Iron Sulfide

Iron Sulfide (FeS), similar to most metal sulfides, is used as a lubricant in the friction materials industry, albeit on the lower end of the efficiency spectrum. 

Compared to Pyrite (FeS₂) it is a less stable compound overall with a sulfur content of around 30%.
What makes it attractive to use is the competitive price when compared to most other metal sulfides.

The major application is in the steel and metal fabrication industry as well as other related industries.