Phenocryst
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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., an 'olivine felsite').
Phenocrysts are commonly found in materials such as felsite and andesite.