Kieserite, an almost white salt that is slowly soluble in cold water and more easily soluble in hot water, forms naturally in marine evaporate deposits where seawater has been concentrated and exposed to prolonged evaporation. A typical analysis of kieserite reveals 15.0% magnesium (25% as magnesium oxide, 75% as magnesium sulfate), 20.0% sulfur (as sulphate), up to 2.5% chlorides, and 12% water. When not stored in closed containers, specimens of kieserite will absorb water from the air and be converted to magnesium sulfate heptahydrate.
