The Mössbauer spectra of hydroxycarbonate green rusts, Clay Mineral

paramagnetic state were fitted using two Fe 2+ and Fe 3+ doublets each. These spectra can be interpreted in terms of an ordered cation arrangement in the pyroaurite-type structure. The inner Fe 2+ doublet seems to be the outcome of excess Fe 3+ in the samples studied. The outer Fe 2+ doublet (quadrupole splitting ~2.8 mm s-1, FWHM = 0.26 mm s-l) is relatively well-defined and can, together with the fact that his doublet disappears on oxidation, possibly serve to identify these compounds in natural systems. Green rusts of the Fe2+(Fe3+,A1)-hydroxycarbonate type have crystal structures resembling that of pyroaurite (Stampfl, 1969; Taylor, 1980). The structures of these compounds consist of Fe 2+ and Fe 3+ and/or A13+ ions octahedrally surrounded by oxygen in brucite-like sheets that are approximately 7.8 A apart, and separated by interlayers containing CO]- and H20. The formulae of such green rusts can, by analogy to that of pyroaurite, be written (Fe3+,A1)2Fe2+(OH)L6CO3.4H20. The Fe2+/Fe 3+ ratio may, however, vary somewhat, and can be as low as 2:1 (Stampfl, 1969; Allmann, 1970; Taylor, 1973). Carbonate-containing green rust has been identified as a corrosion product in municipa