Everything about Dacite totally explained
Dacite 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's largely vitreous, while in others it's 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.
Further Information
Get more info on 'Dacite'.
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