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

摘要

A green rust has been recognised as a new mineral (IMA 2003-057) and given the name fougerite. Its chemical counterpart is Fe~(II-III) hydroxycarbonate GR1 (CO_3~(2-)) 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(CO_3~(2-)) by co-precipitation of Fe~(II) and Fe~(III) cations in carbonated medium shows by using Mossbauer spectroscopy that the domain of existence of GR1(CO_3~(2-)) lies within [0.25, 0.33] for x = {[Fe~(III)]/[Fe_(total)]} with ordered upper limit [Fe_4~(II)Fe_2~(III)(OH)_(12)]~(2+) centre dot [CO_3 approx 3 H_2O]~(2-). GR1(CO_3~(2-)) gets oxidised into ferrihydrite evolving to goethite by aerial oxidation, or into ferric green rust GR1 (CO_3~(2-))*, [Fe_6~(III)(OH)_4(OOH)_4]~(2+) centre dot [CO_3 approx 3 H_2O]~(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 1 ferric in GR1 (CO_3~(2-)), become 3 magnetically split ferric sextets in GR1(CO_3~(2-))*. Structures of ordered GR1(CO_3~(2-)), GR1(CO_3~(2-))* 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 Fe_6~(1-x)~(II)Fe_(6x)~(III)(OH)_(4(4-3x))(OOH)_(2(3x-1))CO_3, the partially deprotonated green rust where 1/3 <= x < 2/3. Substitution of Fe cations by Mg~(II) or Al may occur but the proposal advocating a ferrosic hydroxide Fe(OH)_(2+x) is discarded.