Rock salt (common name for the mineral "halite") is typically formed by the evaporation of salty water (such as sea water) which contains dissolved Na+ (sodium) and Cl- ions (‘chloride ion’ - chlorine anion that forms the negatively charged part of certain salts).
In a solution, sodium and chlorine (the chemical components of halite) are not joined together, but when the solution is dried out, the sodium and chlorine join together and crystallize halite.
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