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The light-colored, aphanitic rock (the volcanic equivalent of syenite), composed largely of alkali feldspar with minor amounts of mafic minerals.



a cenotypal extrusive, usually porphyritic rock. Porphyritic phenocrysts and microlites embedded in volcanic glass are represented by sanidine; neutral and acid plagioclase, biotite, pyroxene, or amphibole are encountered in lesser amounts. Trachyte is the extrusive equivalent of syenite. It consists of up to 60 percent silica and up to 10 percent alkalies. The rock is rough to the touch. There are glassy trachytes, such as obsidians and pumices, and trachytic tuffs. Trachytes grade into liparites, andesites, and basalts. They are found in the Caucasus and, outside the USSR, in Italy and France. Trachyte is a relatively rare rock. (See also.)

References in periodicals archive ?
Rocks were classified as K-feldspar trachytes on modal observations (Table 1), but sample PPG-084i chemical composition suggests high-K calc-alkaline rhyolites (Table 2, Figure 7).
Modal classification (Table 1) indicates K-feldspar trachytes.
Modal (Table 1) and chemical classification (Figure 7) indicate alkaline affinity trachytes with less than 20[degrees]o normative quartz.
2] conditions, slightly Ne-normative basalts can yield highly evolved peralkaline, Hy- and Q-normative trachytes through a two-stage fractionation process occurring in deep and then shallow-level magma chambers.
86]Sr ratios in Kerguelen trachytes supports assimilation of hydrothermally altered oceanic crust in subsurface magma chambers (Gagnevin et al.
Puis, aux stades inter mediaires, la formation de plagioclases reduit considerablement la teneur en Sr et, aux stades finaux, la formation de feldspaths alcalins en extrait le Ba et le Rb alors que se forment les trachytes et les phonolites a partir de magmas alcalins.
Phenocryst-poor latite and trachyte are interpreted as the result of mineral separation, and phenocryst-rich tephrite porphyry and olivine diabase porphyry are the result of phenocryst accumulations in the magma chamber.
From oldest to youngest (Elston and Snider, 1964), they are diabase porphyry, olivine diabase porphyry, fine-grained diabase, latite or trachyte, tephrite porphyry, and rhyolite.
On a total alkali-silica plot (LeMaitre, 1984) compositions span a broad range from tephrite to trachyte (Fig.
Those are hypocrystalline alkaline trachytes and rhyolites (Pardo, 2004), that suggest that volatile release was continuous through the permeable conduit walls while magma rising occurred.