Definitions

1. noun

A crystal formed in a magma chamber and ejected during volcanic eruptions, often larger and more visible than the surrounding groundmass.

“The geologist studied the phenocrysts in the volcanic rock sample to determine its origin.”

2. noun

A large crystal or fragment of a mineral that forms in a magma chamber and appears in a rock after it has cooled.

“The phenocryst of quartz in the granite sample was particularly large and well-formed.”