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Very fine crystalline or glassy rock of volcanic origin, composed chiefly of sodic plagioclase and free silica with subordinate dark-colored minerals.



a magmatic rock that is the effusive equivalent of granodiorite and quartz-diorite. Dacite consists of a finegrained or vitreous basic mass and phenocrysts of plagioclase and quartz, more rarely, of hornblende, biotite, and pyroxene, and sometimes, sanidine. Where albitization processes are widely developed, dacite turns into quartz al-bitophyre. Dacites were ejected by volcanoes in the Paleozoic (in the Urals, for example), Mesozoic, and Cenozoic (in the Caucasus, for example). They are also found among modern lavas on Kamchatka.

References in periodicals archive ?
The comparison of plagioclase factors with the other areas show that our results are meaningful and as a rule, the number of plagioclase nuclei is less than for amphibole in the dacitic rocks.
1980) recognized a unit of dacitic tufts and breccias (their 'Bluff tuff-breccia') overlain by andesitic sedimentary and tuffaceous rocks (the term 'possibly' is used here because descriptions of rock units by different authors are comparable only in a general sense, not in detail).
The Fourchu Group consists of volcanogenic, largely pyroclastic, rocks of dacitic chemical composition (Si02/K20 typically about 70), variably foliated, alternating with basaltic sheets.
In addition, the local intrusions of andesitic and dacitic subvolcanic bodies can be observed in these lithologies (Figure 2b).
Mineralization consists of fine-grained disseminated gold hosted by dacitic to dioritic subvolcanic porphyries.
The entire mineralized drill intersection noted above is contained within oxidized andesitic volcano-clastic and dacitic intrusive rocks that are variably altered with iron oxides, clay, and silica.
The sequence continues with an Eocene-Oligocene unit composed of conglomerates, sandstones, multicolor tuffs, andesites, breccia and ignimbrite that rests unconformably on the Permo-Lower Triassic volcanic complex and is intruded by Miocene intrusive rocks of varied composition (granodioritic, andesitic, dacitic, dioritic, and granitic).
These welded tuff samples have well-preserved pyroclastic flow features and are more likely separate events, which are commonly associated with andesitic and dacitic lava flows (Cas & Wright, 1988).
The Fourchu Group consists of mainly dacitic tuffs and flows, with minor basaltic to rhyolitic tuffs and fiows and tuffaceous sedimentary rocks, and was interpreted by Barr et al.
The primary external building stone is a fine-grained, light grey, dacitic volcanic rock, commonly called Haddington Island andesite.
Occurrence: In hydrothermally altered rhyolitic and dacitic pyroclastic volcanic rocks.
The mineralisation, partly free gold and partly associated with pyrite, is associated with multiple fractures and breccias within two major dacitic intrusions, Cetate and Cirnic.