esquistos

Esquistos, or schists, are metamorphic rocks characterized by a prominent foliation called schistosity, produced by the alignment of platy minerals such as biotite and muscovite. They form under medium-grade regional metamorphism from pre-existing rocks including pelites (clay-rich mudstones and shales), as well as quartzites and, less commonly, basalts and other protoliths. The minerals in schists grow large enough to be visible to the naked eye, giving the rock a glittery or shiny appearance due to mica flakes.

Texture and mineralogy: Schists are typically medium to coarse-grained and display distinct layers of mineral bands. Common mineral assemblages include quartz, feldspar, muscovite, and biotite; higher-grade schists may contain garnet, staurolite, kyanite, or tourmaline. The exact assemblage depends on the protolith and the metamorphic conditions.

Classification: Schist is classified by dominant minerals, such as mica schist (rich in micas), garnet schist, chlorite schist, or amphibole schist. They lie above slate and phyllite and below gneiss in metamorphic-grade sequences.

Occurrence and significance: Schists are widespread in old mountain belts formed by continental collision, including the Appalachians, the Alps, and the Himalayas. They record metamorphic conditions and tectonic histories. Economically, schists are used as decorative or building stones in some locales; in many regions, they serve as indicators in geological mapping.