Structure and crystal chemistry of vivianite-type compounds: Crystal structures of erythrite and annabergite with a Mossbauer study of erythrite

The crystal structures of erythrite from Schneeberg, Saxony, and annabergite from Lavrion, Attika, were refined using single crystal X-ray diffraction data [space group C2/m, Z = 2; a = 10.251(3)Angstrom, b = 13.447(4)Angstrom, c = 4.764(1)Angstrom, beta = 104.98(1)degrees, V = 634.4 Angstrom(3) (erythrite) and a = 10.179(2) Angstrom, b = 13.309(3) Angstrom, c = 4.725(1) Angstrom, beta = 105.00(1)degrees, V = 618.2 Angstrom(3) (annabergite); X-ray data measured up to 2 theta = 65, 70 degrees; 1199, 1434 unique reflections; R = 0.027, 0.015]. Both minerals crystallize in the vivianite structure type, formed by Me(1)O-2(H2O)(4) octahedra and Me(2)(2)O-6(H2O)(4) double octahedral groups, which are linked via XO(4) tetrahedra to complex sheets in (010), further interconnected by hydrogen bonds only. Chemical compositions of the investigated samples, obtained from EDX- and microprobe analyses, are Co2.01Fe0.74Ni0.25(AsO4)(2) . 8H(2)O for erythrite and Ni2.48Mg0.50Fe0.02(AsO4)(2) . 8H(2)O for annabergite. The Me(2+) cation distribution on the Me(1) and Me(2) sites in erythrite and annabergite is discussed based on Mossbauer spectroscopy and site occupancy refinements, respectively. The refinement of the Ni/Mg-ratio in annabergite leads to the formula Ni2.64Mg0.36(AsO4)(2) . 8H(2)O and yields a strong preference of Mg2+ for the Ni(2) site. Mossbauer spectroscopy of erythrite reveals an analogous preference of Fe2+ on the Co(2) site. These findings are further confirmed by crystal chemical considerations.