Applied to an igneous rock rich in silica to the extent of its forming more than about two-thirds of its mass. Such a rock is usually oversaturated with silica so that free silica, quartz, is present. The chief types are granite, adamellite, grano-diorite, microgranite, rhyolite, dacite. ‘Nearly all the important rock-forming minerals are silicates, or compounds of silica with one or more metallic oxides. In these compounds silica may be regarded as playing the part of an acid, while the metallic oxides act as bases. Hence rocks rich in silica are spoken of as “acid”, and those poor in silica but rich in metallic oxides as “basic”‘.
‘Rocks rich in silica were first spoken of as acid and called “acidites” by Von Cotta (1864)’ (Johannsen Petrography, 1939). ‘Acidic’ is sometimes preferred. Either term may refer to a plagioclase felspar towards the albite end of the series. (Attempts that have been made to distinguish uses of ‘acid’ from uses of ‘acidic’ do not seem satisfactory.)
As applied to soils, see pH value.
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