Trachyte
Trachyte (/ˈtreɪkaɪt, ˈtræk-/) 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.
| Igneous rock | |
A cut sample of trachyte, with phenocrysts of sanidine, in a wall in Germany | |
| Composition | |
|---|---|
| Classification | Felsic |
| Primary | Alkali feldspar |
| Secondary | Plagioclase, quartz, or feldspathoid |
| Texture | Aphanitic |
| Equivalents | Intrusive variant is 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.
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