pressure shadow
A pressure shadow (also called strain shadow) is a term used in metamorphic geology to describe a microstructure in deformed rocks that occurs adjacent to a relatively large, undeformed particle, such as a porphyroclast. Pressure shadows often appear in thin sections as pairs of roughly triangular regions that are elongated parallel to the foliation around a clast of a different mineral.{{cite book |last1=Passchier and Trouw |title=Microtectonics |date=2005 |publisher=Springer |isbn=978-3-540-29359-0 |edition=2nd}} Pressure shadows that contain fibrous mineral textures are also termed pressure fringes or strain fringes.{{cite journal|author=Müller, W.|author2=Aerden, D.|author3=Halliday, A. N.|author-link3=Alexander Halliday|year=2000|title=Isotopic dating of strain fringe increments: duration and rates of deformation in shear zones|journal=Science|volume=288|issue=5474|pages=2195–2198|doi=10.1126/science.288.5474.2195|doi-access=free|pmid=10864865 |bibcode=2000Sci...288.2195M }}
Formation
During deformation, minerals can migrate by plastic flow or may grow by diffusive mass transport into the lower-stress regions created by a rigid porphyroclast or porphyroblast.{{cite book |last1=Blenkinsop, T.G. |title=Deformation Microstructures and Mechanisms in Minerals and Rocks |date=2007 |publisher=Kluwer Academic Publishers |isbn=978-0-306-47543-6 |pages=150}}