Crystal structure determination of karibibite, an FE3+arsenite, using electron diffraction tomography

The crystal structure of karibibite, Fe3,3+(As3+O2)4(As2,3+O5)(OH), from the Urucum mine (Minas Gerais, Brazil), was solved and refined from electron diffraction tomography data [R1 = 18.8 % for F>4σ(F)] and further confirmed by synchrotron X-ray diffraction and density functional calculations (DFT). The mineral is orthorhombic, space group Pnma (#62), and the unit cell parameters (synchrotron X-ray diffraction) are a = 7.2558(3), b = 27.992(1), 26 c = 6.5243 (3) Å, and V = 1325.10(8) Å3, with Z = 8. The crystal structure of karibibbite consists of bands of Fe3+O6octahedra 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 3 edge-sharing Fe3+O6octahedra, apex-connected with other ribbons in order to form a kinked bond 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 stoichiometry. Each ribbon of (Fe3+O6) octahedra described before is flanked in its both edges by the (As3+O2)-n n chains. The simultaneous presence of arsenite chains and dimers was 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 = 1/2, yielding probable weak interactions with the O atoms of the facing (AsO2) chain. DFT calculations show that the Fe atoms show the maximum spin polarization consistent with the Fe+3charge state.