Redetermination of metarossite, CaVO·2HO.

The crystal structure of metarossite, ideally CaVO·2HO [chemical name: calcium divanadium(V) hexa-oxide 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 = 0.11 [Kelsey & Barnes (1960 ▸). , 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 (1 = 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 [CaO] polyhedra parallel to [100] that are themselves connected by chains of alternating [VO] trigonal bipyramids parallel to [010]. The two HO mol-ecules are bonded to Ca. Analysis of the displacement parameters show that the [VO] 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, CaVO, implying a possible dehydration pathway for the compounds VO·HO, with = Cu, Cd, Mg or Mn, and = 2 or 4.