Pyrrhotite

Pyrrhotite
Pyrrhotite – Santa Eulalia Mine, Chihuahua, Mexico (7,5x7cm).
General
Category	Mineral
Formula (repeating unit)	Fe1−xS (x = 0 to 0.2)
Strunz classification	2.CC.10
Crystal system	Monoclinic, with hexagonal polytypes
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	A2/a
Unit cell	a = 11.88 Å, b = 6.87 Å, c = 22.79 Å; β = 90.47°; Z = 26
Identification
Color	Bronze, dark brown
Crystal habit	Tabular or prismatic in hexagonal prisms; massive to granular
Cleavage	Absent
Fracture	Uneven
Mohs scale hardness	3.5 – 4.5
Luster	Metallic
Streak	Dark grey – black
Specific gravity	4.58 – 4.65, average = 4.61
Refractive index	Opaque
Fusibility	3
Solubility	Soluble in hydrochloric acid
Other characteristics	Weakly magnetic, strongly magnetic on heating; non-luminescent, non-radioactive
References

Pyrrhotite is an iron sulfide mineral with the formula Fe_(1-x)S (x = 0 to 0.2). 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. The magnetism decreases as the iron content increases, and troilite is non-magnetic.

Pyrrhotite exist as a number of polytypes of hexagonal or monoclinic crystal symmetry; several polytypes often occur within the same specimen. Their structure is based on the NiAs unit cell. As such, Fe occupies an octahedral site and the sulfide centers occupy trigonal prismatic sites.

Materials with the NiAs structure often are non-stoichiometric because they lack up to 1/8th fraction of the metal ions, creating vacancies. One of such structures is pyrrhotite-4C (Fe₇S₈). Here "4" indicates that iron vacancies define a superlattice that is 4 times larger than the unit cell in the "C" direction. The C direction is conventionally chosen parallel to the main symmetry axis of the crystal; this direction usually corresponds to the largest lattice spacing. Other polytypes include: pyrrhotite-5C (Fe₉S₁₀), 6C (Fe₁₁S₁₂), 7C (Fe₉S₁₀) and 11C (Fe₁₀S₁₁). Every polytype can have monoclinic (M) or hexagonal (H) symmetry, and therefore some sources label them, for example, not as 6C, but 6H or 6M depending on the symmetry. The monoclinic forms are stable at temperatures below 254 °C, whereas the hexagonal forms are stable above that temperature. The exception is for those with high iron content, close to the troilite composition (47 to 50% atomic percent iron) which exhibit hexagonal symmetry.