ALSAKHAROVITE-Zn, NaSrKZn(Ti,Nb)_4[Si_4O_(12)]_2(O,OH)_4 centre dot 7H_2O, A NEW MINERAL OF THE LABUNTSOVITE GROUP FROM LOVOZERO MASS IF KOLA PENINSULA

摘要

Alsakharovite-Zn, a new mineral of the labuntsovite group, 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, natrolite, halloysite, rancieite, kuzmenkoite-Zn, tsepinite-Na, vinogradovite, etc. It occurs as coarse flattened-prismatic crystals up to 8 X 2 X 0.5 mm in size. Translucent to transparent; white, pale brown, colorless. Streak white, luster vitreous. Brittle, cleavage not observed, fracture uneven. Mohs' hardness approx 5. D_(meas) 2.90, D_(calc) 2.93 g/cm~3. Optically biaxial (+), alpha 1.680, beta 1.687, gamma 1.785, 2V (meas) 25(10) deg. Chemical composition (electron probe, H_2O by TGA), wt. percent: Na_2O 2.04, K_2O 2.43, CaO 1.48, SrO 4.49, BaO 3.65, MgO 0.04, MnO 0.11, FeO 0.22, ZnO 5.02, Al_2O_3 0.15, SiO_2 39.23, TiO_2 18.89, Nb_2O_5 12.57, H_2O 11.1; Total 101.42. The empirical formula calculated on (Si,Al)_8O_(24)(O,OH)_8 is: (Na_(0.68)Ca_(0.32))_(SIGMA 1.00)(Sr_(0.53)Na_(0.12))_(SIGMA 0.65)(K_(0.63)Ba_(0.29))_(SIGMA 0.92) (Zn_(0.75)Fe_(0.04)Mn_(0.02)Mg_(0.01))_(SIGMA 0.82)(Ti_(2.88)Nb_(1.15))_(SIGMA 4.03)(Si_(7.96)Al_(0.04))_(SIGMA 8)O_(24)[O_(2.58)(OH)_(1.42)]_(SIGMA 4) centre dot 6.80H_2O. Simplified formula - NaSrKZn(Ti,Nb)_4[Si_4O_(12)]_2(O,OH)_4 centre dot 7H_2O (Z = 2). Crystal structure was studied, R = 0.058. Monoclinic, Cm. Observed microtwinning on planes (001) and (-401). Unit cell dimensions: a = 14.495(10), b = 13.945(10), c = 7.838(8) A, beta = 117.75(7) deg, V = 1402(3) A~3. Representative of the gutkovaite structural type. Newly for labuntsovite-group minerals, cation site A2 was found; mainly occupied by Sr. The strongest lines of X-ray powder pattern (d. A-I[hkl]) are: 6.96-7[020, 001]; 3.22-80[402, 421,400]; 3.11-90[041, 022,240]; 2.60-35[151, 241,202], 2.50-40[441, 403], 1.74-30[080, 004], 1.70-40[463, 444,461,442]. IR spectrum is given. Alsakha-rovite-Zn is named in the memory of Russian geologist Aleksey S. Sakharov (1910-1996) who studied Lovozero massif, with consideration of Zn prevailing in D-site. Type specimen is deposited in Fersman Mineralogical Museum, Russian Academy of Sciences, Moscow.