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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