Redetermination of metarossite, CaV⁵⁺₂O₆·2H₂O

The crystal structure of metarossite, ideally CaV₂O₆·2H₂O [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 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 mol-ecules 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·xH₂O, with M = Cu, Cd, Mg or Mn, and x = 2 or 4.