Cassiterite
CASSITERITE (from the Gr., tin), the mineralogical name for tin-stone, the common ore of tin. It consists of tin dioxide, or stannic oxide (SnO2), and crystallizes in the tetragonal system. The crystals are usually 4-sided or 8-sided prisms, striated vertically, and terminated by pyramids (fig. 1). Twins, with characteristic re-entrant angles, such as figs. 2 and 3, are common. Certain slender prismatic crystals, with an acute 8-sided pyramid, are known in Cornwall as "sparable tin," in allusion to their resemblance to sparable nails, whilst very slender crystals are termed needle-tin. Occasionally the mineral occurs in fibrous forms, which pass under the name of "wood-tin," and these, though not unknown in the matrix, are generally found as rolled pebbles. By the disintegration of tin-bearing rocks and vein-stones, the cassiterite passes into the beds of streams as rolled fragments and grains, or even sand, and is then known as stream tin or alluvial tin. This detrital tin-ore was probably used as a source of the metal before the primitive miners had learnt to attack the solid tin-bearing rocks.
Fig. 1.
Pure cassiterite may be colourless, or white, as seen in certain specimens from the Malay Peninsula; but usually the mineral is brown or even black, the colour being referred to the presence of ferric oxide or other impurity. Occasionally the tin-stone is red. In microscopic sections the colour is often seen to be disposed in zones, following the contour of the crystal. A brown variety, with rather resinous lustre, is termed "rosin tin." The usual lustre of crystals of cassiterite is remarkably splendent, even adamantine. The mineral has a high refractive index, and strong bi-refringence. Certain transparent yellow and brown specimens, cut as gem-stones, exhibit considerable brilliancy. The hardness of cassiterite is 6.5, so that it cannot be scratched with a knife, and is nearly as hard as quartz. Its specific gravity is about 7; and in consequence of this high density, the tin-stone is readily separated during the process of dressing, from all the associated minerals, except wolframite, which may, however, be removed by magnetic separators.
Fig. 2. Fig. 3.
Cassiterite usually occurs as veins or impregnations in granitic rocks, and is especially associated with the quartz-mica rock called greisen. The usual associates of the tin-stone are quartz, Tourmaline, apatite, topaz, beryl, fluorite, lithia-mica, wolframite, chalcopyrite, etc. The presence of fluorine in many of these minerals has led to the opinion that the tin has been derived in many cases from an acid or granitic magma by the action of fluorine-bearing vapours, and that cassiterite may have been formed by the interaction of tin fluoride and water vapour. Cassiterite occurs as a pseudomorph after orthoclase felspar in some of the altered granite of Cornwall, and it has occasionally been found as a cementing material in certain brecciated lodes.
Among the localities yielding cassiterite may be mentioned Cornwall, Saxony, Bohemia, Brittany, Galicia in Spain; the Malay peninsula, and the islands of Banca and Billiton; New South Wales, Queensland and Tasmania. Fine examples of wood-tin, occurring with topaz, are found in Durango in Mexico. Deposits of cassiterite under rather exceptional conditions are worked on a large scale in Bolivia; and it is notable that cassiterite is found in Liassic limestone near Campiglia Marittima in Tuscany. Cassiterite has been worked in the York region, Alaska.
(F. W. R.*)
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)
