Shale is a fine‑grained, clastic sedimentary rock formed from mud that is primarily composed of clay minerals, with smaller amounts of quartz, calcite, and organic matter. It characteristically splits easily into thin layers along planes of lamination, a property known as fissility, which makes it one of the most common sedimentary rocks on Earth.

Key points

• Composition: Mostly clay minerals (hydrous aluminum phyllosilicates like kaolinite, illite, and smectite), plus tiny fragments of quartz, calcite, and sometimes organic material.

• Texture and appearance: Very fine-grained; often dull, earthy, or dark-colored when rich in organic matter. The fine particles settle in quiet-water environments such as deep marine basins, lakes, or lagoons.

• Lamination: Shale forms in thin, parallel layers due to its fissility; these laminations are usually less than a centimeter thick.

• Classification notes: Shale is a broader category within sedimentary rocks and can be further described by deposits’ environment (marine, lacustrine, deltaic, terrestrial) and by mineral composition (siliceous, calcareous, micaceous, feldspathic).