Pyrrhotite ( pyrrhos in Greek language meaning "flame-coloured" ) is an iron sulfide mineral with the formula Fe(1−x)S (x = 0 to 0.125). It is an iron-deficient 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.
Structure
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
nickel arsenide unit cell. As such, Fe occupies an octahedral site and the sulfide centers occupy trigonal prismatic sites.
[Shriver, D. F.; Atkins, P. W.; Overton, T. L.; Rourke, J. P.; Weller, M. T.; Armstrong, F. A. "Inorganic Chemistry" W. H. Freeman, New York, 2006. .]
Materials with the NiAs structure often are non-stoichiometric because they lack up to 1/8th fraction of the metal ions, creating Vacancy defect. 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.
Magnetic properties
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 Fe
7S
8 is
ferrimagnetism.
[Sagnotti, L., 2007, Iron Sulfides; in: Encyclopedia of Geomagnetism and Paleomagnetism; (Editors David Gubbins and Emilio Herrero-Bervera), Springer, 1054 pp., p. 454-459.] 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.
Monoclinic pyrrhotite undergoes a magnetic transition known as the Besnus transition at 30 K that leads to a loss of magnetic remanence.
The saturation magnetization of pyrrhotite is 0.12 tesla.
Identification
Physical properties
Pyrrhotite is brassy, bronze, or dark brown in color with a metallic luster and uneven or subconchoidal fracture.
Pyrrhotite may be confused with other brassy sulfide minerals like
pyrite,
chalcopyrite, or
pentlandite. Certain diagnostic characteristics can be used for identification in hand samples. Unlike other common brassy-colored
, pyrrhotite is typically magnetic (varies inversely with iron content).
On the
Mohs scale, pyrrhotite ranges from 3.5 to 4,
compared to 6 to 6.5 for pyrite.
Streak can be used when properties between pyrrhotite and other sulfide minerals are similar. Pyrrhotite displays a dark grey to black streak.
Pyrite will display a greenish black to brownish black streak,
chalcopyrite will display a greenish black streak,
and pentlandite leaves a pale bronze-brown streak.
Pyrrhotite generally displays massive to granular
crystal habit, and may show tabular/prismatic or hexagonal crystals which are sometimes
Iridescence.
Diagnostic characteristics in hand sample include: brassy/bronze color with a grey/black streak, tabular or hexagonal crystals which show iridescence, subconchoidal fracture, metallic luster, and magnetic.
Optical properties
Pyrrhotite is an opaque mineral and will therefore not transmit light. As a result, pyrrhotite will display extinction when viewed under plane polarized light and cross polarized light, making identification with petrographic polarizing light microscopes difficult. Pyrrhotite, and other opaque minerals can be identified optically using a reflected light ore microscope.
The following optical properties
[Spry, P. G., & Gedlinske, B. (1987). Tables for the determination of common opaque minerals. Economic Geology Pub.] are representative of polished/puck sections using ore microscopy:
Pyrrhotite typically appears as anhedral, granular aggregates and is cream-pink to brownish in color.
Weak to strong reflection
pleochroism which may be seen along grain boundaries.
Pyrrhotite has similar polishing hardness to pentlandite (medium), is softer than pyrite, and harder than chalcopyrite.
Pyrrhotite will not display
Crystal twinning or internal reflections, and its strong
anisotropy from yellow to greenish-gray or grayish-blue is characteristic.
Diagnostic characteristics in polished section include: anhedral aggregates, cream-pink to brown in color and strong anisotropy.
Occurrence
Pyrrhotite is a rather common trace constituent of
mafic igneous rocks especially
. It occurs as segregation deposits in layered intrusions associated with pentlandite, chalcopyrite and other sulfides. It is an important constituent of the
Sudbury Basin (1.85 Ga old
impact event in
Ontario, Canada) where it occurs in masses associated with copper and nickel mineralisation.
