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=== Acid rock ===
In modern usage, the term ''acid rock'', although sometimes used as a synonym, normally now refers specifically to a high-silica-content (greater than 63% SiO<sub>2</sub> by weight) [[volcanic rock]], such as [[rhyolite]].  Older, broader usage is now considered archaic.{{Citation needed|date=March 2019}}  That usage, with the contrasting term "basic rock" (MgO, FeO, [[mafic]]), was based on an ancient concept, dating from the 19th century, that "[[silicon dioxide|silicic acid]]" (H<sub>4</sub>SiO<sub>4</sub> or Si(OH)<sub>4</sub>) was the chief form of silicon occurring in siliceous rocks. Although this intuition makes sense from an [[acid-base]] perspective in [[aquatic chemistry]] considering water-rock interactions and silica dissolution, siliceous rocks are not formed by this protonated monomeric species, but by a tridimensional network of SiO<sub>4</sub><sup>4–</sup> tetrahedra connected to each other. Once released in water and hydrolyzed, these silica entities can indeed form [[silicic acid]] in aqueous solution.