Clays and Clay Minerals, Vol. 55, No. 3, 323–334, 2007.
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Authors
Trolard, Fabienne
Bourrie´, Guilhem
Abdelmoula, Mustapha
Refait, Philippe
Feder, Fre´de´ric
Issue Date
2007
Type
Article
Language
Keywords
Periodicals , Geology , Fe , Fougerite , Gleysol , Green Rust , Hydroxide , Iowaite , Mg , New Mineral , Pyroaurite , Soil
Alternative Title
Fougerite, A New Mineral Of The Pyroaurite-iowaite Group: Description And Crystal Structure
Abstract
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-xFe3+ x Mgy(OH)2+2y]+x[x/n A-n.mH2O]-x where A is the interlayer anion and n its valency, with 1/4 4 x/(1+y) 4 1/3 and m 4 (1-x+y). The structure of green rusts and parent minerals can accommodate a variety of anions, such as OH-, Cl-, CO3 2-, SO4 2-. The structure of the mineral was studied by Mo¨ssbauer, Raman and X-ray absorption spectroscopies (XAS) at the FeK edge. Mo¨ ssbauer spectra of the mineral obtained at 78 K are best fitted with four doublets: D1 and D2 due to Fe2+ (isomer shiftd& 1.27 and 1.25 mm s-1, quadrupole splitting DEQ & 2.86 and 2.48 mm s-1, respectively) and D3 and D4 due toFe 3+ (d& 0.46 mm s-1, DEQ & 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 R3¯ m 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 Fouge`res, 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, Fouge`res-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.
Description
Clays and Clay Minerals, Vol. 55, No. 3, 323–334, 2007. Fougerite, A New Mineral Of The Pyroaurite-iowaite Group: Description And Crystal Structure. Fabienne Trolard; Guilhem Bourrie´; Mustapha Abdelmoula; Philippe Refait; Fre´de´ric Feder. DOI: 10.1346/CCMN.2007.0550308. Copyright © 2007, The Clay Minerals Society.
Citation
Clays and Clay Minerals, Vol. 55, No. 3, 323–334, 2007.
Publisher
The Clay Minerals Society
License
Copyright © 2006-2018, The Clay Minerals Society