Structure and thermodynamics of ferrous, stoichiometric and ferric oxyhydroxycarbonate green rusts; redox flexibility and fougerite mineral

Ferric oxyhydroxycarbonate GR(CO32−)*, FeIII6O12H8CO3, is prepared from FeII–III hydroxycarbonate GR(CO32−), FeII4FeIII2(OH)12CO3, by violent oxidation adding H2O2 or by aerial oxidation once dried. Crystals have practically the same aspect and structural features are mainly conserved as testified by X-ray diffraction. Continuous in situ deprotonation of GR(CO32−) by adding the exact amount of H2O2 to reach any value of x={[FeIII]/[Fetotal]}x={[FeIII]/[Fetotal]} is followed, leading to the general formula FeII6(1−x)FeIII6xO12H2(7−3x)CO3 that can be extended all over the range x∈[0,1]x∈[0,1] giving also a ferrous GR(CO32−)§. Mössbauer spectra allow us to devise a model of the long range order of anions that remains unchanged over the whole range of composition whereas protonation or deprotonation accompanies the progressive filling of the three sublattices by FeIII ions with periodicity a0×3, if a0a0 is that of the hexagonal cation pavement. Monitoring the electrode potential EhEh during oxidation by H2O2 allowed determining the chemical potential μ○[GR∗(x)]μ○[GR∗(x)] for any value of x   leading to two domains approximated by μ○[GR∗(x)]={−618+54x} kJmole−1 in range [(1/3),(2/3)(1/3),(2/3)], and μ○[GR∗(x)]={−632+75x} kJmole−1 in range [(2/3),1][(2/3),1]. Occurrences of fougerite mineral in the field (IMA 2003-057) limited to x∈[(1/3),(2/3)]x∈[(1/3),(2/3)] are discussed in agreement with their Mössbauer spectra identical to those obtained in this study.

Figure optionsDownload as PowerPoint slide