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Red-brown, crystalline solid.decomposes in water.

MgS is a white crystalline material but is often encountered in an impure form that is a brown noncrystalline powder.
The chemical properties of MgS resemble those of related ionic sulfides such as those of Na, Ba, and Ca.
Magnesium Sulfide reacts with oxygen to form the corresponding sulfate, magnesium sulfate. MgS reacts with water to give hydrogen sulfide and magnesium hydroxide.

Magnesium Sulfide can be prepared by the direct reaction of the elements, calcined in an inert atmosphere, at a 1:1.1 molecular ratio:
Mg＋ S＋ heat→MgS
MgS also forms by the reaction of hydrogen sulfide gas with soluble magnesium salts in solution.
Aside from being a component of some slags, MgS is a rare nonterrestial mineral “niningerite” detected in some meteorites. Magnesium sulfide is available commercially from a number of suppliers.