Fougerite

Fougerite is a recently discovered representative of clay minerals, chemically and structurally related to so-called green rust, hydrotalcite-like minerals (hydrotalcite, pyroaurite, iowaite, manasseite) and other layered double hydroxides. The formula of the mineral is complex due to structural variabilites, as is for green rust, thus different formulas can be found elsewhere. The structure is based on brucite-like layers. Fougerite crystallizes in trigonal system. The originally given formula for fougerite is [Fe²⁺_1-xFe³⁺_xMg_y(OH)_2+2y] ^+x [x/n A^-n.mH₂O] ^{-x where A denotes an interlayer anion, n denotes valency, by 1/4 ≤ x/(1+y) ≤ 1/3 and m ≤ (1-x+y) (Trolard et al. 2007).}

The interlayer anions may be OH^-, Cl^-, CO₃^2-, SO₄^2- and other. Fougerite is unique among other clay minerals due to having a positive layer charge.

Fougerite was found in forested soils near Fougères. It possess blue-green to bluish-gray colour. It is regarded as a precursor for other iron hydroxides and related compounds, like goethite, lepidocrocite and ferrihydrite (Trolard et al. 2007, Génin et al. 2005).

ee also

* Iron(II) hydroxide
* Layered double hydroxides
* Clay minerals

References

1. 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

2. 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