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Brassy, tabular crystals of pyrrhotite, with sphalerite and quartz, from Nikolaevskiy Mine, Primorskiy Kray, Russia. Specimen size: 5.3 x 4.1 x 3.8 cm
(repeating unit)
Fe1−xS (x = 0 to 0.2)
IMA symbolPyh[1]
Strunz classification2.CC.10
Crystal systemMonoclinic, with hexagonal polytypes
Crystal classPrismatic (2/m)
(same H-M symbol)
Space groupA2/a
Unit cella = 11.88 Å, b = 6.87 Å,
c = 22.79 Å; β = 90.47°; Z = 26
ColorBronze, dark brown
Crystal habitTabular or prismatic in hexagonal prisms; massive to granular
Mohs scale hardness3.5 – 4.5
StreakDark grey – black
Specific gravity4.58 – 4.65, average = 4.61
Refractive indexOpaque
SolubilitySoluble in hydrochloric acid
Other characteristicsWeakly magnetic, strongly magnetic on heating; non-luminescent, non-radioactive

Pyrrhotite (pyrrhos in Greek meaning "flame-coloured") is an iron sulfide mineral with the formula Fe(1-x)S (x = 0 to 0.2). It is a 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 decreases, and troilite is non-magnetic.[5] Pyrrhotite is generally tabular and brassy/bronze in color with a metallic luster. The mineral occurs with mafic igneous rocks like norites. Pyrrhottie is associated and mined with other sulfide minerals like pentlandite, pyrite, chalcopyrite, and magnetite.

NiAs structure of basic pyrrhotite-1C.
Pyrrhotite with pentlandite (late Paleoproterozoic, 1.85 G… | Flickr
Microscopic image of Pyrrhotite under reflected light


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.[6][page needed]

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][7] 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.[8]

Magnetic properties[edit]

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.[9] 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.[10] Monoclinic pyrrhotite undergoes a magnetic transition known as the Besnus transition at 30 K that leads to a loss of magnetic remanence.[11] The saturation magnetization of pyrrhotite is 0.12 tesla.[12]


Physical Properties[edit]

Pyrrhotite is brassy, bronze, or dark brown in color with a metallic luster and uneven or subconchoidal fracture.[13] 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 sulfide minerals, pyrrhotite is typically magnetic (varies inversely with iron content).[13] On the Mohs hardness scale, pyrrhotite ranges from 3.5 to 4,[14] compared to 6 to 6.5 for pyrite.[15] Streak can be used when properties between pyrrhotite and other sulfide minerals are similar. Pyrrhotite displays a dark grey to black streak.[14] Pyrite will display a greenish black to brownish black streak,[15] chalcopyrite will display a greenish black streak,[16] and pentlandite leaves a pale bronze-brown streak.[17] Pyrrhotite generally displays massive to granular crystal habit, and may show tabular/prismatic or hexagonal crystals which are sometimes iridescent.[13]

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[edit]

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[18] are representative of polished/puck sections using ore microscopy:

Photomicrograph of pyrrhotite under reflected light appearing as cream-pink to beige irregular anhedral masses (5x/0.12 POL).

Pyrrhotite typically appears as anhedral, granular aggregates and is cream-pink to brownish in color.[18] Weak to strong reflection pleochroism which may be seen along grain boundaries.[18] Pyrrhotite has similar polishing hardness to pentlandite (medium), is softer than pyrite, and harder than chalcopyrite.[18] Pyrrhotite will not display twinning or internal reflections, and its strong anisotropy from yellow to greenish-gray or grayish-blue is characteristic.[18]

Diagnostic characteristics in polished section include: anhedral aggregates, cream-pink to brown in color and strong anisotropy.


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.[8] It also occurs in pegmatites and in contact metamorphic zones. Pyrrhotite is often accompanied by pyrite, marcasite and magnetite.

