Fougèrite Explained

Fougèrite
Category:	Hydroxide mineral
Formula:	[Fe<sup>2+</sup><sub>4</sub>Fe<sup>3+</sup><sub>2</sub>(OH)<sub>12</sub>][CO<sub>3</sub>]·3H2O
Imasymbol:	Fgè^[1]
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

Fougèrite is a relatively recently described naturally occurring green rust mineral.^[2] It is the archetype of the fougèrite group in the larger hydrotalcite supergroup of naturally occurring layered double hydroxides.^[3] 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.^[4] 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, Fe^IIIO(OH).^{[5] [6]}

See also

• Clay minerals
• Gley soil
• Iron(II) hydroxide

Notes and References

1. Warr. L.N.. 2021. IMA–CNMNC approved mineral symbols. Mineralogical Magazine. 85. 3. 291–320. 10.1180/mgm.2021.43. 2021MinM...85..291W. 235729616. free.
2. "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. Gruyter.com/document/doi/10.1515/9783110589771-005 DOI 10.1515/9783110589771-005
3. 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
4. http://www.mindat.org/min-27543.html Fougèrite data on Mindat.org
5. 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;
6. 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. .