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 annab ergite from Lavrion, Attika, were refined using single crystal X-ray d iffraction data [space group C2/m, Z = 2; a = 10.251(3)Angstrom, b = 1 3.447(4)Angstrom, c = 4.764(1)Angstrom, beta = 104.98(1)degrees, V = 6 34.4 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 A ngstrom(3) (annabergite); X-ray data measured up to 2 theta = 65, 70 d egrees; 1199, 1434 unique reflections; R = 0.027, 0.015]. Both mineral s 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 in terconnected by hydrogen bonds only. Chemical compositions of the inve stigated 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 t he Me(1) and Me(2) sites in erythrite and annabergite is discussed bas ed on Mossbauer spectroscopy and site occupancy refinements, respectiv ely. The refinement of the Ni/Mg-ratio in annabergite leads to the for mula 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 a n analogous preference of Fe2+ on the Co(2) site. These findings are f urther confirmed by crystal chemical considerations.