Magnesium chloride is a chloride with the chemical formula MgCl?. It is a colorless and deliquescent crystal. Magnesium chloride Magnesium chloride These salts are typical ionic halides and are highly soluble in water. Hydrated magnesium chloride can be extracted from brine or seawater. It usually contains six molecules of water of crystallization. However, it loses its water of crystallization when heated to 95°C. It begins to decompose above 135°C, releasing hydrogen chloride (HCl) gas. It serves as a raw material for the industrial production of magnesium and is found in seawater and brine. Hydrated magnesium chloride is the substance commonly used in prescription oral magnesium supplements.