phenocryst

Phenocryst is a relatively large crystal that forms in silicate magmas before eruption and remains embedded in a finer-grained groundmass in porphyritic igneous rocks. The term contrasts with the groundmass or matrix, which consists of smaller crystals that crystallized after extrusion as the magma rapidly cooled.

Formation and texture: Phenocrysts crystallize during the early stages of cooling within a magma chamber or conduit, while the remaining melt continues to crystallize later into a finer-grained groundmass. During eruption or rapid ascent, the melt is quenched, producing the fine-grained matrix that hosts the phenocrysts. The result is a porphyritic texture in which large crystals are conspicuous.

Mineralogy and size: Phenocrysts commonly include feldspar (plagioclase and/or potassium feldspar), quartz in silica-rich rocks, and ferromagnesian minerals such as olivine, pyroxene, and amphibole; magnetite, biotite, and zircon may occur as accessory minerals. Sizes typically range from a few millimeters to several centimeters, making them readily visible in hand specimens. Microphenocrysts are smaller crystals that are still larger than the groundmass.

Interpretation and use: The composition, zoning, and abundance of phenocrysts record magmatic history, including crystallization sequence, fractional crystallization, magma mixing, and pre-eruption conditions. They aid in interpreting magmatic evolution, cooling history, and tectonic setting within volcanic suites.

Occurrence: Phenocrysts are characteristic of many volcanic rocks, such as basalts, andesites, and rhyolites, as well as some intrusive rocks with porphyritic textures.