Preparation and Thermodynamic Equilibria of Green Rusts in Aqueous Solutions and their Identification as Minerals in Hydromorphic Soils

Abstract

In this paper we consider the properties and formation of materials known as Green Rusts (GRs) and the identification of these minerals in hydromorphic soils. Green Rusts are Fe(II)-Fe(III) compds. (mostly hydroxides). From XRD, two types of GRs are distinguished, GR1 incorporating "planar anions", e.g. Cl- and CO32-, and GR2 incorporating "three-dimensional anions". The prepn. of these compds. in the lab. by controlled oxidn. of a ferrous hydroxide ppt. in anion-contg. aq. solns. allow us, by monitoring the Eh and pH vs. time curves, to elaborate them at stoichiometry with the appropriate Fe2+/(OH-) concn. ratio. The XRD characterization of these Green Rusts, and the corresponding Mossbauer spectra allow us to det. structural information and chem. formulas. Synthetic GRs have been prepd. by oxidn. of Fe(OH)2 incorporating Cl-, SO42- or CO32- anions. The existence of GRs can be predicted from the Eh-pH diagrams drawn previously. Mossbauer and Raman spectra have identified a GR in samples extd. from hydromorphic soils. The mineral named fougerite has an Fe(III)/Fetot ratio that varies from 1/3 to 2/3. It is suspected that bacteria from the hydromorphic soils play a role in the formation of fougerite. [on SciFinder(R)]