Trachyte

Trachyte a volcanic rock or lava, named from the rough surface with which it breaks (Greek trachus = "rough "). It belongs in composition to the intermediate class, consisting essentially of sanidine felspar, and containing from 60 to 64 per cent. of silica. Its other chemical constituents are: alumina, 17 per cent., iron-oxide 6-8 per cent., soda 4 per cent., lime 3-5, potash 2-2.5, magnesia 1. In addition to sandine it contains some triclinic felspar, and usually hornblende, biotite mica, and magnetite, with occasional1y augite, apatite, and tridymite. Its average specific gravity is 2.65. It varies in texture, but is largely glassy with felspar microliths and hornblende needles. It belongs mostly to Tertiary volcanic outpourings, being one of the commonest of modern lavas.