a mineral consisting of silicon oxide or silica, SiO 2, 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 Tpiflv,uos, 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. S.)