Structure and crystal chemistry of vivianite-type compounds: Crystal structures of erythrite and annabergite with a Mössbauer study of erythrite
Wildner, Manfred; Giester, Gerald; Lengauer, Christian L.; Mccammon, Catherine A.
European Journal of Mineralogy Volume 8 Number 1 (1996), p. 187 - 192
published: Feb 22, 1996
manuscript accepted: Oct 23, 1995
manuscript received: Jun 29, 1995
ArtNo. ESP147050801018, Price: 29.00 €
Abstract 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) Å, b = 13.447(4) Å, c = 4.764(1) Å, ß = 104.98(1)°, V = 634.4 Å3 (erythrite) and a = 10.179(2) Å, b = 13.309(3) Å, c = 4.725(1) Å, ß = 105.00(1)°, V = 618.2 Å3 (annabergite); X-ray data measured up to 26 = 65, 70°; 1199, 1434 unique reflections; R = 0.027, 0.015]. Both minerals crystallize in the vivianite structure type, formed by Me(1)O2(H2O)4 octahedra and Me(2)2O6(H2O)4 double octahedral groups, which are linked via XO4 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- 8H2O for erythrite and Ni2.48Mg0.50Fe0.02(AsO4)2- 8H2O for annabergite. The Me2+ cation distribution on the Me(l) 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- 8H2O 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.