Sylvite mineral. This mineral consists of potassium chloride (KCl) and is closely related to the more common rock salt called halite (NaCl, sodium chloride). Both these natural salts have isometric crystal systems and form by evaporation from water containing the dissolved mineral. This sample of sylvite shows bedding (layers) as a result of variable impurities present as the salt precipitated out of solution. The mineral is named after the Dutch scientist Franciscus Sylvius (1614-1672). The sample size is 70 by 80 millimetres.