Xistos

Xistos, known in geology as schist, is a foliated metamorphic rock produced by the metamorphism of mudstone or shale under moderate to high temperatures and pressures. The defining feature is schistosity, a planar cleavage resulting from the preferred alignment of platy minerals such as micas and chlorite, which gives the rock a shiny, layered appearance.

Mineral composition in xistos varies, but common constituents include quartz, feldspar, muscovite, biotite, and chlorite. Some schists contain garnet, staurolite, or kyanite, reflecting higher-grade metamorphism. The texture is coarser than slate, with mineral grains visible to the naked eye, and the rock readily splits along the schistosity plane.

Schist sits in a metamorphic sequence that extends from shale to slate, phyllite, schist, and finally gneiss, representing an increasing metamorphic grade. It forms through regional metamorphism associated with mountain-building processes or nearby igneous intrusions that supply heat and fluids.

Xistos occurs in many orogenic belts around the world, including the Scottish Highlands, the Alps, the Appalachians, and parts of Scandinavia. It commonly appears where shale or mudstone has undergone metamorphism and may be interlayered with other metamorphic rocks.

Uses of xistos include decorative stone and building material. Some schists with a metallic sheen or rich coloration are quarried as dimension stone or used for paving and façades. Because of their foliation, some schists split readily along planes of weakness, influencing their durability for certain applications.

Etymology: the word schist derives from the Greek schistos, meaning split or divided, reflecting its tendency to separate along foliation planes.