What makes limestone clastic

limestone

The term limestone covers many different forms of sedimentary rock formed in sea or fresh water. Limestone mainly consists of the chemical component calcium carbonate (CaCO3), which, depending on the shape of the crystal lattice of the molecules, is in the form of the mineral aragonite or calcite.
Depending on the chemical components it still contains, the rock can be white, gray, yellowish, pink, red or black in color.

The formation of limestones takes place in different processes, so there are chemically precipitated, biogenic and clastic limestones.

Biogenic limestones
Most of the limestone is of biogenic origin. As a rule, they are fine-grained, dense limestones and arise from the deposition of calcareous components and excretions from organisms (e.g. microorganisms, corals, snails, mussels, algae).
When dense limestone is formed, the calcareous components first sink to the bottom of the water, where they form limestone sludge. This lime sludge becomes solid limestone through diagenesis. However, they can only be formed up to a certain depth, the so-called carbonate compensation line. From a depth of 100-200m, the seawater is no longer saturated with dissolved calcium carbonate, as the water pressure and thus the solubility for carbon dioxide increase with increasing depth. Calcium carbonate dissociates in connection with carbon dioxide and water to form calcium and hydrogen carbonate ions, so calcareous components begin to dissolve. From a depth of approx. 5000m, carbonate is completely dissolved and can therefore no longer be deposited as sediment.
Depending on which organisms the limestone is mainly composed of, it is differentiated by name. Accordingly, coral / reef lime is formed on coral reefs, mollusc limestone is formed from the calcareous components of molluscs, foraminiferous limestone (chalk) is formed from the deposits of foraminifera, etc. A lime consisting mainly of mussel shells is called shill. If the rock consists of easily recognizable fragments of fossils, one speaks of fossil limestone. Together with other porous limestones, they are important stores for crude oil and natural gas and are used as indicators in the search for new deposits.

Chemically precipitated limestone
Sea and fresh water always contain a more or less high proportion of dissolved calcium carbonate. If the water is saturated, calcium carbonate precipitates from the saturated aqueous solution. Lime crystals develop, which form a lime sludge or accumulate in another form.
Living beings can be involved in chemical precipitation. When plants breathe, for example, the carbon dioxide content of the water decreases. In order to replace the used carbon dioxide, it is released from the hydrogen carbonate ions. The resulting carbonate ions are much less soluble in water and precipitate as calcium carbonate. Lime is precipitated from evaporating water at hot springs.
Names for chemically precipitated lime are e.g. tufa, travertine, sinter, karst and stalactite.

Clastic limestones
A clastic rock is the creation of a new structure from rock fragments and a precipitated binder (e.g. calcite). Since calcium carbonate dissolves quickly, clastic limestone is made up of coarse-grained particles. Examples of such rocks are the reef slope breccia and the calcarenite.

Because of its good solubility, the limestone is prone to chemical weathering. If this weathering takes place underground, caves are created in which stalactites can then form. Soils rich in nutrients develop on weathered limestone.
Limestone is of great economic importance as a building material in industry (cement), natural stone and fertilizer. As already mentioned above, it is still a storage rock for oil and natural gas.

The rock occurs worldwide. It can be found in very old deposits (Jura) as well as very young geological formations.
Landscapes made of limestone are e.g. the Swabian Alb and the Limestone Alps.

Climbing on limestone
The solubility of calcium carbonate creates holes in places where magnesium-rich water replaced the calcium in the calcite and converted it to dolomite. The process is based on the decrease in volume of the rock during the transformation of calcite to dolomite, because the mineral has a smaller structure. So fine holes are formed in the rock, where carbon dioxide acid water can dissolve further calcium carbonate from the stone and thus create larger holes.
Holes can also be caused by continuously dripping water (drip holes in the limestone). These drip holes should be used with caution, however, as their edges are sharp!
Lime sinters can be found in warm areas and are also often used for climbing. In Germany you can find limestone formations in the Ith, Frankenjura and on the Swabian Alp.

Already knew?
Human sweat has an acidic pH. Since calcium carbonate reacts with acid, the acidic hand sweat causes the limestone to dissolve. The otherwise critically viewed chalk can be of use here, because it neutralizes the acidity of the hand perspiration and thus helps to protect the limestone at least from this 'attack'.