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Appears in Collections:Biological and Environmental Sciences Journal Articles
Peer Review Status: Refereed
Title: Total X-ray Scattering, EXAFS, and Mössbauer Spectroscopy Analyses of Amorphous Ferric Arsenate and Amorphous Ferric Phosphate
Authors: Mikutta, Christian
Schröder, Christian
Michel, F Marc
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Keywords: amorphous ferric arsenate
amorphous ferric phosphate
mine waste
Mössbauer spectroscopy
total X-ray scattering
X-ray absorption spectroscopy
Issue Date: Sep-2014
Publisher: Elsevier
Citation: Mikutta C, Schröder C & Michel FM (2014) Total X-ray Scattering, EXAFS, and Mössbauer Spectroscopy Analyses of Amorphous Ferric Arsenate and Amorphous Ferric Phosphate, Geochimica et Cosmochimica Acta, 140, pp. 708-719.
Abstract: Amorphous ferric arsenate (AFA, FeAsO 4•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.
Type: Journal Article
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Rights: Because the funder, Swiss National Science Foundation has a policy that articles must be archived in an Open Access archive, the publisher, Elsevier, does not allow this work to be made publicly available in this Repository. Please use the Request a Copy feature at the foot of the Repository record to request a copy directly from the author. You can only request a copy if you wish to use this work for your own research or private study.
Affiliation: ETH Zurich
Biological and Environmental Sciences
Virginia Polytechnic Institute And State University

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