A growth of fibrous quartz, calcite, chlorite, or muscovite, normal (or sometimes parallel) to the face of a porphyroblast (usually pyrite or magnetite) in a regional metamorphic rock. During metamorphism and deformation minerals dissolved from the region of high pressure, where the matrix is squeezed against the crystal, are redeposited in the pressure shadow zone at the side of the crystal. The minerals nucleate on the face of the porphyroblast and regrow in a fixed crystallographic orientation with respect to the edge of the large porphyroblast. This results in the straight fibres which often intersect at segment junctions. Where the porphyroblast is rotated during the metamorphic event the pressure-fringe minerals grow as curved fibres, thus tracing the stages of detachment between porphyroblast and matrix.
Subjects: Earth Sciences and Geography.