Phenocrysts ("FEEN-o-crists") are mineral crystals in an igneous rock that stand out because of their large size. A rock with phenocrysts is said to have a porphyritic texture, or it may be called a specific kind of porphyry. Phenocrysts often take on the full crystal form typical of the mineral (euhedral), but they may also be partially crystalline (subhedral) or formless (anhedral) depending on the conditions in the magma while they were crystallizing.
Phenocrysts are interesting to igneous petrologists because they clearly began growing early in the cooling history of the rock, before the fine-grained groundmass crystallized around them. Or they may have arrived by gravity, either floating to the top or sinking to the bottom of a magma chamber.
Phenocryst is scientific Greek meaning "crystal that shows." The term is used only in igneous rocks. Unusually large crystals or grains in sedimentary rocks are called phenoclasts, and those in metamorphic rocks are called porphyroblasts. (Learn more in Crysts, Blasts and Clasts.)
This set of photos shows some of the typical minerals that occur as phenocrysts as well as the variety of igneous rocks they occur in. Usually phenocrysts consist of common minerals, but any of the primary minerals may be found as phenocrysts.
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