Geochimica et Cosmochimica Acta, Sept 1, 2014, Vol.140, p.708(12)
To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.gca.2014.05.040 Byline: Christian Mikutta, Christian Schroder, F. Marc Michel Abstract: Amorphous ferric arsenate (AFA, FeAsO.sub.4*xH.sub.2O) 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.sub.6 octahedra (r.sub.Fe-Fe [approximately equal to]3.6A) to which arsenate is attached as a monodentate binuclear.sup.2C complex ('chain model'). Here, we analyzed the structure of AFA and analogously prepared amorphous ferric phosphates (AFP, FePO.sub.4*xH.sub.2O) by a combination of high-energy total X-ray scattering, Fe K-edge X-ray absorption spectroscopy, and.sup.57Fe Mossbauer 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 8A. The PDFs of AFA lacked Fe-Fe pair correlations at r [approximately equal to]3.6A indicative of single corner-sharing FeO.sub.6 octahedra, which strongly supports a local scorodite (FeAsO.sub.4*2H.sub.2O) structure. Likewise, the PDFs and Fe K-edge extended X-ray absorption fine structure data of AFP were consistent with a local strengite (FePO.sub.4*2H.sub.2O) structure of isolated FeO.sub.6 octahedra being corner-linked to PO.sub.4 tetrahedra (r.sub.Fe-P =3.25(1)A). Mossbauer spectroscopy analyses of AFA and AFP indicated a strong superparamagnetism. While AFA only showed a weak onset of magnetic hyperfine splitting at 5K, magnetic ordering of AFP was completely absent at this temperature. Mossbauer 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. Article History: Received 17 January 2014; Accepted 24 May 2014 Article Note: (miscellaneous) Associate editor: Jean Francois Boily
Phosphates -- Analysis ; Spectroscopy -- Analysis ; Iron Compounds -- Analysis
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