Cassiterite

From Wikipedia, the free encyclopedia
  (Redirected from Tin ore)
Jump to: navigation, search
Cassiterite
4447M-cassiterite.jpg
Cassiterite with muscovite, from Xuebaoding, Huya, Pingwu, Mianyang, Sichuan, China (size: 100 x 95 mm, 1128 g)
General
Category Oxide minerals
Formula
(repeating unit)
SnO2
Strunz classification 04.DB.05
Crystal symmetry Tetragonal 4/m 2/m 2/m
Unit cell a = 4.7382(4) Å, c = 3.1871(1) Å; Z=2
Identification
Color Black, brownish black, reddish brown, red, yellow, gray, white; rarely colorless
Crystal habit Pyramidic, prismatic, radially fibrous botryoidal crusts and concretionary masses; coarse to fine granular, massive
Crystal system Tetragonal - Ditetragonal Dipyramidal 4/m 2/m 2/m
Twinning Very common on {011}, as contact and penetration twins, geniculated; lamellar
Cleavage {100} imperfect, {110} indistinct; partings on {111} or {011}
Fracture Subconchoidal to uneven
Tenacity Brittle
Mohs scale hardness 6–7
Luster Adamantine to adamantine metallic, splendent; may be greasy on fractures
Streak White to brownish
Diaphaneity Transparent when light colored, dark material nearly opaque; commonly zoned
Specific gravity 6.98 - 7.1
Optical properties Uniaxial (+)
Refractive index nω = 1.990 - 2.010 nε = 2.093 - 2.100
Birefringence δ = 0.103
Pleochroism Pleochroic haloes have been observed. Dichroic in yellow, green, red, brown, usually weak, or absent, but strong at times
Fusibility infusible
Solubility insoluble
References [1][2][3][4]

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 has been the chief tin ore throughout ancient history and remains the most important source of tin today.[1]

Occurrence[edit]

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.[5][6]

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[edit]

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.

Etymology[edit]

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.

References[edit]

  1. ^ a b Handbook of Mineralogy
  2. ^ Mindat
  3. ^ Webmineral
  4. ^ 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. 
  5. ^ Watt, Louise (2008-11-01). "Mining for minerals fuels Congo conflict". Yahoo News! (Yahoo! Inc). Associated Press. Retrieved 2009-09-03. 
  6. ^ Polgreen, Lydia (2008-11-16). "Congo's Riches, Looted by Renegade Troops". New York Times. Retrieved 2008-11-16. 
Stalactitic-botryoidal, banded, "wood tin" cassiterite, 5.0 x 4.9 x 3.3 cm, Durango, Mexico
Close up of cassiterite crystals