Magnesium Sulphate aka Epsom Salt MgSO4 * 7H20 is an important mineral for its effect on mash and wort pH. It is used as a brewing salt in brewing to adjust the ion content of the wort and enhance enzyme action in the mash to promote a desired flavour profile in the beer. Lowers pH by a small amount.
Epsom is used to add sulfate and magnesium ions to brewing water. It is often used at half the levels of Gypsum. 1 gram in 1 gallon changes the salt levels by 103 ppm sulfate, 26 ppm Magnesium and adds 108 ppm to the hardness.