Phenocrysts - Larger crystals in a groundmass of fine crystals.
Unusually large, often euhedral, crystals in an otherwise fine-grained igneous rock. The resultant texture is a porphyritic texture. Phenocrysts are the result of slower crystallisation of the magma than the fin-grained matrix. See Porphyritic texture.The... More
Crystals in a lava, pumice, or scoria which form by slow cooling of the magma in the magma chamber. When the magma rises to the surface it cools more rapidly and the remaining melt either chills to a glass (as in pumice or scoria) or crystallises to a fine mesh of microscopic crystal called the groundmass (as in many lavas). Phenocrysts in Montserrat rocks may reach a millimetre or more in size, and may sometimes occur in clusters.