Limestone

Limestone is a sedimentary rock composed largely of the mineral calcite (calcium carbonate: CaCO₃). The deposition of limestone strata is often a by-product and indicator of biological activity in the geologic record. Calcium (along with nitrogen, phosphorus, and potassium) is a key mineral to plant nutrition: soils overlying limestone bedrock tend to be pre-fertilized with calcium. Limestone is an important stone for masonry and architecture, vying with only granite and sandstone to be the most commonly used architectural stone. Limestone is a key ingredient of quicklime, mortar, cement, and concrete. The solubility of limestone in water and weak acid solutions leads to important phenomena.

Limestone makes up about 10% of the total volume of all sedimentary rocks. Limestones may also form in both lacustrine and evaporite depositional environments. Calcite can be either dissolved by groundwater or precipitated by groundwater, depending on several factors including the water temperature, and dissolved ion concentrations. Calcite exhibits an unusual characteristic called retrograde solubility in which it becomes less soluble in water as the temperature increases.

Karst topography and caves develop in carbonate rocks due to their solubility in dilute acidic groundwater. Cooling groundwater or mixing of different groundwaters will also create conditions suitable for cave formation. Coastal limestones are often eroded by organisms which bore into the rock by various means. This process is known as bioerosion. It is most common in the tropics, and it is known throughout the fossil record.

Limestone is partially soluble, especially in acid, and therefore forms many erosional landforms. These include limestone pavements, pot holes, cenotes, caves and gorges. Such erosion landscapes are known as karsts. Limestone is less resistant than most igneous rocks, but more resistant than most other sedimentary rocks. Limestone is therefore usually associated with hills and downland and occurs in regions with other sedimentary rocks, typically clays.