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Fougerite, a new mineral of the pyroaurite-ivowaite group : description and crystal structure

Trolard F., Bourrié G., Abdelmoula M., Refait P., Feder F.. 2007. Clays and Clay Minerals, 55 (3) : p. 323-324.

Fougerite (IMA 2003-057) is a mixed M(II)-M(III) hydroxysalt of the green rust group, where M(II) can be Fe or Mg, and M(III) is Fe. The general structural formula is: [Fe2+1-[khi]Fe3+[khi]Mg[gamma](OH)2+2[gamma]]+[khi][[khi]/n A-n.mH2O]-[khi]-[khi] where A is the interlayer anion and n its valency, with 1/4 <= [khi]/(1+[gamma]) <= 1/3 and m <= (1-[khi]+[gamma]). The structure of green rusts and parent minerals can accommodate a variety of anions, such as OH-, C1-, CO23-, SO24-. The structure of the mineral was studied by Mössbauer, Raman and X-ray absorption spectroscopies (XAS) at the FeK edge. Mössbauer spectra of the mineral obtained at 78 K are best fitted with four doublets: D1 and D2 due to Fe2+ (isomer shift [delta] = 1.27 and 1.25 mm s-1, quadrupole splitting [delta]EQ = 2.86 and 2.48 mm s-1, respectively) and D3 and D4 due to Fe3+ ([delta]= 0.46 mm s-1, [delta]EQ = 0.48 and 0.97 mm s-1, respectively). Microprobe Raman spectra obtained with a laser at 514.53 nm show the characteristic bands of synthetic green rusts at 427 and 518 cm-1. X-ray absorption spectroscopy shows that Mg is present in the mineral in addition to Fe, that the space group is Ram and the lattice parameter a = 0.30-0.32 nm. The mineral forms by partial oxidation and hydrolysis of aqueous Fe2+, to give small crystals (400-500 nm) in the form of hexagonal plates. The mineral is unstable in air and transforms to lepidocrocite or goethite. The name is for the locality of the occurrence, a forested Gleysol near Fougères, Brittany, France. Its characteristic blue-green color (5BG6/1 in the Munsell system) has long been used as a universal criterion in soil classification to identify Gleysols. From a thermodynamic model of soil-solution equilibria, it was proposed that for the eponymous mineral, Fougères-fougerite, OH- may be the interlayer anion. In other environments, the interlayer anion may be different, and other varieties of fougerite may exist. Fougerite plays a key role in the pathways of formation of Fe oxides. (Résumé d'auteur)

Thématique : Chimie et physique du sol

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