Trachyte's an igneous, volcanic rock with an aphanitic to porphyritic texture. The mineral assemblage consists of essential alkali feldspar; relatively minor plagioclase and quartz or a feldspathoid such as nepheline may also be present. (See the QAPF diagram). Biotite, clinopyroxene and olivine are common accessory minerals. Chemically, trachyte contains less SiO2 than rhyolite and more (Na2O plus K2O) than dacite. These chemical differences are consistent with the position of trachyte in the T… (More on Trachyte)