Trachyte

Trachyte is an igneous volcanic rock with an aphanitic to porphyritic texture. It is the volcanic equivalent of syenite. 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.
Chemical composition
Chemically, trachyte contains 60 to 65% silica content; less SiO2 than rhyolite and more (Na2O plus K2O) than dacite. These chemical differences are consistent with the position of trachyte in the TAS classification, and they account for the feldspar-rich mineralogy of the rock type.
Mineralogy
Trachytes usually consist mainly of sanidine feldspar. Very often they have minute irregular steam cavities which make the broken surfaces of specimens of these rocks rough and irregular, and from this character they have derived their...