A
phenocryst is a relatively large and usually conspicuous
crystal distinctly larger than the grains of the rock
groundmass of an
igneous rock. 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. Phenocrysts often have
euhedral forms, either due to early growth within a
magma, or by post-emplacement
recrystallization. Normally the term
phenocryst is not used unless the crystals are directly observable, which is sometimes stated as greater than .5 millimeter in diameter. Phenocrysts below this level, but still larger than the groundmass crystals, are termed
microphenocrysts. Very large phenocrysts are termed
megaphenocrysts. Some rocks contain both microphenocrysts and megaphenocrysts. In
metamorphic rocks, crystals similar to phenocrysts are called
porphyroblasts.
