Example of phenocrysts in rhomb porphyry from the Oslo rift area in Norway

A phenocryst is a relatively large and usually conspicuous crystal formed in the mass of a porphyritic igneous rock. For example, olivine may form the primary phenocrysts of some materials, and as such is used to define the subtype of that material (e.g., an 'olivine felsite'). Image File history File links Rhomb_porphyr_Oslo_rift_Norway. ... Image File history File links Rhomb_porphyr_Oslo_rift_Norway. ... Quartz crystal A crystal is a solid in which the constituent atoms, molecules, or ions are packed in a regularly ordered, repeating pattern extending in all three spatial dimensions. ... (For other meanings of Porphyr, see Porphyry) The baptismal font in the Cathedral of Magdeburg is made of rose porphyry from a site near Assuan, Egypt Porphyry is a very hard red, green or purple igneous rock consisting of large-grained crystals, such as feldspar or quartz, dispersed in a... Igneous rocks are formed when molten rock (magma) cools and solidifies, with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks. ... Olivine The mineral olivine is a magnesium iron silicate with the formula (Mg,Fe)2SiO4 in which the ratio of magnesium and iron varies between the two endmembers of the series: forsterite (Mg-rich) and fayalite (Fe-rich). ...

Phenocrysts are commonly found in materials such as felsite and andesite. Felsite is a very fine to cryptocrystalline igneous rock that may or may not contain larger crystals, called phenocrysts, that are typical of many porphyritic igneous rocks. ... A sample of andesite (dark groundmass) with amygdaloidal vesicules filled with zeolite. ...