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Troilite

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Troilite
Polished and etched surface of the Mundrabilla meteorite from Australia. The darker brownish areas with striations are troilite with exolved daubreelite.
General
CategorySulfide mineral
Formula
(repeating unit)		FeS
Strunz classification2.CC.10
Crystal systemHexagonal
Space groupHexagonal dihexagonal dipyramidal
H-M symbol: (6/m 2/m 2/m)
Space group: P 63/mmc
Unit cella = 5.958 Å, c = 11.74 Å; Z = 12
Identification
ColorPale gray brown
Crystal habitMassive, granular; nodular; platey to tabular
CleavageNone
FractureIrregular
Mohs scale hardness3.5 - 4.0
LusterMetallic
StreakGray black
DiaphaneityOpaque
Specific gravity4.67–4.79
Alters toTarnishes on exposure to air
References[1][2][3]

Troilite is a rare iron sulfide mineral with the simple formula of FeS. It is the iron rich endmember of the pyrrhotite group. Pyrrhotite has the formula Fe(1-x)S (x = 0 to 0.2) which is iron deficient. As troilite lacks the iron deficiency which gives pyrrhotite its characteristic magnetism, troilite is non-magnetic.[2]

Troilite can be found as a native mineral on Earth, but is more abundant in meteorites, in particular those originating from the Moon and Mars. Uniform presence of troilite on the Moon and possibly on Mars has been confirmed by the Apollo, Viking and Phobos space probes. The relative intensities of isotopes of sulfur are rather constant in meteorites as compared to the Earth minerals, and therefore troilite from Canyon Diablo meteorite is chosen as the international sulfur isotope ratio standard.

Structure

Troilite has hexagonal structure (Pearson symbol hP24, Space group P-62c No 190). Its unit cell is approximately a combination of two vertically stacked basic NiAs-type cells of pyrrhotite, where the top cell is diagonally shifted.[4] For this reason, troilite is sometimes called pyrrhotite-2C.[5]

Discovery

A meteorite fall was observed in 1766 at Albareto, Modena, Italy. Samples were collected and studied by Domenico Troili who described the iron sulfide inclusions in the meteorite. These iron sulfides were long considered to be pyrite. In 1862 German mineralogist Gustav Rose analyzed the material and recognized it as stoichiometric FeS and gave it the name troilite in recognition of the work of Domenico Troili.[2][1][6]

Occurrence

Troilite has been reported from a variety of meteorites occurring with daubreelite, chromite, sphalerite, graphite, and a variety of phosphate and silicate minerals.[1] It has also been reported from serpentinite in the Alta mine, Del Norte County, California and in layered igneous intrusions in Western Australia, the Ilimaussaq intrusion of southern Greenland, and at Nordfjellmark, Norway. In the Australian occurrence it is associated with copper, nickel iron ore deposits occurring with pyrrhotite, pentlandite, mackinawite, cubanite, valleriite, chalcopyrite and pyrite.[1]

Troilite is extremely rarely encountered in the Earth's crust. (Even pyrrhotite is relatively rare compared to pyrite and Iron(II) sulfate minerals) Most troilite on Earth is of meteoritic origin. One iron meteorite, Mundrabilla contains 25 to 35 volume percent troilite.[7] The most famous troilite-containing meteorite is Canyon Diablo. Canyon Diablo Troilite (CDT) is used as a standard of relative concentration of different isotopes of sulfur.[8] Meteoritic standard was chosen because of the constancy of the sulfur isotopic ratio in meteorites, whereas the sulfur isotopic composition in Earth materials varies due to the bacterial activity. In particular, bacteria reduce 32
SO2−
4 1.07 times faster than 34
SO2−
4 that may increase the 34
S/32
S ratio by up to 10%.[9]

Troilite is the most common sulfide mineral at the lunar surface. It forms about one percent of the lunar crust and is present in any rock or meteorite originating from moon. In particular, all basalts brought by the Apollo 11, 12, 15 and 16 missions contain about 1% of troilite.[4][10][11][12]

Troilite is regularly found in Martian meteorites (i.e. those originating from Mars). Similar to the Moon's surface and meteorites, the fraction of troilite in Martian meteorites is close to 1%.[13][14]

Based on observations by the Voyager spacecraft in 1979 and Galileo in 1996, troilite might also be present in the rocks of Jupiter’s satellites Ganymede and Callisto.[15] Whereas experimental data for Jupiter's moons are yet very limited, the theoretical modeling assumes large percentage of troilite (~22.5%) in the core of those moons.[16]

References

  1. ^ a b c d Handbook of Mineralogy
  2. ^ a b c Troilite on Mindat.org
  3. ^ Troilite on Webmineral
  4. ^ a b Evans, Ht, Jr (1970). "Lunar Troilite: Crystallography". Science. 167 (3918): 621–623. Bibcode:1970Sci...167..621E. doi:10.1126/science.167.3918.621. ISSN 0036-8075. PMID 17781520. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  5. ^ Hubert Lloyd Barnes (1997). Geochemistry of hydrothermal ore deposits. John Wiley and Sons. pp. 382–390. ISBN 047157144X.
  6. ^ Gerald Joseph Home McCall, A. J. Bowden, Richard John Howarth (2006). The history of meteoritics and key meteorite collections. Geological Society. pp. 206–207. ISBN 1862391947.{{cite book}}: CS1 maint: multiple names: authors list (link)
  7. ^ Vagn Buchwald (1975). Handbook of Iron Meteorites. Univ of California. ISBN 0520029348.
  8. ^ Julian E. Andrews (2004). An introduction to environmental chemistry. Wiley-Blackwell. p. 269. ISBN 0632059052.
  9. ^ Kurt Konhauser (2007). Introduction to geomicrobiology. Wiley-Blackwell. p. 320. ISBN 0632054549.
  10. ^ Haloda, Jakub; Týcová, Patricie; Korotev, Randy L.; Fernandes, Vera A.; Burgess, Ray; Thöni, Martin; Jelenc, Monika; Jakeš, Petr; Gabzdyl, Pavel (2009). "Petrology, geochemistry, and age of low-Ti mare-basalt meteorite Northeast Africa 003-A: A possible member of the Apollo 15 mare basaltic suite". Geochimica et Cosmochimica Acta. 73 (11): 3450. Bibcode:2009GeCoA..73.3450H. doi:10.1016/j.gca.2009.03.003.
  11. ^ Grant Heiken ; David Vaniman ; Bevan M. French (1991). Lunar sourcebook. CUP Archive. p. 150. ISBN 0521334446.{{cite book}}: CS1 maint: multiple names: authors list (link)
  12. ^ L. A. Tayrol; Williams, K. L. (1973). "Cu-Fe-S Phases in Lunar Rocks" (PDF). American Mineralogist. 58: 952. Bibcode:1973AmMin..58..952T.
  13. ^ Yanai, Keizo (1997). "General view of twelve martian meteorites". Mineralogical Journal. 19 (2): 65. doi:10.2465/minerj.19.65.
  14. ^ Yu, Y; Gee, J (2005). "Spinel in Martian meteorite SaU 008: implications for Martian magnetism" (PDF). Earth and Planetary Science Letters. 232 (3–4): 287. Bibcode:2005E&PSL.232..287Y. doi:10.1016/j.epsl.2004.12.015.
  15. ^ "Troilite". Mindat.org. Retrieved 2009-07-07.
  16. ^ Fran Bagenal, Timothy E. Dowling, William B. McKinnon (2007). Jupiter. Cambridge University Press. p. 286. ISBN 0521035457.{{cite book}}: CS1 maint: multiple names: authors list (link)
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