Trachyte

trachyte, light-coloured, very fine-grained extrusive igneous rock that is composed chiefly of alkali feldspar with minor amounts of dark-coloured minerals such as biotite, amphibole, or pyroxene. Compositionally, trachyte is the volcanic equivalent of the plutonic (intrusive) rock syenite. Most trachytes show porphyritic texture in which abundant, large, well-formed crystals (phenocrysts) of early generation are embedded in a very fine-grained matrix (groundmass). The phenocrysts are usually sanidine, a glassy potash feldspar commonly rich in sodium, and may range up to two to five centimetres (one to two inches) across; smaller phenocrysts of other minerals may also occur. Rapid cooling and solidification of trachytic lava produces the fine texture of the groundmass, and cooling may be so rapid locally that small quantities of glass are formed.

Trachyte is commonly associated with other lavas in volcanic regions and is thought to have been formed by the crystallization and abstraction of iron, magnesium, and calcium minerals from a parent basaltic lava.

Two types of trachyte are commonly recognized. In potash, or normal, trachyte, sanidine or orthoclase is the dominant feldspar, and the plagioclase feldspar is generally oligoclase. In soda, or alkali, trachyte, both the alkali feldspar and the dark minerals are rich in sodium. As the ratio of alkali feldspar to plagioclase decreases, trachyte passes into latite. Brown biotite is a common constituent, as is amphibole. Normal trachytes may contain diopside; alkali types may show pyroxene crystals with diopsidic cores.

Silica-rich trachyte may have small amounts of quartz that is interstitial to feldspar; tridymite and cristobalite are commonly found lining the small cavities that developed as expanding gas bubbles in the molten lava. As the free silica content increases, trachyte passes into rhyolite. Marked decrease in silica favours the formation of such feldspathoids as leucite, nepheline, sodalite, and analcime; as the quantity of feldspathoids increases, trachyte passes into phonolite.

Like many volcanic rocks, trachyte shows a streaked or banded structure due to flowage of the congealing lava. This structure may be revealed by a conspicuous parallel arrangement of large tabular phenocrysts. Microscopic examination of thin sections reveals the trachytic texture of the fine matrix; tiny lath-shaped sanidine crystals are closely packed in parallel fashion and form flow lines that tend to wrap around the large phenocrysts.