Definition of phenocryst in English:


Pronunciation /ˈfɛnə(ʊ)krɪst//ˈfiːnə(ʊ)krɪst/


  • 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.’
    • ‘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.’
    • ‘Orthoclase occurred as remnant phenocrysts in a groundmass of chloritic material resulting from alteration of iron-rich felsite cobbles.’
    • ‘Pyroxene mineralogy evolves along two paths with near end-member hedenbergite phenocrysts in the groundmass and aegirine solid solution in miaroles.’
    • ‘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.’


Late 19th century: from French phénocryste, from Greek phainein ‘to show’ + krustallos ‘crystal’.