Shale, the most common type of sedimentary rock. Geologists estimate that between 70 and 83 per cent of the earth's sedimentary rock is shale. Shale is made up of particles too small to be seen individually. It tends to split into thin layers parallel or almost parallel to the plane in which the particles were deposited. Although most shale is gray or black, shale of other colors, such as red or yellow, is also found.
Shale is composed primarily of clay minerals and quartz. Various other minerals, such as carbonates and feldspars, as well as organic materials, may also be present. Ferruginous shales contain iron oxides. Calcareous shales contain calcite. Shale is formed from waterborne deposits of silt and mud that are compacted into rock by the pressure of overlying layers of sediments. Many shales contain abundant fossils. When shale is subjected to sufficient heat and pressure, it becomes slate.
Shale is used primarily as a source of clay minerals such as kaolin. Oil shale contains hydrocarbons that can be distilled to form substitutes for petroleum.