Rakovanite, Na3{H3[V10O 28]}·15H2O, a new member of the pascoite family with a protonated decavanadate polyanion

Abstract

Rakovanite, Na3{H3[V10O 28]}·15H2O, is a new mineral species from the Sunday and the West Sunday mines, Slick Rock district, San Miguel County, Colorado, USA; the mineral is the natural analog of the previously synthesized phase. Rakovanite is orange, with an orange-yellow streak. Crystals of rakovanite are up to 1 mm in maximum dimension and vary in habit from bločky to prismatic on [100]. The mineral is transparent, with a subadamantine luster; it does not fluoresce in long- or short-wave ultraviolet radiation. Rakovanite displays brittle tenacity, and has a Mohs hardness of 1. No cleavage or parting was observed, and the mineral displays a conchoidal fracture. The density calculated from the ideal formula and the single-crystal cell data is 2.407 g cm-3. Rakovanite is biaxial (+), with α589nm = 1.776(5), β589nm = 1.803(5), γ589nm = 1.910(6). The measured values of 2V are: 2Vz 540nm= 58(1); 2Vz589nm = 56(1); 2Vz650nm = 53(1)°. Dispersion is r Y > X. Its chemical composition was obtained by electron-probe microanalysis and the crystal-structure refinement; the empirical formula (V = 10 apfu) is (Na 2.90K0.07Ca0.01Al0.01)Σ2. 99{H2.98[V10O28]}· 15H2O, and the simplified formula is Na3{H3[V10O 28]}· 15H2O. Rakovanite is monoclinic, space group P21/n, with a 12.0248(17), b 17.121(3), c 18.140(3) Å, β 106.242(8)°, and Z = 4; the strongest four lines in the powder-diffraction pattern [d in Å(I)(hkl)] are: 11.270(100)(101), 7.696(81)(021), 8.709(78)(002), and 6.892(63)( 120). The atomic arrangement of rakovanite has been refined to R1 = 0.0383; the structural unit in the phase is formed of a triply-protonated decavanadate polyanion, {H3[V 10O28]}. The structural units are linked by the interstitial complexes, the (Na3·15H2O) contents of which balance the +3 charges of the decavanadate polyanion with a NaO 4(H2O)2 group, a Na2(H 2O)10dimer, and three H2O molecules that do not bond to the interstitial cations.