A phenocryst is a relatively large and usually conspicuous crystal formed in the mass of a porphyritic igneous rock. For example, olivine may form the primary phenocrysts of some materials, and as such is used to define the subtype of that material (e.g., a 'porphyritic olivine basalt').

Phenocrysts are commonly found in materials such as felsite and andesite.

Petrology

Fenokristall | Fenocristal