Noonkanbahite, BaKNaTi _2(Si_4O_(12)O_2, a new mineral species:descrp tion and crystal structure

摘要

Noonkanbahite, ideally BaKNaTi_2(Si_4O_(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 x 0.7 cm x 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)°,α1.730(5), β 1.740(5) and γ 1.765(5), dispersion is medium, γ < v. In transmitted plane-polarized light, noonkanbahite is strongly pleochroic, with 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_2O_5 3.18, Al_2O_3 0.17, Fe_2O_3 (recalculated from FeO) 5.63, MnO 0.32, MgO 0.53, BaO 20.60, CaO 1.36, K_2O 5.32, Na_2O 6.14, F 0.78, H_2O 0.58, sum 98.39 wt.%, (H_2O determined by SIMS). The formula unit, calculated on the basis of 14 anions (O+OH+F), is ( Ba_(0.85)K_(0.13))∑_(0.98)(K_(0.59)Na_(0.26)Ca_(0.15))∑_(1.00)Na(Ti_(1.23)FeO_(0.45)~(3+)Nb_(0.15)Mg_(0.08)Mn_(0.03)Zr(0.02)Al_(0.01))∑_(1.97) (Si_(3.99)Al_(0.01)O_(12))(O_(1.33)OH_(0.41)F_(0.26))∑_(2.00), Z = 4.The crystal structure was refined to R_1 = 2.8% for 970 unique (F_o > 4σF) reflections collected on a Bruker single-crystal P4 diffractometer with a CCD detector and MoKa X-radiation. The crystal structure of noonkanbahite is isostructural with that of batisite, ideally BaNa_2Ti_2(Si_4O_(12))O_2, and scherbakovite, ideally K_2NaTi_2(Si_4O_(12))O(OH). There are two octahedrally coordinated sites, M(1) and M(2), occupied by (Ti_(1.23)Fe_(0.45)~(3+)Nb_(0.15)Mg_(0.08)Mn_(0.03)Zr_(0.02)Al_(0.01)), ideally Tie a.p.f.u. There are three interstitial A sites, ~([19])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_4O_(12))O_2, is a K analogue of batisite, BaNa_2Ti_2(Si_4O_(12))O_2, and a Ba analogue of shcherbakovite, K_2NaTi_2(Si_4O_(12))O(OH).