Pyrrhotite
Template:Short description Template:Infobox mineral
Pyrrhotite (pyrrhos in Greek 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.<ref>Template:Cite book</ref> Pyrrhotite is generally tabular and brassy/bronze in color with a metallic luster. The mineral occurs with mafic igneous rocks like norites, and may form from pyrite during metamorphic processes.<ref name="Mauk" /> Pyrrhotite is associated and mined with other sulfide minerals like pentlandite, pyrite, chalcopyrite, and magnetite, and has been found globally.
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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.<ref>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. Template:ISBN.Template:Page needed</ref>
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.<ref name="mindat" /><ref name="structure">Template:Cite book</ref> 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.<ref name="Klein">Template:Cite book</ref>
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.<ref>Sagnotti, L., 2007, Iron Sulfides; in: Encyclopedia of Geomagnetism and Paleomagnetism; (Editors David Gubbins and Emilio Herrero-Bervera), Springer, 1054 pp., p. 454-459.</ref> 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.<ref>Template:Cite book</ref> Monoclinic pyrrhotite undergoes a magnetic transition known as the Besnus transition at 30 K that leads to a loss of magnetic remanence.<ref>Template:Cite journal</ref> The saturation magnetization of pyrrhotite is 0.12 tesla.<ref>Template:Cite book</ref>
Identification
Physical properties
Pyrrhotite is brassy, bronze, or dark brown in color with a metallic luster and uneven or subconchoidal fracture.<ref name="geology.com">Template:Cite web</ref> 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).<ref name="geology.com" /> On the Mohs hardness scale, pyrrhotite ranges from 3.5 to 4,<ref name="mindat2">Template:Cite web</ref> compared to 6 to 6.5 for pyrite.<ref name="rruff.info">Template:Cite web</ref> Streak can be used when properties between pyrrhotite and other sulfide minerals are similar. Pyrrhotite displays a dark grey to black streak.<ref name="mindat2" /> Pyrite will display a greenish black to brownish black streak,<ref name="rruff.info" /> chalcopyrite will display a greenish black streak,<ref>Template:Cite web</ref> and pentlandite leaves a pale bronze-brown streak.<ref>Template:Cite web</ref> Pyrrhotite generally displays massive to granular crystal habit, and may show tabular/prismatic or hexagonal crystals which are sometimes iridescent.<ref name="geology.com" />
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.<ref>Template:Cite web</ref> The following optical properties<ref name="Spry">Spry, P. G., & Gedlinske, B. (1987). Tables for the determination of common opaque minerals. Economic Geology Pub.</ref> are representative of polished/puck sections using ore microscopy:
Pyrrhotite typically appears as anhedral, granular aggregates and is cream-pink to brownish in color.<ref name="Spry" /> Weak to strong reflection pleochroism which may be seen along grain boundaries.<ref name="Spry" /> Pyrrhotite has similar polishing hardness to pentlandite (medium), is softer than pyrite, and harder than chalcopyrite.<ref name="Spry" /> Pyrrhotite will not display twinning or internal reflections, and its strong anisotropy from yellow to greenish-gray or grayish-blue is characteristic.<ref name="Spry" />
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 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.<ref name=Klein/> It also occurs in pegmatites and in contact metamorphic zones. Pyrrhotite is often accompanied by pyrite, marcasite and magnetite.
Formation
Pyrrhotite requires both iron and sulfur to form.<ref name="Mauk" /> Iron is the fourth most abundant element in the Earth's continental crust (average abundance of 5.63% or 56,300 mg/kg in the crust),<ref name="Abundance of Elements in the Earth">"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.</ref> and so the majority of rocks have sufficient iron abundance to form pyrrhotite.<ref name="Mauk" /> However, because sulfur is less abundant (average abundance of 0.035% or 350 mg/kg in the crust),<ref name="Abundance of Elements in the Earth" /> the formation of pyrrhotite is generally controlled by sulfur abundance.<ref name="Mauk" /> Also, the mineral pyrite is both the most common and most abundant sulfide mineral in the Earth's crust.<ref name="Mauk" /> If rocks containing pyrite undergo metamorphism, there is a gradual release of volatile components like water and sulfur from pyrite.<ref name="Mauk" /> The loss of sulfur causes pyrite to recrystallize into pyrrhotite.<ref name="Mauk" />
Pyrite also decomposes into pyrrhotite in hot reductive technogenic environments, such as blast furnaces<ref>Template:Cite journal</ref> and direct coal liquefaction (in which it is an important catalyst).<ref>Template:Cite journal</ref>
Pyrrhotite can also form near black smoker hydrothermal vents.<ref name="Mauk" /> Black smokers release high sulfur concentrations onto the sea floor, and when the surrounding rocks are metamorphosed, pyrrhotite can crystallize.<ref name="Mauk" /> Later tectonic processes uplift the metamorphic rocks and expose pyrrhotite to the Earth's surface.<ref name="Mauk" />
Distribution
United States
