Redetermination of metarossite, CaV(5+) (2)O(6)·2H(2)O

The crystal structure of metarossite, ideally CaV(2)O(6)·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 [CaO(8)] polyhedra parallel to [100] that are themselves connected by chains of alternating [VO(5)] trigonal bipyramids parallel to [010]. The two H(2)O mol­ecules are bonded to Ca. Analysis of the displacement parameters show that the [VO(5)] 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, CaV(2)O(6), implying a possible dehydration pathway for the compounds M (2+)V(2)O(6)·xH(2)O, with M = Cu, Cd, Mg or Mn, and x = 2 or 4.