Phyllonites

Phyllonites are a type of metamorphic rock that forms under conditions of intense shear stress and moderate to low temperatures. They are characterized by a distinct foliation, a layered or banded appearance, which is a result of the alignment of platy or elongated minerals during the deformation process. This foliation is typically very fine-grained and can give the rock a silky or satiny sheen.

The mineralogy of phyllonites is dominated by fine-grained micas, such as sericite or muscovite, along with quartz and often chlorite. The presence of these minerals contributes to the rock's platy texture. Phyllonites are commonly found in fault zones and shear zones, where rocks have been subjected to significant tectonic forces. Their formation often involves the mylonitization of precursor rocks like schists, phyllites, or even shales.

The texture of phyllonites is a key diagnostic feature. It is defined by a pervasive foliation that typically shows asymmetric folds and a characteristic microstructure known as a crenulation cleavage. This texture indicates that the rock has undergone significant ductile deformation. Due to their brittle nature, phyllonites can fracture easily, and their presence in fault zones can influence the mechanical behavior of these structures. They are important indicators of the intensity and style of deformation within the Earth's crust.