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Concept
Trachyte
Summary
Trachyte (ˈtreɪkaɪt,_ˈtrækaɪt) is an extrusive igneous rock composed mostly of alkali feldspar. It is usually light-colored and aphanitic (fine-grained), with minor amounts of mafic minerals, and is formed by the rapid cooling of lava enriched with silica and alkali metals. It is the volcanic equivalent of syenite.
Trachyte is common wherever alkali magma is erupted, including in late stages of ocean island volcanism and in continental rift valleys, above mantle plumes, and in areas of back-arc extension. Trachyte has also been found in Gale crater on Mars.
Trachyte has been used as decorative building stone and was extensively used as dimension stone in the Roman Empire and the Republic of Venice.
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. 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. Trachydacite occupies the same field in the TAS diagram as trachyte, but is distinguished from trachyte by a normative quartz content over 20%. Trachydacite is not a recognized rock type in the QAPF classification, where rocks rich in alkali feldspar and with quartz over 20% would be classified as rhyolites.
The mineral assemblage of trachytes consists of essential alkali feldspar. Relatively minor plagioclase and quartz or a feldspathoid such as nepheline may also be present. This is reflected in the position of the trachyte fields in the QAPF diagram. Biotite, clinopyroxene and olivine are common accessory minerals. The plagioclase is typically sodium-rich oligoclase. The alkali feldspar is typically also sodium-rich sanidine (anorthoclase) and is often cryptoperthitic, with alternating microscopic bands of sodium feldspar (albite) and potassium feldspar (sanidine).
Trachytes are typically fine-grained and light-colored, but can be black if they consist mostly of glass.
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