Pyrrhotite
>
Pyrrhotite is an iron sulfide mineral with the formula Fe₍₁₋ₓ₎S. It is a nonstoichiometric variant of FeS, the mineral known as troilite.
Pyrrhotite is also called magnetic pyrite, because the color is similar to pyrite and it is weakly magnetic. Wikipedia
Molar mass: 85.12 gm
Cleavage: Absent
Color: Bronze, dark brown
Crystal class: Prismatic (2/m); (same H-M symbol)
Crystal habit: Tabular or prismatic in hexagonal prisms; massive to granular
Crystal system: Monoclinic, with hexagonal polytypes
Formula (repeating unit): Fe1−xS (x = 0 to 0.125)