Cassiterite

Cassiterite
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 system	Tetragonal - Ditetragonal Dipyramidal 4/m 2/m 2/m
Space group	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
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

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.

Occurrence

Cassiterite bipyramids, edge length ca. 30mm, 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.

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 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, Maakhir region of Somalia, and Russia. Hydraulic mining methods are used to concentrate the fine particles of ore owing to its high specific gravity of about 7.0 G.

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.

Etymology

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

Stalactitic-botryoidal, banded, "wood tin" cassiterite, 5.0 x 4.9 x 3.3 cm, Durango, Mexico

Close up of cassiterite crystals