schistosity

Schistosity is a planar metamorphic fabric characteristic of schists, produced when platy minerals such as biotite, muscovite, chlorite, and talc become aligned and elongated through differential stress and recrystallization. This alignment creates a distinct foliation plane in the rock, along which the minerals are predominantly oriented, giving the rock a shiny, layered appearance.

Schistosity forms during regional metamorphism at moderate to high temperatures and pressures. As rocks are subjected to directed stress, platy minerals recrystallize and grow parallel to the stress field, and sometimes new minerals crystallize in parallel arrays. With increasing metamorphic grade, mineral grains become coarser and more conspicuously aligned, transitioning from slate or phyllite to schist; higher grades can produce garnet, staurolite, kyanite, or sillimanite-bearing assemblages within the schist.

In hand specimens, schistose rocks display visible sheets of mica and other flat minerals that reflect light, giving a glossy or glittering texture. The schistosity plane is the primary foliation plane, though the rocks may also show lineation or minor folds. Schists differ from slates, which exhibit finer, brittle cleavage, and from gneisses, which display compositional banding from mineral segregation.

While schistosity denotes texture, the corresponding rock is called schist. The presence of coarser mineral grains and a pronounced, well-defined planar fabric distinguishes schists from lower-grade foliated rocks and helps indicate metamorphic conditions and history.