A large or conspicuous crystal in a porphyritic rock, distinct from the groundmass.
- 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.
- 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.
Late 19th century: from French phénocryste, from Greek phainein 'to show' + krustallos 'crystal'.
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