TRIDYMITE  , a

mineral consisting of silicon oxide or silica, SiO2, but differing from quartz in crystalline form . The crystals are small, thin hexagonal plates or scales, which are usually twinned together in groups of three; hence the name of the mineral, from Greek rplSvilos, triplet . The apparent hexagonal plates are themselves pseudo-symmetric twins of optically biaxial material, and the exact crystalline form is doubtful . The plates are colourless and transparent and have a vitreous lustre . The hardness is 7 and the specific gravity 2.3 (that of quartz being 2.65) . Unlike quartz, it is soluble in a boiling solution of sodium carbonate . Tridymite occurs in the cavities of acid volcanic rocks (rhyolite, trachyte and andesite); the best-known localities are Cerro San Cristobal near Pachuca in Mexico, the Euganean Hills near Padua, and the Siebengebirge on the Rhine . Probably identical with tridymite is the form of silica known as asmanite, found in the meteorite which fell at Breitenbach in the Erzgebirge, Bohemia . (L . J .