Of minerals containing this element mention may be made of cassiterite or tinstone, Sn02, tin pyrites, Cu 4 SnS 4 + (Fe,Zn) 2 SnS 4; the metal also occurs in some epidotes, and in company with columbium, tantalum and other metals.
Of these "tinstone" is of the greatest commercial importance.
Throughout the world, primary deposits of tinstone are in or closely connected with granite or acid eruptive rocks of the same type, its mineral associates being tourmaline, fluorspar, topaz, wolfram and arsenical pyrites, and the invariable gangue being quartz: the only exception to this mode of occurrence is to be found in Bolivia, where the tin ore occurs intimately associated with silver ores, bismuth ores and various sulphides, whilst the gangue includes barytes and certain carbonates.
The first stage has for its purpose the production of a fairly pure tinstone; the second the conversion of the oxide into metallic tin; and the third preparing a tin pure enough for commercial purposes.
The alluvial extracted, which in the Malay Peninsula and Archipelago carries from 5 to 60 lb of tinstone (or "black tin," as it is termed by Cornish miners) to the cubic yard of gravel, is washed in various simple sluicing appliances, by which the lighter clay, sand and stones are removed and tinstone is left behind comparatively pure, containing usually 65 to 75% of metallic tin (chemically pure tinstone contains 78.7%).
Lode tin, as tinstone derived from primary deposits is often termed, is mined in the ordinary method, the very hard gangue in which it occurs necessitating a liberal use of explosives.
In diameter, leaves the stamps in suspension in water, and passes through a series of troughs in which the heavier mineral is collected; this then passes through a series of washing operations, which leaves a mixture consisting chiefly of tinstone and arsenical pyrites, which is calcined and washed again, until finally black tin containing about 60 to 65% of metal is left.
Of these sodium stannate, Na2Sn03, is produced industrially by heating tin with Chile saltpetre and caustic soda, or by fusing very finely powdered tinstone with caustic soda in iron vessels.
In the alluvial deposits the associated minerals are chiefly those of great density and hardness, such as platinum, osmiridium and other metals of the platinum group, tinstone, chromic, magnetic and brown iron ores, diamond, ruby and sapphire, zircon, topaz, garnet, &c. which represent the more durable original constituents of the rocks whose distintegration has furnished the detritus.