The crystal structure of baricite, (Mg1.70Fe1.30)(PO4)(2)center dot 8H2O, the magnesium-dominant member of the vivianite group

Abstract

The crystal structure of baricite, the Mg-dominant analogue of vivianite, (Mg1.70Fe1.30-x2+Fex3+)(PO4)(2)(OH)(x). (8-x)H2O, a 10.085(2), b 13.390(3), c 4.6713(9) Angstrom, beta 104.96(3)degrees, V 609.4(1) Angstrom (3), space group C2/m, Z = 2, rho (calc) = 2.440 g/cm(3), has been determined [automated single-crystal diffractometer, MoK alpha, graphite monochromator, image-plate area-detector system, T = 193 K, 5714 reflections, wR(2) = 0. 106 for all 1244 unique reflections, R = 0.043 for 965 observed reflections with I less than or equal to 2 sigma (I)]. A refinement of site occupancies shows that Fe and Mg are partially ordered in the structure. According to X-ray and Mossbauer data, the position at the center of symmetry (M1) is mainly occupied by Fe, whereas Mg prefers the position at the two-fold axis (An); Fe atoms occupy two-thirds of the M1 and one-third of M2 octahedra. The positions of four independent H atoms were obtained from difference-Fourier syntheses and were refined under isotropic approximation. The crystal structure of baricite consists of two type of octahedral structural units: isolated M1O(2)(H2O)(4) octahedra and clusters of two edge-sharing polyhedra M2O6(H2O)(4). These octahedral units are connected in the structure by orthophosphate tetrahedra to form layers two octahedra thick parallel to the ac plane. In the b direction, these layers are held together by hydrogen bonds. The occurrence in nature of an oxidized form, a triclinic symplesite-like phase, is considered to be highly probable.