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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.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Porphyroblast". A list of authors is available in Wikipedia.|