Schistose

Schistose refers to a foliated metamorphic texture in which a pronounced planar fabric, called schistosity, develops as platy minerals such as muscovite, biotite, chlorite, or other phyllosilicates align parallel to each other. Rocks with this texture are typically described as schists, distinguished by visible mineral grains that are large enough to be seen unaided.

Schistosity forms under medium- to high-grade metamorphism, where differential stress promotes realignment and growth of sheet-like minerals. The resulting texture is characterized by a visibly layered or flaky fabric that can split along schistose planes. The mineral composition of schists varies, giving rise to subtypes such as mica schists (dominated by muscovite and/or biotite), chlorite schists, and garnet-bearing mica schists.

Common rock types exhibiting schistose textures include micaschists, chlorite schists, and garnet-mica schists. In hand sample, the foliation is evident and often shiny due to the aligned mica grains. Schists differ from related foliated rocks by the size and prominence of the foliation: schistose textures are coarser and more conspicuous than the slaty cleavage seen in slate, and are typically finer than the banded structures of gneiss.

Etymology and usage: the term derives from German schist, meaning split. Schistose texture is a key descriptor in petrography and regional metamorphism, indicating a metamorphic history of appreciable differential stress and mica-rich mineralogy.