Quantitative Estimation of Fougerite Green Rust in Soils and Sediments by Citrate-Bicarbonate Kinetic Extractions

Fougerite (IMA 203-057), from green rust (GR) group, is difficult to quantify due to its reactivity and its small concentration in soils and sediments. Chemical extractions with citrate-bicarbonate (CB) reagent, in kinetic mode, can be used for a pre-diagnosis. Performed by steps (0, 1, 6, 48, 168 and 504 h), the proposed protocol was applied on samples from Gleysol of Fougere's forest with mineralogical controls by Mossbauer and XRD (X-ray diffraction) after each step of extraction. In less than 6 h, the first fraction extracted is composed of 70% Si, 80% Al, 23% Fe and 80% Mg of total element extractable by CB and is ascribed to the indefinable mineral mixture Si-Al-Fe named by Tamm. Between 6 and 168 h, the second fraction extracted is composed of Fe and Mg with a constant mole ratio Fe/Mg equal to 10 and is ascribed to the fougerite-GR phase. Analysis of XRD pattern and of Mossbauer spectra confirms: (i) all the other mineral phases containing Al, Mg, Si were not dissolved by CB after 6 h; (ii) the CB treatment extracts fougerite-GR completely. The residual fraction is composed of components not dissolved by CB extraction. Thus, the selectivity of CB can be used to quantitatively estimate the presence of fougerite-GRs in soils and sediments.