Dacite

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Dacite (pronounced /deɪsaɪt/) is an igneous, volcanic rock with a high iron content. It is intermediate in compositions between andesite and rhyolite, and, like andesite, it consists mostly of plagioclase feldspar with biotite, hornblende, and pyroxene (augite and/or enstatite). It has an aphanitic to porphyritic texture with quartz as rounded, corroded phenocrysts, or as an element of the ground-mass. The relative proportions of feldspars and quartz in dacite, and in many other volcanic rocks, are illustrated in the QAPF diagram. Dacite is also defined by silica and alkali contents in the TAS classification.

The plagioclase ranges from oligoclase to andesine and labradorite, and is often very zoned. 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 groundmass of these rocks is often microcrystalline, with a web of minute feldspars mixed with interstitial grains of quartz or tridymite; but in many dacites it is largely vitreous, while in others it is felsitic or cryptocrystalline. In 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.

Dacites are mostly associated with andesites and trachytes, and form lava flows, dikes, and in some cases form massive intrusions in the centers of volcanoes. Dacite is an important rock type at Mount St. Helens.

The word dacite comes from Dacia, a province of the Roman Empire which lay between the Danube River and Carpathian Mountains (now modern Romania) where the rock was first described.

This article incorporates text from the Encyclopædia Britannica Eleventh Edition, a publication now in the public domain.