porphyroblasts

Porphyroblasts are relatively large crystals that crystallize in a metamorphic rock and stand out from the finer-grained surrounding matrix. The term is used in metamorphic petrology to describe such crystals whose size is conspicuously larger than the matrix minerals; porphyroblasts are not to be confused with phenocrysts in igneous rocks.

Porphyroblasts form when minerals crystallize during metamorphism from a mineralogical assemblage that is being recrystallized under changing temperature and pressure. They may grow by consuming or replacing preexisting grains or by nucleating anew in the deforming rock. Growth can be favored along specific chemical reactions, and porphyroblasts may develop through inclusion-rich cores and reaction rims as the surrounding matrix changes composition during metamorphism.

Textures include euhedral to subhedral porphyroblasts with internal inclusions of other minerals. Reaction rims or corona textures around the porphyroblast record local changes in mineral stability and composition during metamorphism. The presence of porphyroblasts can influence the optical and mechanical properties of the rock and can reflect the deformation fabric.

Common porphyroblasts include garnet, staurolite, kyanite, and sillimanite, as well as cordierite, in various metamorphic rocks. Porphyroblasts are used to interpret metamorphic conditions and histories because their composition, zoning, and rims can serve as geothermobarometers and record pressure–temperature paths and metasomatic events. They are especially characteristic of schists and gneisses but occur in many metamorphic rocks.

Size ranges from millimeters to centimeters; larger ones are sometimes called megaporphyroblasts.