Chlorine abundance in the lithosphere is estimated to be about 0.045% by weight. Due to its strong chemical affinity for the other elements, it never occurs in the free state, except as a minor constituent of the gaseous output from volcanic eruptions.
Rock salt or halite (NaCl) , sylvite (KCl) and carnallite (MgCl2.KCl.6H2O) are among the most common minerals consisting of chlorides. Beside being dispersed in the lithosphere, chlorine salts are also dissolved in the hydrosphere. In the hydrosphere as well as in the lithosphere sodium chloride, NaCl, is by far the most common of alkali metals: hence its name of "common salt". The average concentration in seawater is approximately 2,6%. Due to its unreplaceable function in human diet it is also known as "table salt" and the uneven distribution of its deposits over the earth's crust has played an essential role in conditioning the pattern of civilization in time and space. The occasional use of this commodity as a valuable currency is still reminded by the word "salary".