Speciation of iron;: characterisation and structure of green rusts and FeII-III oxyhydroxycarbonate fougerite

Fe-II-Fe-III, hydroxysalt green rusts (GR) are very reactive layered double hydroxides that can be characterised mainly by XRD and Mossbauer spectroscopy. Mossbauer 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 FeII-II(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) over barm and P (3) over bar m1 space groups, respectively. Cation and anion distributions within Fe layers and interlayers match abundances of Mossbauer spectrum components. In the view of that, it can be suggested that the fougerite mineral is partially deprotonated FeII-III, hydroxycarbonate with a general formula Fe-6(l-x)(II) (Fe6xO12H2(7-3x)CO3)-O-III, 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.