Chalk is what make up the Cliffs of Dover. Here is a description of what chalk is:
Mineral Chalk can be described as a white, porous and soft sedimentary carbonate rock. The chemical composition of chalk is calcium carbonate (CaCO3) , which is limestone made of mineral calcite. The formation of calcium carbonate occurs in marine conditions through the accumulation of small calcite shells called coccoliths which are shed by micro-organisms called coccolithophores. This is a very common substance which is found in rocks as minerals such as calcite and aragonite which is the main ingredient in pearls and most shell fish and snails. Mineral chalk is found in agricultural lime and is formed when calcium ions which is found in hard water reacts with carbonate ions to form limestone. Flint which is a type of chert is found as bands parallel to nodules which is embedded in chalk. This could be a derivation of sponge spicules or any other siliceous organisms as water gets expelled during compaction.
Looking into the chemistry of mineral chalk or calcium carbonate, it is seen that it has the properties of any other carbonate where it reacts with an acid to release carbon dioxide. Also mineral chalk when heated releases carbon dioxide which is called thermal decomposition. Calcium oxide is also formed in the process. Calcium bi carbonate is formed when calcium carbonate reacts with water which is saturated with carbon dioxide. This reaction leads to erosion of carbonate rocks to form caverns. At times the calcium carbonate could be found in the hexahydrate form CaCO3·6H2O which is only stable below 6 °C.
When considering the structure of mineral chalk , it is thermodynamically stable as calcium carbonate under normal conditions and normally present as hexagonal β-CaCO3. μ-CaCO3 is found as the mineral vaterite. Other forms are also found such as aragonite. The geological sources of calcite, vaterite and aragonite forms pure calcium carbonate minerals. The industrially important sources include marble, limestone and chalk.
Extensive deposits of chalk date from the Cretaceous Period and the name is derived from the word creta which is a Latin word for chalk. Such forms of mineral chalk deposits are seen in Sweden and England. Like all firms of high purity limestone, chalk is used extensively in making cement and lime. When chalk is ground finely and purified it is used for whiting and as a filler, pigment as well as cosmetics, plastics, paints and paper. The main use for chalk whiting is in making of putty as it has good plasticity, aging qualities and oil absorption.