Trachyte

Trachyte has a silica content of 60 to 65% and an alkali oxide content of over 7%. This gives it less SiO2 than rhyolite and more (Na2O plus K2O) than dacite.

When polished, trachyte is often used as a decorative construction material, often as flooring or countertops or even tombstones. It is also suitable for constructing walls and can be found as the foundational stone for many houses. This attractive stone may also be carved for use in statues and monuments.

Trachytes are well represented among the Cenozoic volcanic rocks of Europe. In the United States, trachytes crop out extensively in the Davis Mountains, Chisos Mountains, and Big Bend Ranch State Park in the Big Bend (Texas) region, as well as southern Nevada and South Dakota (Black Hills). There is one known voluminous flow from Pu'u Wa'awa'a on the north flank of Hualalai in Hawaii.

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.

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 name. It was first given to certain rocks of this class from Auvergne, and was long used in a much wider sense than that defined above; in fact it included quartz-trachytes (now known as liparites and rhyolites) and oligoclase-trachytes, which are now more properly assigned to andesites.