THE VOLCANIC EQUIVALENT of syenite, trachyte is commonly porphyritic. Some porphyritic trachytes contain phenocrysts of the mineral sanidine that are up to 2in (5cm) across. Dark, mafic minerals such as biotite, amphibole, and pyroxene can be present in small quantities. Trachyte is similar to rhyolite in color and occurrence, but contains very little or no quartz. Quartz-free trachytes, such as phonolite, can contain feldspathoids such as leucite, nepheline, and sodalite. Unlike andesite, trachyte contains no phenocrysts of plagioclase feldspar. Trachyte has a characteristically rough texture, and its name comes from the Greek for rough.
Trachyte occurs with other alkali-rich felsic, intermediate, and mafic volcanics on both continents and ocean islands. Localities include Ascension Island; Cripple Creek, Colorado, USA; the Auvergne in France; and the Rhine district of central Europe.
- Rock type Intermediate, volcanic, igneous
- Major minerals Sanidine, oligoclase
- Minor minerals Feldspathoids, quartz, hornblende, pyroxene, biotite
- Color Off-white, gray, pale yellow, pink
- Texture Fine to medium, porphyritic
Tough and resistant volcanic rocks such as trachyte have been used for millennia as paving stones. In addition to trachyte, granite, basalt, and gabbro can still be found forming the surfaces of streets and roads. Individual stones may have been used and reused for centuries. In some places the cobbled surfaces of Roman roads still remain, grooved by the passage of countless wheels of all varieties. Although labor-intensive to mine and shape, the resulting surface has unsurpassed wear-resistance.
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