Phyllonitic

Phyllonitic is a term used in geology to describe a type of metamorphic rock that has undergone intense shearing and deformation. These rocks are characterized by a foliation, which is a layered or planar arrangement of minerals, often appearing as a fine-grained, silky or glossy surface. This foliation is typically the result of recrystallization and the alignment of platy minerals, such as micas and chlorites, parallel to the shear planes.

The texture of phyllonite is often described as schistose or phyllitic, indicating a high degree of mineral alignment. The original rock type before metamorphism can vary, but it is commonly derived from sedimentary rocks like shales or mudstones, or from fine-grained igneous rocks. The intense shearing forces cause the rock to become highly ductile, leading to the development of this characteristic fabric.

Phyllonitic rocks are typically found in areas of significant tectonic activity, such as along fault zones or in orogenic belts where crustal shortening and thickening have occurred. The presence of phyllonite is a strong indicator of intense localized deformation and the movement of large rock masses. While the term itself describes the texture and fabric, it is often used in conjunction with the original rock composition to provide a more complete description, for example, "phyllonitic schist" or "phyllonitic slate."