Noonkanbahite, BaKNaTi2(Si4O12)O2, a new mineral species: description and crystal structure

Noonkanbahite, ideally BaKNaTi 2 (Si 4 O 12 )O 2 , is described as a new mineral species. At Liley [Lohley], Eifel Mountains, Germany (the holotype locality), it occurs as sprays of prismatic crystals (up to 8 mm) or single prismatic crystals (up to 4 mm) on walls of cavities in alkaline igneous rocks. At Murun, Siberia, Russia, noonkanbahite forms coarse lamellar crystals up to 0.05 cm × 0.7 cm × 1.5 cm embedded in kalsilite syenite. Noonkanbahite is brittle, H = 6, D obs. = 3.39(1), D calc. = 3.49 g/cm 3 , has a vitreous lustre and does not fluoresce in ultraviolet light. It has poor cleavage on {010} and {100} and weak parting on {011}. Noonkanbahite is biaxial positive with 2V obs. = 75(2)°, 2V calc. = 72.7(9)°, α 1.730(5), β 1.740(5) and γ 1.765(5), dispersion is medium, r X colourless, Y yellowish, Z straw-yellow; X = a , Y = b , Z = c . Noonkanbahite is orthorhombic, space group Imma , a = 8.0884(4), b = 10.4970(5), c = 13.9372(6) A, V = 1183.3(1) A 3 , Z = 4. The strongest ten X-ray diffraction lines in the powder pattern [ d in A( I )( hkl )] are: 2.907(100)(222), 8.353(50)(001), 3.196(50)(220), 2.097(50)(242), 2.241(40)(215), 2.179(40)(035), 3.377(30)(031), 2.694(30)(015), 2.304(30)(233), and 1.564(30)(064). Electron microprobe analysis gives SiO 2 37.82, TiO 2 15.54, ZrO 2 0.42, Nb 2 O 5 3.18, Al 2 O 3 0.17, Fe 2 O 3 (recalculated from FeO) 5.63, MnO 0.32, MgO 0.53, BaO 20.60, CaO 1.36, K 2 O 5.32, Na 2 O 6.14, F 0.78, H 2 O 0.58, sum 98.39 wt.%, (H 2 O determined by SIMS). The formula unit, calculated on the basis of 14 anions (O+OH+F), is (Ba 0.85 K 0.13 ) Σ0.98 (K 0.59 Na 0.26 Ca 0.15 ) Σ1.00 Na(Ti 1.23 \(\mathrm{Fe}_{0.45}^{3+}\) Nb 0.15 Mg 0.08 Mn 0.03 Zr 0.02 Al 0.01 ) Σ1.97 (Si 3.99 Al 0.01 O 12 )(O 1.33 OH 0.41 F 0.26 ) Σ2.00 , Z = 4. The crystal structure was refined to R 1 = 2.8% for 970 unique ( F 0 > 4σ F ) reflections collected on a Bruker single-crystal P 4 diffractometer with a CCD detector and Mo K α X-radiation. The crystal structure of noonkanbahite is isostructural with that of batisite, ideally BaNa 2 Ti 2 (Si 4 O 12 )O 2 , and scherbakovite, ideally K 2 NaTi 2 (Si 4 O 12 )O(OH). There are two octahedrally coordinated sites, M (1) and M (2), occupied by (Ti 1.23 \(\mathrm{Fe}_{0.45}^{3+}\) Nb 0.15 Mg 0.08 Mn 0.03 Zr 0.02 Al 0.01 ), ideally Ti 2 a.p.f.u. There are three interstitial A sites, [9] A (1), [8] A (2) and [6] A (3), occupied by Ba, K and Na, respectively. Si tetrahedra and M octahedra form a framework with interstitial cages occupied by Ba, K and Na atoms at the A sites. Noonkanbahite, BaKNaTi 2 (Si 4 O 12 )O 2 , is a K analogue of batisite, BaNa 2 Ti 2 (Si 4 O 12 )O 2 , and a Ba analogue of shcherbakovite, K 2 NaTi 2 (Si 4 O 12 )O(OH).