pyrrhotite


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Related to pyrrhotite: cinnabar, arsenopyrite, pentlandite, molybdenite

pyrrhotite

(pĭr`ətīt) or

magnetic pyrites,

bronze-yellow to bronze-red mineral, a sulfide of iron sometimes containing nickel. It tarnishes easily and is somewhat magnetic. It crystallizes in the hexagonal system, but appears usually in massive form, and occurs chiefly in basic igneous rocks. In some areas, e.g., at Sudbury, Ont., Canada, it is associated with pentlandite, an important ore of nickel. It is found also in the United States, Europe, and South America.

Pyrrhotite

 

(also pyrrhotine or magnetic pyrites), a sulfide mineral with composition Fe1-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 Fe0.875S (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 S2– 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

pyrrhotite

[′pir·ə‚tīt]
(mineralogy)
Fe1-x S 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).
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This project will help professors at the Department of Geology and Geological Engineering at the Faculty of Science and Engineering of the Universit Laval and co-holders of the research chair, Jose Duchesne and Benot Fournier, as well as NRC scientists, to establish acceptable limits for the content of various sulphurs, including pyrrhotite, in Canadian concrete aggregates.
Caption: Figure 4: Microphotographs showing intergrowth of minerals (a-e), photographs of rock cores (f-i); (a) subhedral-anhedral pyrite and chalcopyrite; (b) subhedral-anhedral galena and sphalerite; (c) quartz coexisting with molybdenite; (d) metasomatic magnetite by pyrrhotite and chalcopyrite; (e) solid solution separated sphalerite and chalcopyrite; (f) mass chalcopyrite in quartz porphyry; (g) vein quartz coexisting with chalcopyrite; (h) vein molybdenite in quartzporphyry; (i) conglomeration molybdenite in quartz porphyry (Py: pyrite; Ccp: chalcopyrite; Gn: galena; Mag: magnetite; Mlb: molybdenite; Po: pyrrhotite; Sp: sphalerite; Qtz: quartz).
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Chalcopyrite is associated with pyrrhotite in the core, whereas sphalerite is associated with pyrite on the margins.
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The GHTZ is further characterized by layers rich in pyrite and pyrrhotite that may play an important role in Hg cycling when sulphide mineral oxidation allows the release of metals in the environment (Fox et al.
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.
The haul includes two of the largest crystals of their kind ever uncovered in Switzerland--a 12.7 centimetre pyrrhotite (iron sulphide), and a 3.7cm fluorapophyllite (sillicate).