Tin has two allotropic forms: grey tin and white tin. On warming, grey tin or a tin, changes at 13.2oC into white or ß tin, the ordinary form of the metal.
Colour: White tin- Silvery- white metal. Grey tin- Grey metal
Natural occurrence: The principal tin bearing ore is casserite (SnO2). Tin makes up only about 0.001% of the earth's crust and is chiefly mined in Malaysia. Ores are also found abundantly in Cornwall, Germany, Bolivia, Brazil and Australia.
White tin has an irregular, close packed, highly crystalline structure. Due to the breaking of these crystals a "tin cry" is heard when a bar is bent. White tin has a tetragonal structure and a co-ordination number of 6.
Grey tin has a cubic diamond structure.
Uses and industrial applications: All uses of tin are of the white form:
Isolation: The ore, cassiterite, is first ground and washed to remove impurities, then roasted to oxidise the sulphides of iron and copper. It is washed again and is reduced by carbon in a reverberatory furnace. The molten tin that collects on the bottom is drawn off and moulded into blocks. It is remelted at low temperatures so that the impurities form an insoluble mass.
SnO2 + 2C Sn + 2C0
Tin may also be purified by electrolysis.
Interconversions: Grey tin can be converted to white tin by heating it past 13.2oC. White tin returns to grey tin as it cools below 13.2oC. The metallic surface becomes covered with a grey powder, which is easily rubbed off. The grey patches gradually spread and eventually the object covered may totally lose its structural integrity and fall to pieces. Due to the spreading nature of the condition it is often called tin disease or tin pest. In the cold cathedrals of northern Europe, tin disease was a serious problem in the last century when organ pipes were commonly made of tin. Tin disease was, on occasion, responsible for the complete disintegration of organ pipes in some of these cathedrals in long, cold winters. This transformation can be prevented by alloying tin with antimony or bismuth.