The crystal structure of sarmientite, Fe₂³⁺ (AsO₄)(SO₄)(OH)·5H₂O, solved ab initio from laboratory powder diffraction data

Abstract

The crystal structure of sarmientite, Fe23+ (AsO4)(SO4)(OH)·5H2O, from the type locality (Santa Elena mine, San Juan Province, Argentina), was solved and refined from in-house powder diffraction data (CuKα1,2 radiation). It is monoclinic, space group P21/n, with unit-cell dimensions a = 6.5298(1), b = 18.5228(4), c = 9.6344(3) Å, β = 97.444(2)º, V = 1155.5(5) Å3, and Z = 4. The structure model was derived from cluster-based Patterson-function direct methods and refined by means of the Rietveld method to Rwp = 0.0733 (X2 = 2.20). The structure consists of pairs of octahedral-tetrahedral (Fe−As) chains at (y,z) = (0,0) and (½,½), running along a. There are two symmetry-independent octahedral Fe sites. The Fe1 octahedra share two corners with the neighbouring arsenate groups. Both individual chains are related by a symmetry centre and joined by two symmetry-related Fe2 octahedra. Each Fe2 octahedron shares three corners with double-chain polyhedra (O3, O4 with arsenate groups; the O8 hydroxyl group with the Fe1 octahedron) and one corner (O11) with the monodentate sulfate group. The coordination of the Fe2 octahedron is completed by two H2O molecules (O9 and O10). There is also a complex network of H bonds that connects polyhedra within and among chains. Raman and infrared spectra show that (SO4)2− tetrahedra are strongly distorted.

Application of Patterson-function direct methods to materials characterization

“…In spite of being relatively new, the cluster-based S-FFT algorithm has already solved some rather difficult unknown crystal structures from conventional laboratory PD data, e.g. those of the highly hydrated minerals sanjuanite, Al 2 (PO 4 )-(SO 4 )(OH)Á9H 2 O, Z = 4, space group P2 1 /n (Colombo et al, 2011), and sarmientite, Fe 2 3+ (AsO 4 )(SO 4 )(OH)Á5H 2 O, Z = 4, space group P2 1 /n, V = 1156 Å 3 (Colombo et al, 2014), or the triclinic crystal structure of a new partially deprotonated mixed-valence manganese(II,III) hydroxide arsenate related to sarkinite (de Pedro et al, 2012). Cluster-based S-FFT has also determined the frameworks of hybrid materials like calcium hydroxyphosphonoacetates (Colodrero et al, 2011), magnesium tetraphosphonate (Colodrero et al, 2012) Perspective view, along the b axis, of the monoclinic unit cell of ibuprofen (Freer et al, 1993), with atomic positions taken directly from the clusterbased DM Fourier map (a = 12.46, b = 8.03, c = 13.53 Å , = 112.95 , V = 1246 Å 3 ).…”