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Calcium Sulfide

Calcium sulfide, CaS, crystallizes in cubes like rock salt and occurs as oldhamite in nature. CaS has an odor of rotten eggs, which stems from H2S formed by hydrolysis of the calcium sulfide.

CaS is produced by reduction of calcium sulfate with charcoal:

CaSO4 + 2 C CaS + 2 CO2

It also occurs as a by-product of the Leblanc process, which was in heavy use in the 19th century before the invention of the Solvay process.

CaS can be oxidized by calcium sulfate to form calcium oxide and sulfur dioxide:

3 CaSO4 + CaS 4 CaO + 4 SO2

Calcium sulfide decomposes upon contact with water:

CaS + H2O Ca(SH)(OH)
Ca(SH)(OH) + H2O Ca(OH)2 + H2S

In the presence of traces of heavy metals CaS becomes phosphorescent (like many other sulfides such as zinc sulfide or cadmium sulfide).