CASSITERITE, the mineralogical name of tin-stone, SnO,, the common ore of tin (from the Gr. aaaairEpos, tin). It crystallizes in the tetragonal system, usually in prisms with four or eight sides, terminated by pyramids. Twinning is common, as shown by re-entrant angles on the crystals, certain slender pris matic crystals are called "sparable tin" and "needle tin" in Corn wall, where the usual name for the ordinary mineral is "black tin." The banded and fibrous mineral of the same composition known as "wood tin" is probably formed by precipitation from the colloidal state. It is common in Cornwall and Bolivia. Cassiterite varies from colourless to dark brown or black, the darker shades being commonest. It has a brilliant adamantine lustre : hardness about 6.5 and density 7. It is also very stable and therefore readily concentrated in residual and alluvial deposits, from which in fact most of the ore is obtained. Primary cassiterite occurs in disseminations, segregations, lodes and veins in and around gran ites, in association with tourmaline, topaz, lithia-mica, fluorspar, beryl, and other minerals of pneumatolysis (q.v.), and is often accompanied by wolf ramite. It is believed to be formed at a late stage in the cooling of the granite by a reaction between tin fluoride and water.
Cassiterite is found in all parts of the world.