Porphyroblasts (POR-fer-o-blasts) are the name for especially large crystals in metamorphic rocks, analogous to phenocrysts in igneous rocks. They are mineral grains that form during the process of metamorphism. But outside of research papers, most geologists just call them crystals. Learn more about the terminology of large mineral particles in Crysts, Blasts and Clasts. Porphyroblasts 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 rock during metamorphism.
This set of photos shows some of the typical minerals that occur as porphyroblasts as well as the variety of metamorphic rocks they occur in. Usually porphyroblasts are common minerals, but any of the metamorphic minerals may be found as porphyroblasts.
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- Graphic Index
- Text Index
