Geochimica et Cosmochimica Acta, 01 September 2014, Vol.140, pp.708-719
Amorphous ferric arsenate (AFA, FeAsO · H O) 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 FeO octahedra ( ∼3.6 Å) to which arsenate is attached as a monodentate binuclear C complex (‘chain model’). Here, we analyzed the structure of AFA and analogously prepared amorphous ferric phosphates (AFP, FePO · H O) by a combination of high-energy total X-ray scattering, Fe K-edge X-ray absorption spectroscopy, and Fe 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 ∼3.6 Å indicative of single corner-sharing FeO octahedra, which strongly supports a local scorodite (FeAsO ·2H O) structure. Likewise, the PDFs and Fe K-edge extended X-ray absorption fine structure data of AFP were consistent with a local strengite (FePO ·2H O) structure of isolated FeO octahedra being corner-linked to PO tetrahedra ( = 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.
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