The tin occurs in the form of cassiterite, and is found chiefly in or near the crystalline rocks, especially the granite.
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.
- This, if the term is taken to include the hydrates, exists in a variety of forms. (I) Tinstone (see above and also Cassiterite) is proof against all acids.
Rutile assumes tetragonal forms isomorphous with cassiterite, SnO 2 (and also zircon, ZrSiO 4); anatase is also tetragonal, and brookite or thorhombic. Rutile is the most stable and anatase the least, a character reflected in the decrease in density from rutile (4.2) and brookite (4.0) to anatase (3.9).
In Cornwall and Devon it is associated with cassiterite in the tinlodes, but is also found in the copper-lodes: well crystallized specimens have been obtained from the neighbourhood of Tavistock, Redruth and St Agnes.
Zirconia can be obtained crystalline, in a form isomorphous with cassiterite and rutile, by fusing the amorphous modification with borax, and dissolving out with sulphuric acid.
It is an amorphous white powder; but it may also be obtained in crystals isomorphous with cassiterite by heating the amorphous form with borax to a very high temperature.