Cassiterite

Cassiterite is a tin oxide mineral, SnO2. It is generally opaque, but it is translucent in thin crystals. Its luster and multiple crystal faces produce a desirable gem. Cassiterite was the chief tin ore throughout ancient history and remains the most important source of tin today.

Cassiterite
Cassiterite with muscovite, from Xuebaoding, Huya, Pingwu, Mianyang, Sichuan, China (size: 100 x 95 mm, 1128 g)
General
CategoryOxide minerals
Formula
(repeating unit)		SnO2
Strunz classification4.DB.05
Crystal systemTetragonal
Crystal classDitetragonal dipyramidal (4/mmm)
H-M symbol: (4/m 2/m 2/m)
Space groupP42/mnm
Unit cella = 4.7382(4) Å,
c = 3.1871(1) Å; Z = 2
Identification
ColorBlack, brownish black, reddish brown, brown, red, yellow, gray, white; rarely colorless
Crystal habitPyramidic, prismatic, radially fibrous botryoidal crusts and concretionary masses; coarse to fine granular, massive
TwinningVery common on {011}, as contact and penetration twins, geniculated; lamellar
Cleavage{100} imperfect, {110} indistinct; partings on {111} or {011}
FractureSubconchoidal to uneven
TenacityBrittle
Mohs scale hardness6–7
LusterAdamantine to adamantine metallic, splendent; may be greasy on fractures
StreakWhite to brownish
DiaphaneityTransparent when light colored, dark material nearly opaque; commonly zoned
Specific gravity6.98–7.1
Optical propertiesUniaxial (+)
Refractive indexnω = 1.990–2.010 nε = 2.093–2.100
Birefringenceδ = 0.103
PleochroismPleochroic haloes have been observed. Dichroic in yellow, green, red, brown, usually weak, or absent, but strong at times
Fusibilityinfusible
Solubilityinsoluble
References[1][2][3][4][5]

Occurrence
Cassiterite bipyramids, edge length ca. 30 mm, Sichuan, China

Most sources of cassiterite today are found in alluvial or placer deposits containing the resistant weathered grains. The best sources of primary cassiterite are found in the tin mines of Bolivia, where it is found in hydrothermal veins. Rwanda has a nascent cassiterite mining industry. Fighting over cassiterite deposits (particularly in Walikale) is a major cause of the conflict waged in eastern parts of the Democratic Republic of the Congo.[6][7] This has led to cassiterite being considered a conflict mineral.

Cassiterite is a widespread minor constituent of igneous rocks. The Bolivian 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 mineral occurs extensively in Cornwall as surface deposits on Bodmin Moor, for example, where there are extensive traces of an hydraulic mining method known as streaming. The current major tin production comes from placer or alluvial deposits in Malaysia, Thailand, Indonesia, the Maakhir region of Somalia, and Russia. Hydraulic mining methods are used to concentrate mined ore, a process which relies on the high specific gravity of the SnO2 ore, of about 7.0.

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 are found.

Stalactitic-botryoidal, banded, "wood tin" cassiterite, 5.0 x 4.9 x 3.3 cm, Durango, Mexico
Close up of cassiterite crystals, Blue Tier tinfield, Tasmania, Australia

Etymology

The name derives from the Greek kassiteros for "tin": this comes from the Phoenician word Cassiterid referring to the islands of Ireland and Great 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.

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References
  1. Mineralienatlas
  2. Handbook of Mineralogy
  3. Mindat
  4. Webmineral
  5. Hurlbut, Cornelius S.; Klein, Cornelis (1985). Manual of Mineralogy (20th ed.). New York: John Wiley and Sons. pp. 306–307. ISBN 0-471-80580-7.
  6. Watt, Louise (2008-11-01). "Mining for minerals fuels Congo conflict". Yahoo News!. Yahoo! Inc. Associated Press. Retrieved 2009-09-03.
  7. Polgreen, Lydia (2008-11-16). "Congo's Riches, Looted by Renegade Troops". The New York Times. Retrieved 2008-11-16.
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