Jump to content

Pyrrhotite: Difference between revisions

From Wikipedia, the free encyclopedia
Browse history interactively
← Previous editNext edit →
Content deleted Content added
VisualWikitext
Ol Evene (talk | contribs)
61 edits
Edited lead section to update info and match major subsections as suggested by peer-review.
Ol Evene (talk | contribs)
61 edits
mNo edit summary
Line 38: Line 38:
}}
}}


'''Pyrrhotite''' (''pyrrhos'' in Greek meaning "flame-coloured"'')'' is an [[iron sulfide]] [[mineral]] with the formula Fe<sub>(1-x)</sub>S (x = 0 to 0.2). It is a [[nonstoichiometric compound|nonstoichiometric]] variant of FeS, the mineral known as [[troilite]].
'''Pyrrhotite''' (''[[Pyrrhus of Epirus|pyrrhos]]'' in Greek meaning "flame-coloured"'')'' is an [[iron sulfide]] [[mineral]] with the formula Fe<sub>(1-x)</sub>S (x = 0 to 0.2). It is a [[nonstoichiometric compound|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.<ref name=Vaughan>Vaughan, D. J.; Craig, J. R. "Mineral Chemistry of Metal Sulfides" Cambridge University Press, Cambridge: 1978. {{ISBN|0521214890}}.{{page needed|date=July 2015}}</ref>{{page needed|date=July 2015}} Pyrrhotite is generally tabular and brassy/bronze in color with a [[Lustre (mineralogy)|metallic luster]]. The mineral occurs with [[Mafic|mafic igneous rocks]] like [[Norite|norites]]. Pyrrhottie is associated and mined with other sulfide minerals like [[pentlandite]], [[pyrite]], [[chalcopyrite]], and [[magnetite]].
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.<ref name=Vaughan>Vaughan, D. J.; Craig, J. R. "Mineral Chemistry of Metal Sulfides" Cambridge University Press, Cambridge: 1978. {{ISBN|0521214890}}.{{page needed|date=July 2015}}</ref>{{page needed|date=July 2015}} Pyrrhotite is generally tabular and brassy/bronze in color with a [[Lustre (mineralogy)|metallic luster]]. The mineral occurs with [[Mafic|mafic igneous rocks]] like [[Norite|norites]]. Pyrrhottie is associated and mined with other sulfide minerals like [[pentlandite]], [[pyrite]], [[chalcopyrite]], and [[magnetite]].
[[File:Iron(II)-sulfide-unit-cell-3D-balls.png|thumb|[[Nickel arsenide#Crystal structure|NiAs structure]] of basic pyrrhotite-1C.]]
[[File:Iron(II)-sulfide-unit-cell-3D-balls.png|thumb|[[Nickel arsenide#Crystal structure|NiAs structure]] of basic pyrrhotite-1C.]]

Revision as of 00:00, 25 March 2023

Pyrrhotite
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
General
CategoryMineral
Formula
(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
Identification
ColorBronze, dark brown
Crystal habitTabular or prismatic in hexagonal prisms; massive to granular
CleavageAbsent
FractureUneven
Mohs scale hardness3.5 – 4.5
LusterMetallic
StreakDark grey – black
Specific gravity4.58 – 4.65, average = 4.61
Refractive indexOpaque
Fusibility3
SolubilitySoluble in hydrochloric acid
Other characteristicsWeakly magnetic, strongly magnetic on heating; non-luminescent, non-radioactive
References[2][3][4]

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][page needed] 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

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 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

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]

Identification

Physical Properties

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 samples include: brassy/bronze colour when paired with a grey/black streak, tabular or hexagonal crystals which show iridescence, subconchoidal fracture, metallic luster, and magnestism.

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 (which contains an upper reflecting polarizing light illuminator). As such, 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 which display strong anisotropy.

Occurrence

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. Pyrrhotite does not have specific applications. It is mined primarily because it is associated with pentlandite, sulfide mineral that can contain significant amounts of nickel and cobalt.[2]

Etymology and history

The name pyrrhotite is derived from Greek pyrrhos, flame-colored.[2]

Issues

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.[19][20][21]

References

  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". Mindat.org. Retrieved 2009-07-07.
  3. ^ "Pyrrhotite" (PDF). Rruff.geo.arizona.edu. Retrieved 2015-07-10.
  4. ^ "Pyrrhotite Mineral Data". Webmineral.com. Retrieved 2015-07-10.
  5. ^ Vaughan, D. J.; Craig, J. R. "Mineral Chemistry of Metal Sulfides" Cambridge University Press, Cambridge: 1978. ISBN 0521214890.[page needed]
  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.{{cite journal}}: CS1 maint: unflagged free DOI (link)
  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". geology.com. Retrieved 2023-02-20.
  14. ^ a b "Pyrrhotite". Mindat.org. Retrieved 2009-07-07.
  15. ^ a b "Pyrite" (PDF). rruff.info. 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. ^ 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.
  20. ^ "Crumbling Foundations". nbcconnecticut.com. Retrieved 2016-06-08.
  21. ^ "U.S. GAO - Crumbling Foundations: Extent of Homes with Defective Concrete Is Not Fully Known and Federal Options to Aid Homeowners Are Limited". gao.gov. Retrieved 2021-02-22.
Retrieved from "https://en.wikipedia.org/w/index.php?title=Pyrrhotite&oldid=1146452817"