[ It also occurs in and in contact metamorphic rock zones. Pyrrhotite is often accompanied by pyrite, marcasite and magnetite.
]
Formation
Pyrrhotite requires both iron and sulfur to form.["Abundance of Elements in the Earth’s Crust and in the Sea," in CRC Handbook of Chemistry and Physics, 103rd Edition (Internet Version 2022), John R. Rumble, ed., CRC Press/Taylor & Francis, Boca Raton, FL.] and so the majority of rocks have sufficient iron abundance to form pyrrhotite.
Pyrite also decomposes into pyrrhotite in hot reductive technogenic environments, such as
Pyrrhotite can also form near black smoker hydrothermal vents.
Distribution
United States
Pyrrhotite occurs in a variety of locations in the United States.[
]
/ref> In the eastern United States, pyrrhotite occurs in highly Metamorphic rock rock that forms a belt along the Appalachian Mountains.
Mining locations worldwide
The following are some of the locations worldwide where pyrrhotite has been reported during mining:
Canada
| +
!Location
!Mine
!Main Target Commodities |
| British Columbia, Riondel | Bluebell Mine | Cadmium, Copper, Gold, Lead, Silver, Zinc |
| Quebec | Henderson No. 2 mine (Copper Rand mine)[Tavchandjian, O. (1992). Analyse quantitative de la distribution spatiale de la fracturation et de la minéralisation dans les zones de cisaillement: applications aux gisements du complexe du lac Dore (Chicougamau-Québec). Université du Québec à Chicoutimi.] | Copper, Gold |
| Quebec | B&B Quarry, Sharwinigan | Crushed rock (Gabbro) for construction |
| Quebec | Maskimo Quarry, Sharwinigan | Crushed rock (Gabbro) for construction |
US
| +
!Location
!Mine
!Main Target Commodities |
| Connecticut | Becker Quarry (Becker's Quarry) | Not given, but abundant quartz, kyanite, and garnet are worthy of mentioning.
Note: This was a quarry producing crushed rock aggregate for use in construction |
Australia
Brazil
Italy
| +
!Location
!Mine
!Main Target Commodities |
| Tuscany | Bottino Mine | Silver, |
Kosovo
| +
!Location
!Mine
!Main Target Commodities |
| Mitrovica District | Trepça Mine | Lead, Silver, Silver |
Etymology and history
Named in 1847 by Ours-Pierre-Armand Petit-Dufrénoy.
Sulfide oxidation in construction aggregates leading to concrete degradation
Pyrrhotite-containing rocks cannot be used as aggregates for concrete, because sulfate ions released by pyrrhotite Redox readily cause an internal sulfate attack (ISA) in concrete. This is an insidious and severe form of concrete degradation.
Uses
Other than a source of sulfur, pyrrhotite does not have specific applications.[Haldar, S. K. (2017). Platinum-nickel-chromium deposits : Geology, exploration and reserve base. Elsevier. p.24. ISBN 978-0-12-802041-8.] It is generally not a valuable mineral unless significant nickel, copper, or other metals are present.[Kolahdoozan, M. & Yen, W.T. (2002). Pyrrhotite – An important gangue and a source for environmental pollution. Green Processing 2002 – Proceedings: International Conference on the Sustainable Processing of Minerals. 245–249.] Iron is seldom extracted from pyrrhotite due to a complicated Metallurgy.
Mineral abbreviations
| +Table of pyrrhotite mineral abbreviations. Note: only use official IMA-CNMNC symbol listed in bold text.
! Abbreviation
! Source |
| Pyh | IMA-CNMNC |
| Po
/ref> |
Synonyms
+ Synonyms of the mineral pyrrhotite. | Magnetic pyrite | Magnetopyrite | Magnetic pyrites |
| Pyrrhotine | Pyrrohotite | Magnetic iron pyrites |
| Dipyrite | Kroeberite | Vattenkies |
External links
