Magnesium sulfate (or magnesium sulphate) is an
inorganic salt (
chemical compound) containing
magnesium,
sulfur and
oxygen, with the formula MgSO
4. It is often encountered as the heptahydrate
sulfate mineral epsomite (MgSO
4·7H
2O), commonly called
Epsom salt, taking its name from a bitter saline spring in
Epsom in
Surrey, England, where the salt was produced from the springs that arise where the porous chalk of the
North Downs meets non-porous London clay. The monohydrate, MgSO
4·H
2O is found as the mineral
kieserite. The overall global annual usage in the mid-1970s of the monohydrate was 2.3 million tons, of which the majority was used in agriculture.