Fougerite and FeII-III hydroxycarbonate green rust; ordering drprotonation 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 Fell and Fe-III cations in carbonated medium shows by using Mossbauer spectroscopy that the domain of existence of GR1(CO32-) lies within [0.25, 0.33] for x = {[Fe-III]/[Fe-total]) with ordered upper limit [(Fe4Fe2III)-Fe-II(OH)(12)](2+)center dot[CO3 center dot similar to 3 H2O](2-). GR1(CO32-) gets oxidised into ferrihydrite evolving to goethite by aerial oxidation, or into ferric green rust GR1(CO32-)*, [Fe-6(III)(OH)(4)(OOH)(4)](2+). [CO3 center dot similar to 3 H2O](2-) by OH- deprotonation. A mass balance of iron ox(yhydrox)ides is drawn accordingly in the carbonated medium. Mossbauer spectra measured at 12 K show quite different magnetic properties and the three quadrupole doublets, comprising 2 ferrous and I ferric in GR1 (CO32-), become 3 magnetically split ferric sextets in GR1 (CO32-) *. Structures of ordered GR1 (CO32-),GRI(CO32-)* and GRI(Cl-) hydroxychloride are drawn. Extension to other hydrotalcite-like compounds is proposed whereas occurrences of fougerite mixed with clay minerals are presented. Fougerite is Fe-6(1-x(II))Fe-6x(III)(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 Mg-II or Al-III may occur but the proposal advocating a ferrosic hydroxide Fe(OH)((2+x)) is discarded. (c) 2005 Elsevier SAS. All rights reserved.