RAKOVANITE, Na3 {middle dot}15H2O, A NEW MEMBER OF THE PASCOITE FAMILY WITH A PROTONATED DECAVANADATE POLYANION

Rakovanite, Na 3 {H 3 [V 10 O 28 ]}·15H 2 O, 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 blocky 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 α 589nm = 1.776(5), β 589nm = 1.803(5), γ 589nm = 1.910(6). The measured values of 2 V are: 2 Vz 540nm = 58(1); 2 Vz 589nm = 56(1); 2 Vz 650nm = 53(1)°. Dispersion is r v , strong, parallel; orientation: X = b ; Z c = 8 ° in obtuse β. The pleochroic scheme in rakovanite is: X light yellow, Y orangish yellow, Z yellowish orange, Z > Y > X . Its chemical composition was obtained by electron-probe microanalysis and the crystal-structure refinement; the empirical formula (V = 10 apfu ) is (Na 2.90 K 0.07 Ca 0.01 Al 0.01 ) ∑2.99 {H 2.98 [V 10 O 28 ]}·15H 2 O, and the simplified formula is Na 3 {H 3 [V 10 O 28 ]}·15H 2 O. Rakovanite is monoclinic, space group P 2 1 / n , with a 12.0248(17), b 17.121(3), c 18.140(3) A, β 106.242(8)°, and Z = 4; the strongest four lines in the powder-diffraction pattern [ d in A(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 R 1 = 0.0383; the structural unit in the phase is formed of a triply-protonated decavanadate polyanion, {H 3 [V 10 O 28 ]}. The structural units are linked by the interstitial complexes, the (Na 3 ·15H 2 O) contents of which balance the +3 charges of the decavanadate polyanion with a NaO 4 (H 2 O) 2 group, a Na 2 (H 2 O) 10 dimer, and three H 2 O molecules that do not bond to the interstitial cations.