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Phenocryst identification and geology

A relatively large, conspicuous crystal embedded in a finer-grained groundmass in a porphyritic igneous rock. Phenocrysts formed during a period of slow cooling at depth, before the remaining magma was erupted or rapidly cooled, producing the fine-grained matrix. Phenocryst mineralogy and composition provide information about the magma's crystallization history and storage conditions. Field: petrology. Example: Large feldspar phenocrysts up to 5 cm in a rhyolite porphyry indicate that the magma spent considerable time crystallizing in a magma chamber before erupting and rapidly quenching the remaining liquid to a fine-grained or glassy groundmass.. Related terms: Groundmass, Porphyritic, Crystal, Magma.

Sources, scope & corrections

These authoritative resources support category-level nomenclature, terminology, identification methods, and safety. When a specimen-specific safety warning appears, the public-health source listed here directly supports that precaution. They are not line-by-line citations for every other property on this entry, and natural specimens vary.

Editorial owner
GeoMiner
Last updated
2026-03-30T15:19:42.055955+00:00
Review status
Published; not independently peer-reviewed
  • USGS Thesaurus — U.S. Geological Survey. Controlled earth-science terminology used to describe USGS information resources.
  • Glossary of Geologic Terms — U.S. National Park Service. Geologic definitions prepared by the NPS Geologic Resources Inventory.

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Common questions

What does Phenocryst mean in geology?
A relatively large, conspicuous crystal embedded in a finer-grained groundmass in a porphyritic igneous rock. Phenocrysts formed during a period of slow cooling at depth, before the remaining magma was erupted or rapidly cooled, producing the fine-grained matrix. Phenocryst mineralogy and composition provide information about the magma's crystallization history and storage conditions.
What is an example of Phenocryst?
Large feldspar phenocrysts up to 5 cm in a rhyolite porphyry indicate that the magma spent considerable time crystallizing in a magma chamber before erupting and rapidly quenching the remaining liquid to a fine-grained or glassy groundmass.
Which earth-science field uses the term Phenocryst?
Phenocryst is used in petrology.
Which terms are related to Phenocryst?
Related terms in this reference are Groundmass, Porphyritic, Crystal, Magma.

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