Mossbauer and XAS study of a green rust mineral; the partial substitution of Fe2+ by Mg2+

Layered double hydroxysalt green rusts, GRs, are very reactive compounds an d their general formula, [Fe-(1-x)(2+) Fe-x(3+) (OH)(2)](x+) [x/n A(n-).m H 2O](x-), where x is the ratio Fe3+/Fe-tot, reflects the structure in which brucite-like layers alternate with interlayers of anions A(n-) and water mo lecules. A GR mineral was extracted from hydromorphic soils in Fougeres (Fr ance) and studied by X-ray absorption spectroscopy (XAS) and transmission M ossbauer spectroscopy (TMS). The XAS spectrum at the Fe K absorption edge o f this mineral proved to be very similar to that of synthetic GRs. However, the radial distribution function obtained for the GR mineral proved to be intermediate between those of GR(CO32-) and pyroaurite, that is between the Fe2+-Fe3+ and Mg2+-Fe3+ hydroxycarbonates. Consequently, a partial substit ution of Fe2+ by Mg2+ occurs, leading to the general formula of [Fe(1-x)2+M gy2+Fex3+ (OH)((2+2y))](x+) [x/n A(n-).m H2O](x-) where A(n-) is the interl ayer anion. Unfortunately, the XAS spectra of various GR proved to be indep endent of the interlayer anion, and the nature of the anions present in the mineral GR could not be determined. The Mossbauer spectrum of the mineral, measured at 77 K, is composed of four quadrupole doublets: D, and D, due t o Fe2+ [delta congruent to 1.26 mm/s and DeltaE(Q) congruent to 2.5 and 2.9 mm/s, respectively] and D-3 and D-4 due to Fe3+ [delta congruent to 0.46 m m/s and DeltaE(Q) congruent to 0.5 and 1.0 mm/s, respectively]. Finally, sy nthetic Mg2+-Fe2+-Fe3+ hydroxycarbonates could be prepared by coprecipitati on from Mg and Fe salts and lead to Mossbauer spectra similar to that of th e mineral. In particular, the partial substitution of Fe2+ by Mg2+ proved t o be consistent with the existence of the unusual doubler D-4 .