Phenocryst

Such rocks that have a distinct difference in the size of the crystals are called porphyries, and the adjective porphyritic is used to describe them.

Normally the term phenocryst is not used unless the crystals are directly observable, which is sometimes stated as greater than 0.5 mm (0.020 in) in diameter.

[6] Categorizing a rock as aphyric or as sparsely phyric is often a question of whether a significant number of crystals exceed the minimum size.

[7] Geologists use phenocrysts to help determine rock origins and transformations because crystal formation partly depends on pressure and temperature.

Plagioclase phenocrysts often exhibit zoning with a more calcic core surrounded by progressively more sodic rinds.

Granites often have large feldspathic phenocrysts. This granite, from the Swiss side of the Mont Blanc massif, has large white phenocrysts of plagioclase (that have trapezoid shapes when cut through). 1 euro coin ( diameter 2.3 cm) for scale.
Photomicrograph of a porphyritic-aphanitic felsic rock, from the Middle Eocene in the Blue Ridge Mountains of Virginia. Plagioclase phenocrysts (white) and hornblende phenocryst (dark; intergrown with plagioclase) are set in a fine matrix of plagioclase laths that show flow structure.