Pyrrhotite

Pyrrhotite
Pyrrhotite – Santa Eulalia Mine (Chihuahua) Mexico (7,5x7cm)
General
Category	Mineral
Chemical formula	Fe1-xS (x = 0 to 0.2)
Strunz classification	02.CC.10
Crystal symmetry	Monoclinic prismatic H-M symbol: (2/m) 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
Crystal system	Monoclinic, with hexagonal polytypes
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 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 increases as the iron content decreases, and the troilite is non-magnetic.

Etymology and history

The name pyrrhotite is derived from Greek pyrrhos, flame-colored.

Crystal structure

NiAs structure of basic pyrrhotite-1C

Pyrrhotite has a number of polytypes of hexagonal or monoclinic crystal symmetry; several polytypes often occur within the same specimen. Their crystalline structure is based on the NiAs unit cell, where metal occurs in octahedral coordination and anions in trigonal prismatic arrangement. An important feature of this structure is an ability to omit metal atoms with the total fraction up to 1/8, thereby creating iron vacancies. One of such structures is pyrrhotite-4C (Fe₇S₈). Here "4" indicates that iron vacancies form a superlattice which 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 (Fec₁₁S₁₂), 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.

Magnetic properties

The ideal FeS lattice, such as that of troilite, is non-magnetic. The ferromagnetism which is widely observed in pyrrhotite is therefore attributed to the presence of relatively large concentrations of iron vacancies (up to 20%) in the crystal structure. Vacancies lower the crystal symmetry. Therefore, monoclinic forms of pyrrhotite are in general more defect-rich than the more symmetrical hexagonal forms, and thus are more magnetic. Upon heating to 320 °C, pyrrhotite loses its magnetism, but also starts decomposing to magnetite. The saturation magnetization of pyrrhotite is 0.12 tesla.

Occurrence

Pyrrhotite is a rather common trace constituent of mafic igneous rocks especially norites. It occurs as segregation deposits in layered intrusions associated with pentlandite, chalcopyrite and other sulfides. It is an important constituent of the Sudbury intrusion where it occurs in masses associated with copper and nickel mineralisation. It also occurs in pegmatites and in contact metamorphic zones. Pyrrhotite is often accompanied by pyrite, marcasite and magnetite. Pyrrhotite does not have specific applications. It is mined primarily because it is associated with pentlandite, sulfide mineral that can contain significant amounts of nickel and cobalt.