Magnesium sulfate or magnesium sulphate (in English-speaking countries other than the US) is a chemical compound, a salt with the formula MgSO4, consisting of magnesium cations Mg2+ (20.19% by mass) and sulfate anions SO2−4. It is a white crystalline solid, soluble in water but not in ethanol. Magnesium sulfate is usually encountered in the form of a hydrate MgSO4·nH2O, for various values of n between 1 and 11. The most common is the heptahydrate MgSO4·7H2O, known as Epsom salt, which is a household chemical with many traditional uses, including bath salts.The main use of magnesium sulfate is in agriculture, to correct soils deficient in magnesium (an essential plant nutrient because of the role of magnesium in chlorophyll and photosynthesis). The monohydrate is favored for this use; by the mid 1970s, its production was 2.3 million tons per year. The anhydrous form and several hydrates occur in nature as minerals, and the salt is a significant component of the water from some springs. Hydrates
Magnesium sulfate can crystallize as several hydrates, including:

Anhydrous, MgSO4; unstable in nature, hydrates to form epsomite. Monohydrate, MgSO4·H2O; kieserite, monoclinic. MgSO4·1.25H2O or 4MgSO4·5H2O. Dihydrate, MgSO4·2H2O; orthorhombic. MgSO4·2.5H2O or 2MgSO4·5H2O. Trihydrate, MgSO4·3H2O. Tetrahydrate, MgSO4·4H2O; starkeyite, monoclinic. Pentahydrate, MgSO4·5H2O; pentahydrite, triclinic. Hexahydrate, MgSO4·6H2O; hexahydrite, monoclinic. Heptahydrate, MgSO4·7H2O ("Epsom salt"); epsomite, orthorhombic. Enneahydrate, MgSO4·9H2O, monoclinic. Decahydrate, MgSO4·10H2O. Undecahydrate, MgSO4·11H2O; meridianiite, triclinic.As of 2017, the existence of the decahydrate apparently has not been confirmed.All the hydrates lose water upon heating. Above 320 °C, only the anhydrous form is stable.
Magnesium (or sodium) sulfate is also used for testing aggregates for soundness in accordance with ASTM C88 standard, when there are no service records of the material exposed to actual weathering conditions. The test is accomplished by repeated immersion in saturated solutions followed by oven drying to dehydrate the salt precipitated in permeable pore spaces. The internal expansive force, derived from the rehydration of the salt upon re-immersion, simulates the expansion of water on freezing. Magnesium sulfate is also used to test the resistance of concrete to external sulfate attack (ESA). Aquaria
Magnesium sulfate heptahydrate is also used to maintain the magnesium concentration in marine aquaria which contain large amounts of stony corals, as it is slowly depleted in their calcification process. In a magnesium-deficient marine aquarium, calcium and alkalinity concentrations are very difficult to control because not enough magnesium is present to stabilize these ions in the saltwater and prevent their spontaneous precipitation into calcium carbonate. Double salts
Double salts containing magnesium sulfate exist. There are several known as sodium magnesium sulfates and potassium magnesium sulfates. A mixed copper-magnesium sulfate heptahydrate (Mg,Cu)SO4·7H2O was recently found to occur in mine tailings and has been given the mineral name alpersite. See also
Calcium sulfate
Magnesium chloride

References
External links
International Chemical Safety Cards—Magnesium Sulfate