Total X-ray scattering, EXAFS, and Mössbauer spectroscopy analyses of amorphous ferric arsenate and amorphous ferric phosphate

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 ∼3.6 Å) to which arsenate is attached as a monodentate binuclear 2C complex ('chain model'). Here, we analyzed the structure of AFA and analogously prepared amorphous ferric phosphates (AFP, 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 AFA and AFP is about 8 Å. The PDFs of AFA lacked Fe-Fe pair correlations at r ∼3.6 Å indicative of single corner-sharing FeO6 octahedra, which strongly supports a local scorodite (FeAsO4·2H2O) structure. Likewise, the PDFs and Fe K-edge extended X-ray absorption fine structure data of AFP were consistent with a local strengite (FePO4·2H2O) structure of isolated FeO6 octahedra being corner-linked to PO4 tetrahedra (rFe-P = 3.25(1) Å). Mössbauer spectroscopy analyses of AFA and AFP indicated a strong superparamagnetism. While AFA only showed a weak onset of magnetic hyperfine splitting at 5 K, magnetic ordering of AFP 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.