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Crystal structure determination of karibibite, an Fe3+ arsenite, using electron diffraction tomography

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The crystal structure of karibibite, Fe3+ 3(As3+O2)4(As3+ 2O5)(OH), from the Urucum mine (Minas Gerais, Brazil), was solved and refined from electron diffraction tomography data [R 1 = 18.8% for F > 4σ(F)] and further confirmed by synchrotron X-ray diffraction and density functional theory (DFT) calculations. The mineral is orthorhombic, space group Pnma and unit-cell parameters (synchrotron X-ray diffraction) are a = 7.2558(3), b = 27.992(1), c = 6.5243 (3) Å, V = 1325.10(8) Å3, Z = 4. The crystal structure of karibibbite consists of bands of Fe3+O6 octahedra running along a framed by two chains of AsO3 trigonal pyramids at each side, and along c by As2O5 dimers above and below. Each band is composed of ribbons of three edgesharing Fe3+O6 octahedra, apex-connected with other ribbons in order to form a kinked band running along a. The atoms As(2) and As(3), each showing trigonal pyramidal coordination by O, share the O(4) atom to form a dimer. In turn, dimers are connected by the O(3) atoms, defining a zig-zag chain of overall (As3+O2) –n n stoichiometry. Each ribbon of (Fe3+O6) octahedra is flanked on both edges by the (As3+O2) –n n chains. The simultaneous presence of arsenite chains and dimers is previously unknown in compounds with As3+. The lone-electron pairs (4s2) of the As(2) and As(3) atoms project into the interlayer located at y = 0 and y = ½, yielding probable weak interactions with the O atoms of the facing (AsO2) chain.

The DFT calculations show that the Fe atoms have maximum spin polarization, consistent with the Fe3+ state.
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Document Type: Research Article

Publication date: 01 October 2017

This article was made available online on 23 October 2016 as a Fast Track article with title: "Crystal structure determination of karibibite, an FE3+ arsenite, using electron diffraction tomography".

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