Mackinawite

Mackinawite is an iron nickel sulfide mineral with formula (Fe,Ni)1 + xS (where x = 0 to 0.11). The mineral crystallizes in the tetragonal crystal system and has been described as a distorted, close packed, cubic array of S atoms with some of the gaps filled with Fe.[5] Mackinawite occurs as opaque bronze to grey-white tabular crystals and anhedral masses. It has a Mohs hardness of 2.5 and a specific gravity of 4.17. It was first described in 1962 for an occurrence in the Mackinaw mine, Snohomish County, Washington for which it was named.[4]

Mackinawite
General
CategorySulfide mineral
Formula
(repeating unit)		(Fe,Ni)1 + xS (where x = 0 to 0.11)
Strunz classification2.CC.25
Crystal systemTetragonal
Crystal classDitetragonal dipyramidal (4/mmm)
H-M symbol: (4/m 2/m 2/m)
Space groupP4/nmm
Unit cella = 3.67 Å, c = 5.03 Å; Z = 2
Identification
Formula mass85.42 g/mol
ColorBronze to white grey
Crystal habitAs well-formed thin tabular crystals; massive, fine-feathery
CleavagePerfect on {001}
Mohs scale hardness2.5
LusterMetallic
StreakBlack
DiaphaneityOpaque
Specific gravity4.17
References[1][2][3][4]

Occurrence

Mackinawite occurs in serpentinized peridotites as a hydrothermal alteration product, in meteorites, and in association with chalcopyrite, cubanite, pentlandite, pyrrhotite, greigite, maucherite, and troilite.[2] Mackinawite also occurs in reducing environments such as freshwater and marine sediments as a result of the metabolism of iron and sulfate-reducing bacteria.

In anoxic environments, mackinawite is formed by the reaction of HS− with either Fe2+ ions or with Fe metal.[6] Mackinawite is a metastable mineral that occurs predominantly as a poorly crystalline precipitate.[7] After the initiation of precipitation, mackinawite can take up to 2 years to form at 25 °C.[8] It has been reported that mackinawite can be stable for up to 16 weeks at temperatures up to 100 °C at pH values from 3-12.[9] Laboratories have also produced synthetic mackinawite to study its formation using several different methods such as reacting sulfide with metallic iron or a solution of ferrous iron, growing sulfide reducing bacteria using Fe2+, and electrochemically.[10][6][11][9][12]

Transformations in the environment

Depending on the redox conditions mackinawite can form more stable phases such as greigite[13] and ultimately pyrite,[14] an important mineral in anoxic aqueous settings that is preserved in sedimentary deposits, especially black shale.[9][15][16][17][18][19] While it has been determined that mackinawite is a necessary precursor to pyrite, the pathway of iron sulfide mineral formation from aqueous species to solid mineral is still nebulous. Many iron sulfide minerals may exist in the transition between poorly ordered mackinawite and crystalline pyrite such as greigite, smithite, and pyrrhotite;[20][21] however, studies have also indicated that pyrite formation from mackinawite can occur where oxidation has commenced and the sulfur present is in intermediate oxidation states (-1 to +6) and intermediate sulfur species such as elemental sulfur or polysulfides and surface oxidized monosulfide species, such as oxidized mackinawite or greigite are present.[9]

