Iron-chromium hydroxides are important solid phases governing the aqueous concentrations of Cr(III) in soils and fly ashes. Although direct identification of noncrystalline (Fe,Cr)(OH)3 is difficult, the infrared spectra of noncrystalline Fe(OH)3 and Cr(OH)3, coprecipitated (Fe,Cr)(OH)3, and physical mixtures of Fe(OH)3 and Cr(OH)3 can be distinguished on the basis of the asymmetric stretching doublet (v3) of structural carbonate anions. As the Cr mole fraction of the coprecipitated (Fe,Cr)(OH)3 increases, the position of the low-frequency v3 peak (v3″) changes progressively to higher frequencies, and the carbonate v3 splitting decreases. No change in carbonate v3 splitting or v3″ location was observed for physical mixtures of Fe(OH)3 and Cr(OH)3. The changes in v3 splitting are believed to be caused by different degrees of polarization of the carbonate ligand by the Fe and Cr cations.

Pure Cr(OH)3 exhibits a strong affinity for carbonate and H2O and tends to remain noncrystalline even at very high pHs. In contrast, pure Fe(OH)3 gradually converts to crystalline goethite at high pH, to the exclusion of much of the H2O and carbonate. The presence of Cr in (Fe,Cr)(OH)3 solid solutions seems to inhibit the transformation to crystalline goethite. The strong association of carbonate with Cr and the kinetic inertness of Cr(III) inner-sphere complexes in general may account for the maintenance of noncrystalline solid-solution materials in lieu of transformation to a crystalline end product.