Porphyroblast

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An almandine-garnet growing as a porphyroblast in a quartzitic gneiss. This particular garnet measures 3 cm. Location: Paraíba, Brazil.
An almandine-garnet growing as a porphyroblast in a quartzitic gneiss. This particular garnet measures 3 cm. Location: Paraíba, Brazil.
Purple pyrope porphyroblasts in a peridotite, coin of 1 euro for scale. The yellow-brown groundmass is weathered olivine. Location: Otrøy, Western Gneiss Region, Caledonides, Norway.
Purple pyrope porphyroblasts in a peridotite, coin of 1 euro for scale. The yellow-brown groundmass is weathered olivine. Location: Otrøy, Western Gneiss Region, Caledonides, Norway.

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.

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.

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