Synthetic solid solutions formed between goethite and diaspore

Abstract

The substitutional solid solutions of goethite (a‐FeOOH) and diaspore (α‐AlOOH) have been investigated by X‐ray powder diffraction and chemical analysis in order to establish the solubility limits of aluminium in the goethite structure. α‐Fe1‐xAIxOOH samples were obtained by ageing of coprecipitated amorphous (Fe,Al) hydroxides at pH 13 in NaOH and Na[Al(OH)4] solutions at 25°C (G25 series) and 70°C (G70 series), respectively. Chemical analysis reveals that the aluminium content of Al goethites is consistent with Al concentration in the coprecipitated hydroxides up to about 10 mol% Al. In Al goethites obtained from (Fe,Al) hydroxides with higher Al:Fe ratios, the relative Al content is considerably lower. It was found that pure ferrihydrite stored in the aluminate solution is also transformed into Al goethite. The X‐ray diffraction results show a significant divergence of the unitcell parameters from Vegard's law. This points out the limited ranges of the solid solution between goethite and diaspore and suggests the formation of defect, cation‐deficient structures.

Diffuse Reflectance Spectra of Al Substituted Goethite: A Ligand Field Approach

“…A similar relation between 10 Dq and x could be established for the nearly complete solid-solution selies of Cr~AI2_~O 3 (Reinen, 1969), confirming the earlier prediction of Orgel (1957) that the introduction of the larger Cr ion would lead to site compression around Cr rather than to relaxation of the host lattice. It is well known that the smaller ionic radius of AI(III) (53.5 pm as compared to 64.5 pm of Fe(IIl); Shannon, 1976) reduces the lattice of Al-substituted goethite (Schulze, 1984;Wolska and Schwertmann, 1993;Schwertmann and Carlson, 1994). Accordingly, the UCLs (in nm) along the crystallographic axes a, b and c decrease: UCLb and UCL C follow closely the corresponding Vegard lines (hypothetical straight lines between the UCLs of unsubstituted goethite and those of diaspore), The larger deviation between the observed UCLa and the Vegard line was explained by an average increase of the H-bond lengths due to structural disorder (Schulze and Schwertmann, 1984).…”

Coprecipitation in the barite isostructural family: 1. binary mixing properties

“…A class of common and important coprecipitation reactions involves iron and manganese oxyhydroxides (Cornell and Schwertmann, 1996;Hem and Lind, 1991). Laboratory experiments show that Al (Shulze and Schwertmann, 1984;Wolska and Schwertmann, 1993), Cr (Manceau et al, 1992;Schwertmann et al, 1989), Mn, Ni, and Co (Cornell, 1991;Stiers and Schwertmann, 1985), UO 2 2ϩ (Bruno et al, 1995;Duff et al, 2002), and Cu 2ϩ , Zn, and Cd (Gerth, 1990) can substitute for Fe 3ϩ in goethite, Cr 3ϩ and Fe 3ϩ coprecipitate to form an amorphous solid solution (Amonette and Rai, 1990;Sass and Rai, 1987). Jarosite solid solutions are found to precipitate in acid mine drainage environments (Alpers et al, 1989).…”

Distinguishing Between Surface and Bulk Dehydration-Dehydroxylation Reactions in Synthetic Goethites by High-Resolution Thermogravimetric Analysis

“…Possible deviations for pure goethites include excess structural OH and Fe cation deficiencies (Wolska and Schwertmann, 1993). In addition, structural defects such as stacking faults or intergrowths were also identified (Schwertmann, 1984a;Mann et al, 1985;Carnell and Giovanoli, 1986).…”