IMA 2014-019 = bavsiite

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Marco E. Ciriotti
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IMA 2014-019 = bavsiite

Messaggio da Marco E. Ciriotti » gio 12 set, 2019 10:46

Prossima pubblicazione.

▪ Bojar, H.-P., Walter, F., Baumgartner, J. (2019): Bavsiite, Ba2V2O2[Si4O12], mineral data and crystal structure. Mineralogical Magazine, 83, (in press).

Bavsiite 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, barite, celsian, diopside, fresnoite, mica, suzukiite, walstromite, witherite and minerals of the cerchiarite - group. Bavsiite is optical uniaxial (+), ω = 1.725(3) and ε = 1.750(3) (589 nm) and pleochroic. Electron microprobe analyses yielded the empirical formula Na0.02Ba1.98Ti0.16Fe2+0.03V4+1.80Al0.05Si4.00O14 based on 14 oxygens, the simplified chemical formula is Ba2V2Si4O14. Bavsiite is tetragonal, space group I4/m, a = 7.043(1) Å, c = 11.444(2) Å, 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) Å, c = 11.470(1) Å. The eight strongest lines of the powder X-ray diffraction pattern are [d, Å (I,%) (hkl)] are 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), 1.763 (25) (040). The crystal structure was refined to R = 0.0159 based upon 312 unique reflections with I>2(I). The crystal structure of bavsiite comprises unbranched single [Si4O12]8- rings connected by [VO5]6- square pyramids and BaO12 polyhedra. It can also be considered as cage-like [Si4V2O18]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]. Bavsiite is polymorphic to suzukiite, BaVSi2O7, which is orthorhombic.
Marco E. Ciriotti

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