Total X-ray Scattering, EXAFS, and Mössbauer Spectroscopy Analyses of Amorphous Ferric Arsenate and Amorphous Ferric Phosphate

Abstract

Amorphous ferric arsenate (AFA, FeAsO4•xH2O) is an important As precipitate in a range of oxic As-rich environments, especially acidic sulfide-bearing mine wastes. Its structure has been proposed to consist of small polymers of single corner-sharing FeO6 octahedra (rFe-Fe ~3.6Å) to which arsenate is attached as a monodentate binuclear 2C complex (‘chain model’). Here, we analyzed the structure of AFAs and analogously prepared amorphous ferric phosphates (AFPs, FePO4•xH2O) by a combination of high-energy total X-ray scattering, Fe K-edge X-ray absorption spectroscopy, and 57Fe Mössbauer spectroscopy. Pair distribution function (PDF) analysis of total X-ray scattering data revealed that the coherently scattering domain size of AFAs and AFPs is about 8Å. The PDFs of AFA lacked Fe-Fe pair correlations at r ~3.6Å indicative of single-corner sharing FeO6 octahedra, which strongly supports a local scorodite (FeAsO4•2H2O) structure. Likewise, PDF analyses as well as wavelet-transform and shell-fit analyses of Fe K-edge extended X-ray absorption fine structure data of the AFPs suggest a local strengite (FePO4•2H2O) structure with isolated FeO6 octahedra being corner-linked to PO4 tetrahedra (rFe-P = 3.25(1)Å). Mössbauer spectroscopy analyses of AFAs and AFPs indicated a strong superparamagnetism. While the AFAs only showed a weak onset of magnetic hyperfine splitting at 5 K, magnetic ordering of the AFPs was completely absent at this temperature. Mössbauer spectroscopy may thus offer a convenient way to identify and quantify AFA and AFP in mineral mixtures containing poorly crystalline Fe(III)-oxyhydroxides. In summary, our results imply a close structural relationship between AFA and AFP and suggest that these amorphous materials serve as templates for the formation of scorodite and strengite (phosphosiderite) in strongly acidic low-temperature environments.