Porphyroblast

    A porphyroblast is a large mineral crystal in a metamorphic rock which has grown within the finer grained groundmass as a result of static recrystallisation in the rock. Therefore porphyroblasts are almost always nice euhedral crystals.

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The most common porphyroblasts in metapelites (metamorphosed mudstones and siltstones) are garnets and staurolites, which stand out in well foliated metapelites (such as schists) against the platy mica matrix.

A similar type of crystal is a phenocryst, a large crystal in an igneous rock. Porphyroblasts are often confused with porphyroclasts, which can also be large outstanding crystals, but which are older than the matrix of the rock.

A rock which has many porphyroblasts is described as having a porphyroblastic texture.

See also

• List of rock textures
• Rock microstructure