The acidic igneous rocks (commonly referred to as “acid” rocks and rarely as acidites) comprise those rocks, whatever their origin and whatever level of the crust in which they crystallize, that are sufficiently silica-rich to contain modal (actual) or normative (theoretical) quartz in quantities of about 10% or more. (Persilicic is a little-used term equivalent to acidic as applied to igneous rocks). Among the volcanic rocks, rhyolite, obsidian, and acidic tuff are the predominant representatives; among the deep-seated rocks, granite and granodiorite are most common.
The term acid (acidic) (along with intermediate, basic, and ultrabasic) is an unfortunate and archaic term and should be replaced by such acronyms as “felsic” or “sialic.” This nomenclature appears to have originated with Elie de Beamont in 1847. The Sauerstoffs-quotient (“oxygen quotient” or “acidity ratio”) of Bishof and Roth in the middle of the 19th century provides an insight into the terminology. By this...
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Ward, R.F. (1989). Acid(IC) igneous rocks . In: Petrology. Encyclopedia of Earth Science. Springer, Boston, MA. https://doi.org/10.1007/0-387-30845-8_2
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