Pyrrhotite occurs in a variety of locations in the United States.<ref name="Mauk" /><ref>Template:Cite book</ref><ref>U.S. Geological Survey, 2019, Mineral Resource Data System: accessed April 11, 2023, at http://mrdata.usgs.gov/mrds/.</ref><ref>Mindat.org, 2019, Mines, minerals and more: accessed April 11, 2023, at https://mindat.org/.</ref> In the eastern United States, pyrrhotite occurs in highly metamorphosed rock that forms a belt along the Appalachian Mountains.<ref name="Mauk" /> Pyrrhotite-bearing rocks are generally unseen in the central United States as the area is unmetamorphosed and underlain by sedimentary rocks which do not contain pyrrhotite.<ref name="Mauk" /> Discontinuous belts that contain pyrrhotite are present in the western United States along the Sierra Nevada mountain range and Cascade Range extending into the northwestern United States.<ref name="Mauk" /> Pyrrhotite may also be found west and south of Lake Superior.<ref name="Mauk" />
Mining locations worldwide
The following are some of the locations worldwide where pyrrhotite has been reported during mining:<ref name="mindat2" />
Canada
| Location | Mine | Main Target Commodities |
|---|---|---|
| British Columbia, Riondel | Bluebell Mine<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> | Cd, Cu, Au, Pb, Ag, Zn |
| Québec | Henderson No. 2 mine (Copper Rand mine)<ref>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.</ref> | Cu, Au |
| Québec | B&B Quarry, Sharwinigan | Crushed rock (Gabbro) for construction |
| Québec | Maskimo Quarry, Sharwinigan | Crushed rock (Gabbro) for construction |
US
| Location | Mine | Main Target Commodities |
|---|---|---|
| Connecticut | Becker Quarry (Becker's Quarry)<ref>Template:Cite journal</ref> | 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
| Location | Mine | Main Target Commodities |
|---|---|---|
| Tasmania | Renison Bell Mine (Renison Mine)<ref>Template:Cite journal</ref> | Sn |
Brazil
| Location | Mine | Main Target Commodities |
|---|---|---|
| Minas Gerais | Morro Velho mine<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> | Au, iron-ore<ref>Template:Cite web</ref> |
Italy
| Location | Mine | Main Target Commodities |
|---|---|---|
| Tuscany | Bottino Mine<ref>Template:Cite journal</ref> | Ag, sulfides<ref>Template:Cite web</ref> |
Kosovo
| Location | Mine | Main Target Commodities |
|---|---|---|
| Mitrovica District | Trepça Mine<ref>Template:Cite journal</ref> | Pb, Ag, Zn |
Etymology and history
Named in 1847 by Ours-Pierre-Armand Petit-Dufrénoy.<ref>Template:Cite web</ref> "Pyrrhotite" is derived from the Greek word πυρρός, "pyrrhos", meaning flame-colored.<ref name=mindat>Template:Cite web</ref>
Sulfide oxidation in construction aggregates leading to concrete degradation
Template:Main Pyrrhotite-containing rocks cannot be used as aggregates for concrete, because sulfate ions released by pyrrhotite oxidation readily cause an internal sulfate attack (ISA) in concrete. This is an insidious and severe form of concrete degradation.<ref name="USGS.gov-2020">Template:Cite web</ref> Pyrrhotite has been linked to crumbling concrete basements in Quebec, Massachusetts and Connecticut where local quarries extracted insufficiently characterised aggregates for making concrete.<ref name="nytimes" /><ref name="nbcconnecticut" /><ref name="GAO" /> Many houses in Ireland, particularly in County Donegal, have also been affected by pyrrhotite inclusions present in aggregates improperly selected for making concrete.<ref name="Brough">Template:Cite journal</ref><ref name="Leemann">Template:Cite journal</ref> The iron sulfide it contains can naturally react with oxygen and water, and over time pyrrhotite breaks down into sulfuric acid and forms secondary minerals such as ettringite, thaumasite and gypsum.<ref name="USGS.gov-2020" /><ref name="Mauk">Template:Cite book</ref> These newly formed alteration products occupy a larger volume than pyrrhotite. Their expansion induces tensile stress in the concrete matrix, causing the concrete to crack and leading to the failure of home foundations or concrete structures.<ref name=nytimes>Template:Cite news</ref><ref name=nbcconnecticut>Template:Cite web</ref><ref name=GAO>Template:Cite web</ref><ref name="USGS.gov-2020" /><ref name="Mauk" />
Uses
Other than a source of sulfur, pyrrhotite does not have specific applications.<ref name="Haldar">Haldar, S. K. (2017). Platinum-nickel-chromium deposits : Geology, exploration and reserve base. Elsevier. p.24. ISBN 978-0-12-802041-8.</ref> It is generally not a valuable mineral unless significant nickel, copper, or other metals are present.<ref name="Haldar" /><ref>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.</ref> Iron is seldom extracted from pyrrhotite due to a complicated metallurgical process.<ref name="Haldar" /> It is mined primarily because it is associated with pentlandite, Template:Chem2, a sulfide mineral that can contain significant amounts of nickel and cobalt.<ref name="mindat" /> When found in mafic and ultramafic rocks, pyrrhotite can be a good indicator of economic nickel deposits.<ref name="Haldar" />
Mineral abbreviations
| Abbreviation | Source |
|---|---|
| Pyh | IMA-CNMNC<ref>Template:Cite journal</ref> |
| Po | Whitney and Evans, 2010;<ref>Template:Cite journal</ref> The Canadian Mineralogist, 2019.<ref>The Canadian Mineralogist (2019) The Canadian Mineralogist list of symbols for rock- and ore-forming minerals (December 30, 2019). https://www.mineralogicalassociation.ca/wordpress/wp-content/uploads/2020/01/symbols.pdf.</ref> |
Synonyms
| Magnetic pyrite | Magnetopyrite | Magnetic pyrites |
| Pyrrhotine | Pyrrohotite | Magnetic iron pyrites |
| Dipyrite | Kroeberite | Vattenkies |