Pyrrhotite has some unusual characteristics.
First, it has an unusual formula. The amount of sulfur does vary by roughly
20% or 50% to 55 atoms of iron that varies? Really the same difference.
Thus the unusual formula of FE1-xs.
Secondly, it has two symmetries. While this should indicate
that there are two minerals and not one, in the case of pyrrhotite, mineralogists have made an exception. When pyrrhotite is low in sulfur and the formula is closer to true FeS,
then the structure is hexogonal. But when it is high in sulfur, the structure is monoclinic.
Clearly two different symmetries, two different formulae; therefore, two different minerals... except, that in natural pyrrhotite crystals both phases are present in the same.
A GOOD WEBSITE FOR ROCKS WOULD BE The stupid page of rocks