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
IMA symbol	Cst[1]
Strunz classification	4.DB.05
Crystal system	Tetragonal
Crystal class	Ditetragonal dipyramidal (4/mmm) H-M symbol: (4/m 2/m 2/m)
Space group	P42/mnm
Unit cell	a = 4.7382(4) Å, c = 3.1871(1) Å; Z = 2
Identification
Color	Black, brownish black, reddish brown, 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	[2][3][4][5][6]

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.