# Porphyroblast > Mediated Wiki article. Canonical URL: https://mediated.wiki/source/Porphyroblast > Markdown URL: https://mediated.wiki/source/Porphyroblast.md > Source: https://en.wikipedia.org/wiki/Porphyroblast > Source revision: 1297726373 > License: Creative Commons Attribution-ShareAlike 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/) {{Short description|Large mineral crystal in a metamorphic rock}} [[File:Thin section of garnet-mica-schist.jpg|thumb|300px|[Thin section](/source/Thin_section) of a garnet porphyroblast (black) in a mica schist that contains foliated trails of small inclusions (white and grey)]] A '''porphyroblast''' is a large [mineral](/source/mineral) [crystal](/source/crystal) in a [metamorphic rock](/source/metamorphic_rock) which has grown within the finer grained [matrix](/source/Matrix_(geology)). Porphyroblasts are commonly [euhedral](/source/euhedral) crystals, but can also be partly to completely irregular in shape. The most common porphyroblasts in [metapelite](/source/pelite)s (metamorphosed [mudstone](/source/mudstone)s and [siltstone](/source/siltstone)s) are [garnet](/source/garnet)s and [staurolite](/source/staurolite)s, which stand out in well-[foliated](/source/Foliation_(geology)) metapelites (such as [schist](/source/schist)s) against the platy [mica](/source/mica) matrix. A similar type of crystal is a ''[phenocryst](/source/phenocryst)'', a large crystal in an [igneous rock](/source/igneous_rock). Porphyroblasts are often confused with ''[porphyroclast](/source/porphyroclast)s'', which can also be large outstanding crystals, but which are ''older'' than the matrix of the rock.{{cite book | url=https://books.google.com/books?id=2lYCFGeuAVcC&q=porphyroclast&pg=PA140 | title=Microtectonics | publisher=Springer Science & Business Media | author=Passchier C.W. & Trouw R.A.J. | year=2005 | pages=132–141 | isbn=9783540293590}} If a porphyroblastic mineral has small inclusions of minerals within it, the mineral is described as [poikiloblast](/source/poikiloblast)ic. This observation can help interpret deformation history. 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.Hyeong, K.;Jung, W.-S. (2010). The use of garnet porphyroblasts to resolve the metamorphic pressure-temperature-deformation (P-T-d) path: An example from the Imjingang belts, South Korea. Geosciences Journal - GEOSCI J. 14. 111-126. [https://link.springer.com/article/10.1007/s12303-010-0012-6 10.1007/s12303-010-0012-6]Hyeong, K.; Sanislav, I. (2017). Foliation intersection/inflection axes within porphyroblasts (FIAs): a review of advanced applications and significance. Geosciences Journal. 21. 1013-1032. [https://link.springer.com/article/10.1007/s12303-017-0047-z 10.1007/s12303-017-0047-z] 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 curving trails of quartz and other mineral inclusions that record rotation of the crystals relative to their surroundings. However, the question of how much porphyroblasts actually rotate in an external reference frame fixed to the Earth's surface during metamorphism and deformation has long been the subject of debate. The question focused on so-called ''"spiral garnets"'', also known as ''"snowball garnets"'', whose inclusion trails define spiral patterns. These microstructures are interpreted classically as having formed by shearing induced rotation of a growing garnet crystal.{{Cite book|title=Rotated garnets in metamorphic rocks. Special Paper of the Geological Society of America, 129 pp.|last=Rosenfeld|first=J.L.|publisher=Geological Society of America|year=1970}} Later research, however, led to an alternative formation model in which a porphyroblast grows over a developing microfold while maintaining a stable position in the external reference frame.{{Cite journal|author1=T.H. Bell |author2=S.E. Johnson|year=1989|title=Porphyroblast inclusion trails: the key to orogenesis|journal=Journal of Metamorphic Geology |volume=7|issue=3 |pages=279–310 |doi=10.1111/j.1525-1314.1989.tb00598.x|bibcode=1989JMetG...7..279B}} Repetition of this process can then produce complex spiral-shaped patterns. Although many researchers continue to adopt the classic rotational model, most researchers who have published research testing both models by measuring the orientations of porphyroblasts have come to support the modern interpretation.{{Cite journal|url=https://www.sciencedirect.com/journal/tectonophysics/vol/587/suppl/C|title=Deformation, Pophyroblasts and Mountain Building: A Special Issue in Honor of the Career Contributions of T.H. Bell|author1=D. Aerden|author2=S.E. Johnson|author3=K. Michibayashi|journal=Tectonophysics|volume=587 |year=2013}} File:Almandine.jpeg|An [almandine](/source/almandine)-[garnet](/source/garnet) growing as a porphyroblast in a [quartzitic](/source/quartzite) [gneiss](/source/gneiss) File:Garnet Mica Schist Syros Greece.jpg|Dark-coloured porphyroblasts of garnet in mica schist at Syros, Greece File:Almandin.jpg|Large almandine porphyroblasts growing in gray-green schist File:Garnet schist 1 (16735443408).jpg|Garnet porphyroblasts in schist featuring the deflection of the foliation around their space File:Staurolite-ostruzna-czech-republic.jpg|[Staurolite](/source/Staurolite) growing as porphyroblasts in a mica schist File:Garnet - almandine Fe2+3Al2-SiO4-3 (33455134241).jpg|Almandine porphyroblast in a schist, showcasing the curved trail of the surrounding matrix File:Orange spessartine garnets in schist (Outomongu Mine, Namibia) 3.jpg|[Spessartine](/source/Spessartine) porphyroblasts in schist ==See also== {{Commons category|Porphyroblasts}} * [List of rock textures](/source/List_of_rock_textures) * [Rock microstructure](/source/Rock_microstructure) ==References== {{Reflist}} Category:Metamorphic petrology --- Adapted from the Wikipedia article [Porphyroblast](https://en.wikipedia.org/wiki/Porphyroblast) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Porphyroblast?action=history)). 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