title: Redetermination of metarossite, CaV5+2O6·2H2O.
abstract: 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 factorR= 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 compoundsM2+V2O6·xH2O, withM= Cu, Cd, Mg or Mn, andx= 2 or 4.
authors.affiliation: Département des Sciences de la Terre, École Normale Supérieure de Lyon, Site Monod, 15 parvis René Descartes, BP 7000, 69342 Lyon, France; Département des Sciences de la Terre, Université Claude Bernard Lyon 1, 43 Bd du 11 Novembre 1918, 69622 Villeurbanne Cedex, France.
authors.forename: Anaïs
authors.initials: A
authors.lastname: Kobsch
authors.affiliation: Department of Geosciences, University of Arizona, 1040 E. 4th Street, Tucson, AZ 85721-0077, USA.
authors.forename: Robert T
authors.initials: RT
authors.lastname: Downs
authors.affiliation: Lunar and Planetary Laboratory, University of Arizona, 1629 E. University Blv, Tucson, AZ 85721-0092, USA.
authors.forename: Kenneth J
authors.initials: KJ
authors.lastname: Domanik
keywords: brannerite; crystal structure; hydrogen bonds; metarossite; phase transformation; redetermination