Tumchaite Na-2(Zr,Sn)Si4O11 center dot 2H(2)O - A new mineral from carbonatites of the Vuoriyarvi alkali-ultrabasic massif, Murmansk Region, Russia

Tumchaite, Na-2(Zr,Sn)Si4O11. 2H(2)O, is a new species from the Vuoriyarvi alkali-ultrabasic massif, Murmansk Region, Russia, where it occurs as color less to while tabular monoclinic crystals associated with calcite, dolomite , a mineral of the serpentine group and pyrite in the late dolomite-calcite carbonatites. It is transparent to translucent; with vitreous luster; and perfect cleavage on (100). Mohs hardness is 4.5, D-meas is 2.78 (2) g/cm(3) . Tumchaite is optically biaxial (-), with alpha = 1.570 (2), beta = 1.588 (2), gamma = 1.594 (2), 2V(meas) = 60 (5)degrees, and elongation positive, Y = b, c boolean AND Z= 3 degrees. Pleochroism exists, with Y = Z = colorle ss, X = greenish-gray. Electron microprobe analysis gave (wt%): Na2O 13.72, CaO 0.15, SiO2 52.71, TiO2 0.35, ZrO2 20.41, SnO2 5.73, HfO2 0.60, H2O (co mputed assuming 2H(2)O pfu.) 7.86, total 101.53. The X-ray study pointed to space group P2(l)/c, a = 9.144 (4), b = 8.818 (3), c = 7.537 (3) Angstrom, beta = 113.22 (3)degrees, V = 558.49 Angstrom(3), Z = 2. The strongest lin es of the powder diffraction pattern [d in Angstrom (I) (hkl)] are: 8.40 (1 0) (100), 5.38 (9) (11 (1) over bar), 4.00 (8) (111), 3.401 (9) (20 (2) ove r bar), 2.902 (9) (211), 2.691 (9) (13 (1) over bar). The crystal structure of tumchaite was refined to R = 0.043 for 865 F-o > 4 sigma(F-o). The mine ral is isotypic with penkvilksite-1M. The structure is characterized by sil icate sheets parallel (100), formed by alternating clockwise- and countercl ockwise-growing spiral chains of corner-sharing SiO2 tetrahedra. The sheets are connected by octahedra occupied by (Zr, Sn) at 0, 1/2, 0. The Zr/Sn ra tio in the octahedra is 4. Water molecules and Na cations are placed in the cavities of the polyhedral framework. The ideal crystal-chemical formula i s Na, (Zr0.8Sn0.2) [Si4O11]2 . H2O. The mineral is named tumchaite for the river Tumcha near Vuoriyarvi massif.