Chalk is a very pure form of calcium carbonate , and is made in a similar way to limestone . The formation of coal is outlined in the chapter Products from Oil . Rock salt is formed from salts which have crystallized out of sea water , as the water has evaporated . The process is the same as that given for general crystallization where evaporation of water from a saturated solution causes the salt to crystallize . The salt crystals will then settle as a sediment , and compaction turns the salt into a sedimentary rock .
Rock salt is mainly sodium chloride .