Cassiterite

The name cassiterite is derived from the Greek word for tin. Tin-stone, Tinore, Zinnerz & Zinnstein (Germ.). Tennmalm (Swed.). Etain oxyde (Fr.). Visor tin is a miner's term for the notched, elbow-shaped twin. Stream-tin is rolled and worn cassiterite in sand form. Wood-tin is compact and fibrous with concentric bands this resembling wood. Needle-tin ore and sparable tin are varieties with long prismatic or acute pryamidal habits.

Tin oxide, SnO₂. Sn = 78.6 per cent., O = 21.4 per cent. Fe, Ch, and Ta may substitute for Sn. Ainalite is a tantalian variety from Finland.

Tetragonal; ditetragonal dipyramidal. a : c = 1 : 0.6723. Prismatic crystals having dipyramids are common.

Habit : Twinning on {011} very common both in contact and penetrating forms.

Hardness, 6-7. Specific gravity, 6.99;6.8-7.1. Lustre, adamantine to splendent, sometimes tending to be greasy on fracture surfaces. Colour, usually reddish-brown to brownish—black, but rarely colourless or yellow. Streak, white, greyish or brownish. Fracture, subconchoidal or uneven. Tenacity, brittle. Cleavage, {100} imperfect. Magnetism, weakly magnetic when ferriferous almost non-magnetic when iron-free. Fusibility, infusible.

Uniaxial, positive. Colourless to yellow, orange-red, etc., in transmitted light. Refractive indices for a wavelength of γ = 440, ω = 2.0475, ε = 2.1397. Light-grey with strong anisotropism in polished section.

Cassiterite is distinguished by its high specific gravity and by its colour and hardness. It is virtually infusible before the blowpipe. The best and simplest test for tinstone is to place a grain of it in contact with zinc in cold dilute hydrochloric or sulphuric acid (not nitric) when the nascent hydrodgen produced will reduce the oxide to metallic tin. The resulting metallic grains can be polished to yield an unmistakable bright silver-white surface. The minerals commonly mistaken for cassiterite include rutile, wolfram, garnet, ilmenite, zinc blende, etc., but none of them will react to the above test, In a good reducing blow-pipe flame cassiterite can also be reduced to tin on charcoal preferably with sodium carbonate. Insoluble in acids.

The chief primary mode of occurrence is in pneumatolytic veins associated with granites, the main gangue minerals being quartz, fluorspar, topaz, tourmaline. It has also been noted as a primary constituent of igneous rocks and in certain contact-metamorphic rocks. About half of total world production comes from alluvial deposits derived by the disintegration of tinstone veins. The main world sources are Malaya, Bolivia, Indonesia, Belgian Congo and Nigeria.

Cassiterite is practically thc sole ore-mineral of tin, although a small amount of the metal is extracted from tin sulphide minerals. Black tin is a trade term for dressed tin ore (with about 70 per cent. metallic tin). Not to be confused with Block tin, the commercial metal in cast block form.