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Fougèrite

Hydrotalcite group mineral (green rust)

Field	Value
name	Fougèrite
image	Fougerite Concordia Felix-0.jpg
category	Hydroxide mineral
formula	[Fe2+4Fe3+2(OH)12][CO3]·3H2O
IMAsymbol	Fgè
strunz	4.FL.05
system	Trigonal
class	m ( 2/m) – Hexagonal scalenohedral
symmetry	Rm
unit cell	a = 3.125 Å, c = 22.5 Å
colour	Bluish green
luster	Earthy
diaphaneity	Opaque
alteration	Unstable – alters to goethite or limonite
references

| length fast/slow = Fougèrite is a relatively recently described naturally occurring green rust mineral. It is the archetype of the fougèrite group in the larger hydrotalcite supergroup of naturally occurring layered double hydroxides. The structure is based on brucite-like layers containing Fe2+ and Fe3+ cations, O2− and OH− anions, with loosely bound [CO3]2− groups and H2O molecules between the layers. Fougèrite crystallizes in trigonal system. The ideal formula for fougèrite is [Fe2+4Fe3+2(OH)12][CO3]·3H2O. Higher degrees of oxidation produce the other members of the fougèrite group, namely trébeurdenite, [Fe2+2Fe3+4O2(OH)10][CO3]·3H2O and mössbauerite, [Fe3+6O4(OH)8][CO3]·3H2O.

Fougèrite was first found in forested soils near Fougères, Brittany, France, and recognised as a valid mineral species by the International Mineralogical Association in 2002. It is blue-green to bluish-gray in colour, and resembles clay minerals in habit, forming hexagonal platelets of submicron diameter. In this environment, it is intimately intergrown with trébeurdenite, to give varying overall ratios of Fe2+:Fe3+. The existence of two intergrown fixed-composition phases has been demonstrated by Mössbauer spectroscopy. The mineral is unstable in air, and decomposes by oxidation, dehydration and decarbonation, to ferrihydrite, and ultimately to lepidocrocite or goethite, FeIIIO(OH).

References

Warr, L.N.. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine.
"Minerals and the Emergence of Life, pp 149-153 in "Metals, Microbes and Minerals: The Biogeochemical Side of Life" (2021) pp xiv + 341.Walter de Gruyter, Berlin. Authors Duval, Simon; Zuchan, Kilian; Baymann, Frauke; Schoepp-Cothenet, Barbara; Branscomb, Elbert; Russell, Michael, J.; Nitschke, Wolfgang; Editors Kroneck, Peter M.H. and Sosa Torres, Martha. [https://www.de Gruyter.com/document/doi/10.1515/9783110589771-005 DOI 10.1515/9783110589771-005]
Mills, S.J., Christy, A.G., Génin J.-M.R., Kameda, T. and Colombo, F. 2012: ''Nomenclature of the hydrotalcite supergroup: natural layered double hydroxides,'' Mineralogical Magazine, 76, 1289-1336. [http://pubsites.uws.edu.au/ima-cnmnc/Mills%20et%20al%202012%20hydrotalcite%20nomenclature.pdf] {{Webarchive. link. (2013-05-24)
[http://www.mindat.org/min-27543.html Fougèrite data on Mindat.org]
Trolard F., Bourrié G., Abdelmoula M., Refait P. and Feder F. 2007: ''Fougerite, a new mineral of the pyroaurite-iowaite group: description and crystal structure,'' Clays and Clay Minerals, vol. 55, no. 3, p. 323-334; {{doi. 10.1346/CCMN.2007.0550308
Génin J.-M. R., Aïssa R., Géhin A., Abdelmoula M., Benali O., Ernstsen V., Ona-Nguema G., Upadhyay Ch. and Ruby Ch.; 2005: ''Fougerite and Fe^II-III hydroxycarbonate green rust; ordering, deprotonation and/or cation substitution; structure of hydrotalcite-like compounds and mythic ferrosic hydroxide Fe(OH)_2+x,'' Solid State Sciences, vol. 7., no. 5, p. 545-572. {{doi. 10.1016/j.solidstatesciences.2005.02.001.

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