Malaysia, and in Indonesia--are among the most important economic occurrences. In Bolivia beautiful specimens are found in association with a variety of minerals in near-surface and low-pressure but high-temperature veins. In the U.S., cassiterite occurs in pegmatites (of no commercial value), though vein deposits in Virginia and California have been unsuccessfully worked. The small red-brown botryoidal masses of "wood-tin" found attached to rhyolite (and broken loose in placer washings in Durango, Mexico) are very different from the other tin varieties, but once seen they are easily recognized.

Cassiterite
Cassiterite crystals
General
Category	Mineral
Chemical formula	SnO2
Identification
Color	purple, wine, black, reddish brown or yellow
Crystal habit	Pyramidic, prismatic
Crystal system	tetragonal; 4/m 2/m 2/m
Cleavage	good in two directions forming prisms, poor in a third (basal)
Fracture	Subconchoidal to rough
Mohs scale hardness	6 - 7
Luster	adamantine or greasy
Streak	White to brownish
Specific gravity	6.4 - 7.1
Pleochroism	None
Fusibility	infusible
Solubility	insoluble
Other characteristics	high refractive index of approximately 2.0

Cassiterite is a tin oxide mineral, SnO₂. It is generally opaque but is translucent in thin crystals. Its luster and multiple crystal faces produce a desirable gem. Cassiterite is the chief ore of tin today.

Occurrence

Most sources of cassiterite today are found in alluvial or placer deposits containing the resistant weathered grains. The best source of primary cassiterite is the tin mines of Bolivia, where it is found in hydrothermal veins. Fighting over cassiterite deposits is a major cause of the conflict waged in eastern parts of the Democratic Republic of the Congo.^[1][2]

Cassiterite is a widespread minor constituent of igneous rocks. The Bolivia veins and the old exhausted workings of Cornwall, England, are concentrated in high temperature quartz veins and pegmatites associated with granitic intrusives. The veins commonly contain tourmaline, topaz, fluorite, apatite, wolframite, molybdenite, and arsenopyrite. The current major tin production comes from placer or alluvial deposits in Malaysia, Thailand, Indonesia, and Russia. Hydraulic mining methods are used to concentrate the fine particles of ore.

Crystallography

crystal structure of cassiterite

Crystal twinning is common in cassiterite and most aggregate specimens show crystal twins. The typical twin is bent at a near-60-degree angle, forming an "Elbow Twin". Botryoidal or reniform cassiterite is called wood tin.

Cassiterite is also used as a gemstone and collector specimens when quality crystals can be found.

The name derives from the Greek kassiteros for "tin" - or - from the Phoenician word Cassiterid referring to the islands of Ireland and Britain, the ancient sources of tin - or - as Roman Ghirshman (1954) suggests, from the region of the Kassites, an ancient people in west and central Iran.

Crystals of cassiterite tin ore

References

1. ^ Watt, Louise (2008-11-01). "Mining for minerals fuels Congo conflict". Yahoo News!. Associated Press (Yahoo! Inc). http://www.newsvine.com/_news/2008/11/01/2061627-mining-for-minerals-fuels-congo-conflict. Retrieved 2009-09-03. 
2. ^ Polgreen, Lydia (2008-11-16). "Congo's Riches, Looted by Renegade Troops". New York Times. http://www.nytimes.com/2008/11/16/world/africa/16congo.html?pagewanted=1&_r=1&th&emc=th. Retrieved 2008-11-16. 

• Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., John Wiley and Sons, New York, pp. 306-307 ISBN 0-471-80580-7.
• Mineral galleries accessed 4-21-05.
• Mindat accessed 4-21-05.
• Webmineral accessed 4-21-05.

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