limestones

Limestones are a broad class of sedimentary rocks composed primarily of calcium carbonate (CaCO3). The dominant mineral is calcite, though some limestones contain dolomite, or lesser amounts of silica, clay, silt, or organic matter. They typically form in marine settings where calcareous algae, foraminifera, mollusks, and corals accumulate shells and skeletons, or by direct chemical precipitation of calcium carbonate from seawater or groundwater.

Two main pathways yield limestones: biogenic, produced from accumulated organic debris and skeletal fragments (fossiliferous, oolitic, or micritic limestones); and chemical or inorganic precipitation, which forms travertine and speleothems in caves and hot springs. Chalk is a soft, fine-grained biogenic limestone and is largely composed of coccolithophores' remains; oolitic limestones consist of spheroidal grains.

In diagenesis the grains are compacted and cemented, forming a solid rock. Limestone dissolves readily in weak acids, leading to karst landscapes with features such as caves, sinkholes, and stalactites. When subjected to high temperature and pressure during burial, some limestones recrystallize and can metamorphose into marble.

Economically, limestone is a major source of calcium carbonate for cement and lime production, agricultural lime, and a building stone. It occurs worldwide and is especially abundant in carbonate platforms formed during various geological eras. Its fossil content also provides valuable records for paleoenvironments and past climates.