Page 34 - Curriculum Visions Dynamic Book

Tin
Tin is a relatively rare, silvery white, soft metal with a low melting point (232°C). It is mostly found as tin oxide, cassiterite, in veins that are close to igneous rocks (the same conditions that gave rise to lead deposits, see page 7). Tin compounds were probably deposited in these veins by hot fluids rising above molten magma.
Tin is a very soft and weak metal, so it is never used on its own. Tin is also very brittle; a bar of tin will break up into sharp crystals if bent, as well as making a high pitched sound like crying.
The main use of tin has traditionally been as a plating on steel (see page 38), utilising the noncorrosive properties of the metal.
However, tin has many other uses, especially as an alloy. Although tin is a weak metal on its own, an alloy that includes tin
is typically much harder than one without it. There are other advantages, too, such as making the alloy easier to cast. Tin is found
in some brass, bronze and pewter.
The low melting point of tin makes it a useful component of solder and for preparing alloys that have low melting points. For this reason, it can be used in fire-detection systems.
Ore deposits
Tin, like many other poorly reactive metals, such as tungsten, copper and silver, is formed in the metamorphic zones surrounding ancient magma chambers. It was present in the hydrothermal fluids that rose up from the magma chambers as they cooled during the last stages
of volcanic activity. The fluids flowed into fissures that had been produced as the magma chambers domed up the overlying rocks. As the fluids rose in the fissures, they cooled, and a wide variety of minerals solidified as the temperature dropped below their various melting points. Tin-bearing lodes are normally found making a ring of fissures over a magma chamber.
The only lode deposits to have been found are in Cornwall, England and Bolivia. These
rich lodes are mined and the
tin processed by direct smelting. The type of mining involved is shown on these pages.
Elsewhere tin is mined as placer deposits, that is, deposits of sand and clay that contain
tin that has been eroded from lodes and transported by streams. These are very low grade deposits and have to
be processed over a number
of stages.
 Cornish tin mines during the time when they were at full production during the last century.
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