Acidic rock or acid rock refers to the chemical composition of

igneous rock Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma o ...

s that has 63% wt% SiO₂ content. Rocks described as acidic usually contain more than 20% of free

quartz Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical ...

. Typical acidic rocks are

granite Granite () is a coarse-grained ( phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies under ...

or rhyolite. Term is used in chemical classification of

based on the content of

silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is ...

(SiO₂). Due to the fact that chemical analyzes are not always available, especially during the fieldwork, classification based on the mineral (modal) composition is more often used (dividing the igneous rocks into felsic/leucocratic,

mafic A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks incl ...

/melanocratic and

ultramafic Ultramafic rocks (also referred to as ultrabasic rocks, although the terms are not wholly equivalent) are igneous and meta-igneous rocks with a very low silica content (less than 45%), generally >18% MgO, high FeO, low potassium, and are composed ...

). The term "silicic", widely used in North America, has wider meaning, however, is often used as broadly synonymous with "acidic". Terms describing composition of igneous rocks as acidic or basic evolved during the 19th century. It was based on the idea that high silica rocks are acidic and on the contrary the rocks with low silica content are basic. Although this idea is erroneous in a chemical sense (acidic rocks don't have low pH), both terms are used today.

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Igneous Rocks
Petrology {{petrology-stub

See also

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