claystone

Claystone is a fine-grained sedimentary rock dominated by clay-sized particles. It is primarily composed of clay minerals such as kaolinite, illite, and smectite, and may include minor quartz, feldspar, carbonates, and organic matter. Unlike shale, which is typically fissile, claystone is usually non-fissile or only weakly fissile, so it does not readily split along bedding planes.

Claystone forms by lithification of clay-rich sediments deposited in low-energy environments, including lakes, floodplains, marginal seas, and deep basins. Through compaction and cementation, clay flakes bind together to produce a dense rock. Diagenetic processes may introduce cement minerals such as calcite, quartz, or iron oxides, and locally develop authigenic clays.

Colors range from gray and green to red and brown, reflecting the mineralogy and oxidation state. The rock is generally soft when fresh, becoming hard on compaction, and it has low permeability, often acting as an aquitard in sedimentary sequences.

In stratigraphy, claystone is distinguished from shale by its limited fissility, and from mudstone by its predominant clay content and reduced silt and sand. It is widely distributed in sedimentary basins and serves as a source and reservoir rock in some hydrocarbon systems, and as a raw material for ceramic clay and brick industries.