Speciation of iron; characterisation and structure of green rusts and Fe II–III oxyhydroxycarbonate fougerite

Abstract

Fe II–Fe III hydroxysalt green rusts (GR) are very reactive layered double hydroxides that can be characterised mainly by XRD and Mössbauer spectroscopy. Mössbauer spectra of the main compounds presented in this article are reviewed with special emphasis on those of GRs constituted of one or two ferrous doublets and one ferric doublet. The structure of GR has a stacking sequence of alternating brucite-like layers of Fe II–III(OH) 6 octahedrons and anion interlayers as confirmed from XRD patterns. There exist two kinds of structure named green rust one (GR1) and green rust two (GR2) with R 3 ¯ m and P 3 ¯ m 1 space groups, respectively. Cation and anion distributions within Fe layers and interlayers match abundances of Mössbauer spectrum components. In the view of that, it can be suggested that the fougerite mineral is partially deprotonated Fe II–III hydroxycarbonate with a general formula Fe II 6 ( 1 − x ) Fe III 6 x O 12 H 2 ( 7 − 3 x ) CO 3 , where x is within the range [ ( 1 3 ) , ( 2 / 3 ) ]. Cation substitution by Mg II and Al III cannot be excluded relating fougerite with other minerals like pyroaurite and hydrotalcite. To cite this article: J.-M.R. Génin et al., C. R. Geoscience 338 (2006).