 Gray, red, black, altered white/tan, flow-banded pumice dacite Dacite is a high-silica igneous, volcanic (extrusive) rock. It is intermediate in compostion between andesite and rhyolite, and, like andecite, it consists mostly of plagioclase feldspar with biotite, hornblende, augite or enstatite. It has an aphanitic to porphyritic texture, but they contain also quartz as rounded, corroded phenocrysts, or as an element of the ground-mass.
The feldspar content ranges from oligoclase to andesite and labradorite, and is often very zonal; sanidine occurs also in some dacites, and when abundant gives rise to rocks that form transitions to the rhyolites. The biotite is brown; the hornblende brown or greenish brown; and the augite is usually green.
The ground-mass of these rocks is often microcrystalline, with a web of minute feldspars mixed with interstitial grains of quartz; but in many dacites it is largely vitreous, while in others it is felsitic or cryptocrystalline. In the hand specimen many of the hornblende and biotite dacites are grey or pale brown and yellow rocks with white feldspars, and black crystals of biotite and hornblende. Other dacites, especially augite- and enstatite-dacites, are darker colored.
The rocks of this group occur in Hungary, Almeria (Spain), Argyllshire and other parts of Scotland, New Zealand, the Andes, Martinique, Nevada and other districts of western North America, Greece as well as other places. They are mostly associated with andesites and trachytes, and form lava flows, dikes, and, in some cases, they form massive intrusions in the centers of old volcanoes. Among continental petrographers the older dacites (Carboniferous, and others) are often known as porphyrites.
See also
This article incorporates text from the public domain 1911 Encyclopædia Britannica.
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