Redetermination of metarossite, CaV25+O6 center dot 2H(2)O

Abstract

The crystal structure of metarossite, ideally CaV2O6 center dot 2H(2)O [chemical name: calcium divanadium(V) hexaoxide dihydrate], was first determined using precession photographs, with fixed isotropic displacement parameters and without locating the positions of the H atoms, leading to a reliability factor R = 0.11 [Kelsey & Barnes (1960). Can. Mineral. 6, 448- 466]. This communication reports a structure redetermination of this mineral on the basis of single- crystal X- ray diffraction data of a natural sample from the Blue Cap mine, San Juan County, Utah, USA (R1 = 0.036). Our study not only confirms the structural topology reported in the previous study, but also makes possible the refinement of all non- H atoms with anisotropic displacement parameters and all H atoms located. The metarossite structure is characterized by chains of edge- sharing [CaO8] polyhedra parallel to [100] that are themselves connected by chains of alternating [VO5] trigonal bipyramids parallel to [010]. The two H2O molecules are bonded to Ca. Analysis of the displacement parameters show that the [VO5] chains librate around [010]. In addition, we measured the Raman spectrum of metarossite and compared it with IR and Raman data previously reported. Moreover, heating of metarossite led to a loss of water, which results in a transformation to the brannerite- type structure, CaV2O6, implying a possible dehydration pathway for the compounds M2+V2O6 center dot xH(2)O, with M = Cu, Cd, Mg or Mn, and x = 2 or 4.