Fougerite and FeII-III hydroxycarbonate green rust; ordering, deprotonation and/or cation substitution; structure of hydrotalcite-like compounds and mythic ferrosic hydroxide Fe(OH)(2+x)

A green rust has been recognised as a new mineral (IMA 2003-057) and given the name fougerite. Its chemical counterpart is FeII-III hydroxycarbonate GR1(CO32−) compound, which is partially deprotonated since formed by reduction of ferric oxyhydroxides through the activity of dissimilatory iron-reducing bacteria (DIRB) in anaerobic gley soils. Preparation of GR1(CO32−) by co-precipitation of FeII and FeIII cations in carbonated medium shows by using Mössbauer spectroscopy that the domain of existence of GR1(CO32−) lies within [0.25,0.33] for x={[FeIII]/[Fetotal]} with ordered upper limit [FeII4FeIII2(OH)12]2+⋅[CO3⋅∼3H2O]2−. GR1(CO32−) gets oxidised into ferrihydrite evolving to goethite by aerial oxidation, or into ferric green rust GR1(CO32−)∗, [FeIII6(OH)4(OOH)4]2+⋅[CO3⋅∼3H2O]2− by OH− deprotonation. A mass balance of iron ox(yhydrox)ides is drawn accordingly in the carbonated medium. Mössbauer spectra measured at 12 K show quite different magnetic properties and the three quadrupole doublets, comprising 2 ferrous and 1 ferric in GR1(CO32−), become 3 magnetically split ferric sextets in GR1(CO32−)∗. Structures of ordered GR1(CO32−), GR1(CO32−)∗ and GR1(Cl−) hydroxychloride are drawn. Extension to other hydrotalcite-like compounds is proposed whereas occurrences of fougerite mixed with clay minerals are presented. Fougerite is FeII6(1−x)FeIII6x(OH)4(4−3x)(OOH)2(3x−1)CO3, the partially deprotonated green rust where 1/3⩽x<2/3. Substitution of Fe cations by MgII or AlIII may occur but the proposal advocating a ferrosic hydroxide Fe(OH)(2+x) is discarded.