- Porphyroblast
thumb|right|350px|An_almandine -garnet growing as a porphyroblast in a quartziticgneiss . This particular garnet measures 3 cm. Location:Paraíba ,Brazil .] A porphyroblast is a largemineral crystal in ametamorphic rock which has grown within the finer grained groundmass. Porphyroblasts are commonlyeuhedral crystals, but can also be partly to completely irregular in shape.The most common porphyroblasts in meta
pelite s (metamorphosedmudstone s andsiltstone s) aregarnet s andstaurolite s, which stand out in well foliated metapelites (such asschist s) against the platymica matrix.A similar type of crystal is a "
phenocryst ", a large crystal in anigneous rock . Porphyroblasts are often confused with "porphyroclast s", 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".
As porphyroblasts grow, the foliation may be preserved as oriented inclusions trapped by the porphyroblast as it overgrows them, and this is helpful for tracking changing deformation planes.
In metamorphic rocks that experience deformation during metamorphism, porphyroblasts may grow before, during, or after the phase of deformation recorded by the matrix minerals. The relationship of porphyroblast growth to deformation is typically evaluated by comparing the shape orientation of trails of mineral inclusions in the porphyroblast to the matrix fabric.
Some garnet porphyroblasts contain intensely curving trails of quartz and other mineral inclusions, and have been called 'snowball garnets'. The question of whether porphyroblasts (and garnet in particular) rotate during metamorphism and deformation has long been the subject of debate.
ee also
*
List of rock textures
*Rock microstructure
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