THE CRYSTAL STRUCTURE OF BARICITE, (Mg1.70Fe1.30)(PO4)2{middle dot}8H2O, THE MAGNESIUM-DOMINANT MEMBER OF THE VIVIANITE GROUP

Abstract

The crystal structure of baricite, the Mg-dominant analogue of vivianite, (Mg 1.70 Fe 2+ 1.30− x Fe 3+ x )(PO 4 ) 2 (OH) x ·(8− x )H 2 O, a 10.085(2), b 13.390(3), c 4.6713(9) A, β 104.96(3)°, V 609.4(1) A 3 , space group C 2/ m , Z = 2, ρ calc = 2.440 g/cm 3 , has been determined [automated single-crystal diffractometer, Mo K α, graphite monochromator, image-plate area-detector system, T = 193 K, 5714 reflections, w R 2 = 0.106 for all 1244 unique reflections, R = 0.043 for 965 observed reflections with I ≤ 2σ( 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 ( M 1) is mainly occupied by Fe, whereas Mg prefers the position at the two-fold axis ( M 2); Fe atoms occupy two-thirds of the M 1 and one-third of M 2 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 M 1O 2 (H 2 O) 4 octahedra and clusters of two edge-sharing polyhedra M 2O 6 (H 2 O) 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.