Chapman, Carleton A. Department of Geology, University of Illinois, Urbana, Illinois.
Last reviewed:December 2019
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A relatively large crystal embedded in a finer-grained or glassy igneous rock. The presence of phenocrysts gives the rock a porphyritic texture (see illustration). Phenocrysts are represented most commonly by feldspar, quartz, biotite, hornblende, pyroxene and olivine. Strictly speaking, phenocrysts crystallize from molten rock material (lava or magma). They commonly represent an earlier and slower stage of crystallization than does the matrix in which they are embedded. Phenocrysts are to be distinguished from certain relatively large crystals (porphyroblasts) which develop late in solid rock as the result of metamorphism or metasomatism. If the origin of a large crystal is in question, the nongenetic term megacryst should be used. See also: Contact aureole; Igneous rocks; Porphyroblast; Porphyry
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