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phenocryst (plural phenocrysts)

  1. (mineralogy) any relatively large crystal embedded in a more fine-grained or glassy igneous rock
    • 1965, Gordon A Macdonald and Takashi Katsura, Eruption of Lassen Peak, Cascade Range, California, in 1915: Example of Mixed Magmas, Geological Society of America Bulletin 76 (5), pages 475:
      Some of the feldspar phenocrysts are more than 1 cm long, but most are less than 5 mm.