In geology, a crystal in crystalline rock significantly larger than the surrounding groundmass. The presence of phenocrysts marks a porphyritic texture, and it's generally a sign of a sudden change in temperature or pressure while the magma is cooling.
From the greek, phainein, to show + krystallos, crystal.
Phe"no*cryst (?), n. [Gr. &?; to appear + &?; crystal.] (Geol.)
One of the prominent embedded crystals of a porphyry.
© Webster 1913
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