ЛЕПХЕНЕЛЬМИТ-Zn, Ba_2Zn(Ti,Nb)_4[Si_4O_(12)]_2(O,OH)_4 centre dot 7H_2O - НОВЫЙ МИНЕРАЛ ГРУППЫ ЛАБУНЦОВИТА И ЕГО КРИСТАЛЛИЧЕСКАЯ СТРУКТУРА

摘要

Lepkhenelmite-Zn, a new mineral of the labuntsovite group (the kuzmenkoite subgioup), was found in cavities of eudialyte-aegirine-feldspar pegmatite at Lepkhe-Nelm Mt., Lovozero alkaline massif Kola peninsula, Russia. It is a hydrothermal mineral associated with lamprophyllite, natiolne balloysite, rancieite, kuzmenkoite-Zn. tsepinite-Na, paratsepinite-Ba, vinogradovite, apatite, etc Occuis as coarse flattened-prismatic crystals up to 7 X 2.5 x 0.5 mm and crystal clusters up to 1 X 0 5 cm. Transparent. Pale brown. Streak white. Lustre vitreous. Brittle, cleavage not observed, fracture uneven. Mohs' hardness approx5. D(meas.) is 2.96, D(calc.) is 3.07 g/cm~3. Optically biaxial (+), a 1.683, (3 1.692 gamma 1 795, 2V(meas) 25(10) deg, 2V(calc.) 34.5 deg. Chemical composition (electron probe, H_2O by TGA), wt percent: Na_2 0.590, K_2O 1.98, CaO 1.16, SrO 1.79, BaO 11.04, MgO 0.02, MnO 0.81, FeO 0.21, ZnO 3.66, A1_2O_3 0.42 SiO_2 37 01, TiO_2 18.56, Nb_2O_5 10.60, H_2O 11.80, Total 99.65. The empirical formula calculated on (Si,A1)_8O_(24)(O,OH)_8 is: (Ba_(0.92)K_(0.54)Ca_(0.26)Na_(0.24)Sr_(0.22))_(SIGMA 2.18)(Zn_(0.58)Mn_(0.15)Fe_(0.04)Mg_(0.01))_(SIGMA 0.78)(Ti_(2.97)Nb_(1.02))_(SIGMA 3.99)(Si_(7.89)Al_(0.11))_(SIGMA 8)O_(24) [O_(2.01)(OH)_(1.99)]_(SIGMA 4) centre dot 7.39H_2O. Simplified formula is: Ba_2Zn(Ti,Nb)_4[Si_4O_(12)]_2(O,OH) centre dot 7H_2O (ZETA = 2). Lepkhenelmite-Zn forms an isomnorphous series with kuzmenkoite-Zn. Crystal structure was studied, R = 0.099. Monoclinic, Cm. Unit cell dimensions: a = 14.381(3), b= 13.889(3), c = 7.793(2) A, |3 = 117.52(3) deg, V= 1380.4(5) A~3. The strongest lines of X-ray powder pattern (d. A-I[hkl]) are: 6.95 - 37[020, 001], 6.39 - 10[20 - 1, 200], 3.194 - 100[42 - 1, 40 - 2, 400], 3.101 - 22[041, 022], 3.050 - 8[24 - 1, 240], 2.906 - 6[42 - 2, 420], 2.585 - 6[24 - 2, 420]. The IR spectrum is given. Lepkhenelmite-Zn is named for the locality where it was discovered and Zn prevailing in D-site. Type specimen is deposited in Fersman Mineralogieal Museum of Russian Academy of Sciences, Moscow.