Bavsiite, Ba2V2O2[Si4O12], mineral data and crystal structure

Baysiite from the type locality Gun Claim, Yukon Territory, Canada, occurs as millimetre-sized sky-blue platy crystals in a barium-rich low-temperature skarn related to a porphyritic quartz monzonite stock. Associated minerals are alstonite, baryte, celsian, diopside, fresnoite, mica, suzukiite, walstromite, witherite and minerals of the cerchiaraite group. Baysiite is optical uniaxial (+), with omega = 1.725(3) and is an element of = 1.750(3) (589 nm) and pleochroic. Electron microprobe analyses yielded the empirical formula Na0.02Ba1.98Ti0.16Fe0.032+V1.804+Al0.05Si4.00O14 based on 14 oxygen atoms, the simplified chemical formula is Ba2V2Si4O14. Baysiite is tetragonal, space group I4/m, a = 7.043(1), c= 11.444(2) angstrom and Z= 2 obtained from single crystal data at 100 K, which are in good agreement with cell parameters from powder diffraction data at 293 K: a= 7.051(1) A and c= 11.470(1) angstrom. The eight strongest lines of the powder X-ray diffraction pattern are [d, angstrom (I,%)(hkl)]: 3.76(30)(112), 3.36(44)(013), 3.004(100)(022), 2.493(43)(220), 2.486(67)(114), 2.286(24)(222), 1.785(39) (116) and 1.763(25)(040). The crystal structure was refined to R= 0.0159 based upon 312 unique reflections with I > 2 sigma(I). The crystal structure of baysiite comprises unbranched single [Si4O12](8-) rings connected by [VO5](6-) square pyramids and BaO12 polyhedra. It can also be considered as cage-like [Si4V12O18](12-) clusters built by four SiO4 tetrahedra and two VO5 square pyramids. These clusters are cross-linked to form a pseudo-two-dimensional network (2D) parallel to (001) containing empty channels along the a axis and the 2D networks held together by Ba2+ ions located in channels parallel to the c axis. The structural formula is Ba2V2O2 [Si4O12]. Baysiite is polymorphic to suzukiite, BaVSi2O7, which is orthorhombic.