Pyrrhotite

Pyrrhotite exists 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.[7]

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.[2][8] 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.[9]

The ideal FeS lattice, such as that of troilite, is non-magnetic. Magnetic properties vary with Fe content. More Fe-rich, hexagonal pyrrhotites are antiferromagnetic. However, the Fe-deficient, monoclinic Fe₇S₈ is ferrimagnetic.[10] 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.[11] Monoclinic pyrrhotite undergoes a magnetic transition known as the Besnus transition at 30 K that leads to a loss of magnetic remanence.[12] The saturation magnetization of pyrrhotite is 0.12 tesla.[13]

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 (1.85 Ga old meteorite impact crater in Ontario, Canada) where it occurs in masses associated with copper and nickel mineralisation.[9] It also occurs in pegmatites and in contact metamorphic zones. Pyrrhotite is often accompanied by pyrite, marcasite and magnetite.

Pyrrhotite requires both iron and sulfur to form.[6] Iron is the fourth most abundant element in the Earth's continental crust (average abundance of 5.63 % or 56,300 mg/kg in the crust),[20] and so the majority of rocks have sufficient iron abundance to form pyrrhotite.[6] However, because sulfur is less abundant (average abundance of 0.035 % or 350 mg/kg in the crust),[20] the formation of pyrrhotite is generally controlled by sulfur abundance.[6] Also, the mineral pyrite is both the most common and most abundant sulfide mineral in the Earth's crust.[6] If rocks containing pyrite undergo metamorphism, there is a gradual release of volatile components like water and sulfur from pyrite.[6] The loss of sulfur causes pyrite to recrystallize into pyrrhotite.[6]

Pyrrhotite can also form near black smoker hydrothermal vents.[6] Black smokers release high sulfur concentrations onto the sea floor, and when the surrounding rocks are metamorphosed, pyrrhotite can crystallize.[6] Later tectonic processes uplift the metamorphic rocks and expose pyrrhotite to the Earth's surface.[6]

The following are some of the locations worldwide where pyrrhotite has been reported during mining:[15]

Named in 1847 by Ours-Pierre-Armand Petit-Dufrénoy.[34] "Pyrrhotite" is derived from the Greek word πνρρό, "pyrrhos", meaning flame-colored.[2]

If pyrrhotite-containing rocks are crushed and added to concrete filler, pyrrhotite creates a problem in the production of concrete.[35] Pyrrhotite has been linked to crumbling concrete basements in Quebec, Massachusetts and Connecticut when local quarries included it in their concrete mixtures.[36][37][38] Many houses in Ireland have also been affected by inclusion of rocks containing pyrrhotite in concrete blocks. [39] The iron sulfide it contains can naturally react with oxygen and water, and over time pyrrhotite breaks down into sulfuric acid and secondary minerals like gypsum.[35][6] These secondary products occupy a larger volume than pyrrhotite, which expands and cracks the concrete leading to home foundation failure.[36][37][38][35][6]

Other than a source of sulfur, pyrrhotite does not have specific applications.[40] It is generally not a valuable mineral unless significant nickel, copper, or other metals are present.[40][41] Iron is seldom extracted from pyrrhotite due to a complicated metallurgical process[40] It is mined primarily because it is associated with pentlandite, a sulfide mineral that can contain significant amounts of nickel and cobalt.[2] When found in mafic and ultramafic rocks, pyrrhotite can be a good indicator of economic nickel deposits.[40]

• Spencer, Leonard James (1911). "Pyrrhotite" . Encyclopædia Britannica (11th ed.).