See also

References
  1. Mineralienatlas
  2. Handbook of Mineralogy
  3. Webmineral data
  4. Mindat
  5. Taylor, L.A.; Finger, L.W. (1970). "Structural refinement and composition of mackinawite". Carnegie Institute of Washington Geophysical Laboratory Annual Report. 69: 318–322.
  6. Lennie, A.R.; Redfern, A.T.R.; Champness, P.E.; Stoddart, C.P.; Schofield, P.F.; Vaughn, D.J. (1997). "Transformation of mackinawite to greigite: An in situ X-ray powder diffraction and transmission electron microscope study" (PDF). American Mineralogist. 82: 203–309.
  7. Vaughn, D.J.; Craig, J.R. (1978). Mineral chemistry of metal sulfides. Cambridge University Press. ISBN 978-0521214896.
  8. Rickard, D.T. (1995). "Kinetics of FeS precipitation: Part 1. Competing reaction mechanisms". Geochimica et Cosmochimica Acta. 59 (21): 4367–4379. Bibcode:1995GeCoA..59.4367R. doi:10.1016/0016-7037(95)00251-T.
  9. Benning, L.G.; Wilkin, R.T.; Barnes, H.L. (2000). "Reaction pathways in the Fe–S system below 100°C". Chemical Geology. 167 (1–2): 25–51. Bibcode:2000ChGeo.167...25B. doi:10.1016/S0009-2541(99)00198-9.
  10. Yamaguchi, S.; Moori, T. (1972). "Electrochemical Synthesis of Ferromagnetic Fe3 S 4". Journal of the Electrochemical Society. 119 (8): 1062. doi:10.1149/1.2404398.
  11. Mullet, M.; Boursiquot, S.; Abdelmoula, M.; Génin, J.-M.; Ehrhardt, J.-J. (2002). "Surface chemistry and structural properties of mackinawite prepared by reaction of sulfide ions with metallic iron". Geochimica et Cosmochimica Acta. 66 (5): 829–836. Bibcode:2002GeCoA..66..829M. doi:10.1016/S0016-7037(01)00805-5.
  12. Michel, F.M.; Antao, S.M.; Chupas, P.J.; Lee, P.L.; Parise, J.B.; Schoonen, M.A.A. (2005). "Short- to medium-range atomic order and crystallite size of the initial FeS precipitate from pair distribution function analysis". Chemistry of Materials. 17 (25): 6246–6255. doi:10.1021/cm050886b.
  13. Csákberényi-Malasics, D., Rodriguez-Blanco, J.D., Kovács Kis, V., Rečnik, A., Benning, L.G., and Pósfai, M. (2012) Structural properties and transformations of precipitated FeS. Chemical Geology, 294-295, 249-258. doi: 10.1016/j.chemgeo.2011.12.009.
  14. Schoonen, M.A.A. (2004). "Mechanisms of sedimentary pyrite formation". In Amend, J.P.; Edwards, K.J.; Lyons, T.W. (eds.). Sulfur biogeochemistry : past and present. Geological Society of America special papers 379. pp. 117–134. ISBN 9780896299054.
  15. Cahill, C.L.; Benning, L.G.; Barnes, H.L.; Parise, J.B. (2000). "In situ time-resolved X-ray diffraction of iron sulfides during hydrothermal pyrite growth". Chemical Geology. 167 (1–2): 53–63. Bibcode:2000ChGeo.167...53C. doi:10.1016/S0009-2541(99)00199-0.
  16. Rickard, D.T.; Morse, J.W. (2005). "Acid volatile sulfide (AVS)". Marine Chemistry. 97 (3–4): 141–197. doi:10.1016/j.marchem.2005.08.004.
  17. Pósfai, M.; Dunin-Borkowski, R.E. (2006). "Sulfides in Biosystems" (PDF). Reviews in Mineralogy and Geochemistry. 61 (1): 679–714. Bibcode:2006RvMG...61..679P. doi:10.2138/rmg.2006.61.13.
  18. Hunger, S.; Benning, L.G. (2007). "Greigite: a true intermediate on the polysulfide pathway to pyrite". Geochemical Transactions. 8: 1–20. doi:10.1186/1467-4866-8-1. PMC 1847509. PMID 17376247.
  19. Rickard, D.T.; Luther, G.W. (2007). "Chemistry of iron sulfides". Chemical Reviews. 107 (2): 514–562. doi:10.1021/cr0503658. PMID 17261073.
  20. Rickard, D.T. (1969). "The chemistry of iron sulphide formation at low temperatures". Stockholm Contributions in Geology. 20. pp. 67–95.
  21. Wuensch, B.J.; Prewitt, C.T.; Rajamani, V.; Scott, S.D.; Craig, J.R.; Barton, P.B. (1974). Sulfide Mineralogy: Short Course Notes. Mineralogical Society of America. ISBN 978-0939950010.
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