Etymology and history[edit]

Named in 1847 by Ours-Pierre-Armand Petit-Dufrénoy.[19] "Pyrrhotite" is derived from the Greek word πνρρό, "pyrrhos", meaning flame-colored.[2]


Pyrrhotite has been linked to crumbling concrete basements in Quebec, Massachusetts and Connecticut when local quarries included it in their concrete mixtures. The iron sulfide it contains can react with oxygen and water over time to cause swelling and cracking.[20][21][22]

Uses of Pyrrhotite[edit]

Other than a source of sulfur, pyrrhotite does not have specific applications.[23] It is generally not a valuable mineral unless significant nickel, copper, or other metals are present.[23][24] Iron is seldom extracted from pyrrhotite due to a complicated metallurgical process[23] 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.[23]


  1. ^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  2. ^ a b c d "Pyrrhotite". Retrieved 2009-07-07.
  3. ^ "Pyrrhotite" (PDF). Retrieved 2015-07-10.
  4. ^ "Pyrrhotite Mineral Data". Retrieved 2015-07-10.
  5. ^ Haldar, S. K. (2017). Platinum-nickel-chromium deposits : geology, exploration and reserve base. Elsevier. p.12 ISBN 978-0-12-802041-8.
  6. ^ 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. ISBN 0-7167-4878-9.[page needed]
  7. ^ Barnes, Hubert Lloyd (1997). Geochemistry of hydrothermal ore deposits. John Wiley and Sons. pp. 382–390. ISBN 0-471-57144-X.
  8. ^ a b Klein, Cornelis and Cornelius S. Hurlbut, Jr., Manual of Mineralogy, Wiley, 20th ed, 1985, pp. 278-9 ISBN 0-471-80580-7
  9. ^ Sagnotti, L., 2007, Iron Sulfides; in: Encyclopedia of Geomagnetism and Paleomagnetism; (Editors David Gubbins and Emilio Herrero-Bervera), Springer, 1054 pp., p. 454-459.
  10. ^ Atak, Suna; Önal, Güven; Çelik, Mehmet Sabri (1998). Innovations in Mineral and Coal Processing. Taylor & Francis. p. 131. ISBN 90-5809-013-2.
  11. ^ Volk, Michael W.R.; Gilder, Stuart A.; Feinberg, Joshua M. (1 December 2016). "Low-temperature magnetic properties of monoclinic pyrrhotite with particular relevance to the Besnus transition". Geophysical Journal International. 207 (3): 1783–1795. doi:10.1093/gji/ggw376.
  12. ^ Svoboda, Jan (2004). Magnetic techniques for the treatment of materials. Springer. p. 33. ISBN 1-4020-2038-4.
  13. ^ a b c "Pyrrhotite: Physical properties, uses, composition". Retrieved 2023-02-20.
  14. ^ a b "Pyrrhotite". Retrieved 2009-07-07.
  15. ^ a b "Pyrite" (PDF). Retrieved 2023-02-20.
  16. ^ "Chalcopyrite" (PDF). handbookofmineralogy. Retrieved 2023-02-20.
  17. ^ "Pentlandite" (PDF). handbookofmineralogy. Retrieved 2023-02-20.
  18. ^ a b c d e Spry, P. G., & Gedlinske, B. (1987). Tables for the determination of common opaque minerals. Economic Geology Pub.
  19. ^ "Pyrrhotite". Retrieved March 24, 2023.
  20. ^ Hussey, Kristin; Foderaro, Lisa W. (7 June 2016). "With Connecticut Foundations Crumbling, Your Home Is Now Worthless". The New York Times. Retrieved 2016-06-08.
  21. ^ "Crumbling Foundations". Retrieved 2016-06-08.
  22. ^ "U.S. GAO - Crumbling Foundations: Extent of Homes with Defective Concrete Is Not Fully Known and Federal Options to Aid Homeowners Are Limited". Retrieved 2021-02-22.
  23. ^ a b c d Haldar, S. K. (2017). Platinum-nickel-chromium deposits : geology, exploration and reserve base. Elsevier. p.24. ISBN 978-0-12-802041-8.
  24. ^ 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 Proceesing of Minerals. 245-249.

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