pyrrhotite

[′pir·ə‚tīt]

(mineralogy)

Fe_1-xS A common reddish-brown to brownish-bronze mineral that occurs as rounded grains to large masses, more rarely as tabular pseudohexagonal crystals and rosettes; hardness is 4 on Mohs scale, and specific gravity is 4.6 (for the composition Fe₇S₈).

The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

Pyrrhotite

(also pyrrhotine or magnetic pyrites), a sulfide mineral with composition Fe_1-xS and containing admixtures of Ni and Co. Pyrrhotite’s crystal structure has hexagonal close packing of S atoms. It is defective, since not all the octahedral vacancies are occupied by Fe, as a result which certain Fe²⁺ are converted to Fe³⁺. The structural deficiency in Fe varies, and thus the compositions range from Fe_0.875S (Fe₇S₈) to FeS (the stoichiometric composition FeS is known as troilite). The parameters and symmetry of the crystal cell vary according to the Fe deficiency and, when x ~ 0.11–0.2, pyrrhotite passes from the hexagonal modification to the monoclinic.

Pyrrhotite is brownish yellow with dark brown streaks and has a metallic luster. It usually occurs in the form of solid masses or granular deposits composed of intergrowths of both modifications. It has a hardness of 3.5–4.5 on Mohs’ scale and a density of 4,580–4,700 kg/m³. Its magnetic properties vary according to composition: hexagonal pyrrhotites, which are deficient in S, are paramagnetic, while monoclinic varieties, which are rich in S, are ferromagnetic. Some pyrrhotite minerals display distinctive magnetic anisotropy—paramagnetism in one direction and ferromagnetism in the other, perpendicular to the former.

Pyrrhotite is formed from hot solutions during a reduction in the concentration of dissociated S^2– ions. It is widely distributed in hypogenic copper-nickel ore deposits associated with ultrabasic rocks, as well as in contact-metasomatic deposits and hydrothermal masses with copper-polymetallic, sulfide-cassiterite, and other types of mineralization. Pyrrhotite alters to pyrite, marcasite, and limonite in the zone of oxidation.

REFERENCE

Mineraly, vol. 1, Moscow, 1960.

G. P. BARSANOV

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A kinetic/mass transport model was proposed based on oxidation of pyrrhotite. Bayesian statistical analysis using the Metropolis-Hastings algorithm showed that at ambient oxygen concentration the mean (i.e., estimated) values of the initial oxygen flux, the reaction order, the oxygen diffusion coefficient through the oxidation product layer, and the surface reaction rate constant were 1.3 x [10.sup.-7] mol/[m.sup.2].s, 0.55, 2.91 x [10.sup.-13] [m.sup.2]/s and 3.23 x 10-7 [mol.sup.0.45]/[m.sup.0.35].s, respectively.

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