Pyrrhotite is an unusual iron sulfide mineral with a variable iron content: Fe_(1-x)S (x = 0 to 0.2). The FeS endmember is known as troilite. Pyrrhotite is also called magnetic pyrite because the color is similar to pyrite and it is weakly magnetic. The magnetism decreases as the iron content decreases, and the troilite is non-magnetic.