A large or conspicuous crystal in a porphyritic rock, distinct from the groundmass.
- ‘The majority of the rhyolites are porphyritic and contain phenocrysts of one or more of the following: plagioclase, clinopyroxene, magnetite, quartz and sanidine, plus accessory apatite and zircon.’
- ‘Pyroxene mineralogy evolves along two paths with near end-member hedenbergite phenocrysts in the groundmass and aegirine solid solution in miaroles.’
- ‘Bulk tephra can include phenocrysts or xenocrysts, lithic fragments and other inclusions, the proportions of which may vary across the fallout area, making accurate correlation difficult.’
- ‘Rhyolite dykes, which are commonly porphyritic with phenocrysts of either plagioclase or quartz, are intruded by dykes of hornblende tonalite, and the gabbro is likewise intruded by tonalite dykes.’
- ‘Orthoclase occurred as remnant phenocrysts in a groundmass of chloritic material resulting from alteration of iron-rich felsite cobbles.’
Late 19th century: from French phénocryste, from Greek phainein to show + krustallos crystal.