ORGANOVAITE-Zn,K_2Zn(Nb,Ti)_4(Si_4O_(12))_2(O,OH)_4 centre dot 6H_2O, A NEW MINERAL OF THE LABUNTSOVITE GROUP

摘要

Organovaite-Zn a new mineral of the labuntsovite group has been found in large alkaline pegmatite at Karnasurt mount, Lovozero massif, Kola peninsula, Russia. It occurs as coarse prismatic crystals up to 4 X 0.5 X 0.5 mm and massive pseudomorphs after vuonnemite up to 3 X 2 X 0.2 cm, associates with natrolite, microcline, albite, aegirine, organovaite-Mn, beryllite, rancieite, elpidite, komarovite, catapleiite, yofortierite, mangan-neptunite, nontronite, etc. Formed under hydrothermal conditions; a source of Nb is vuonnemite, a source of Zn is sphalerite. Translucent; colour pink, pinkish-brown, white; streak white; lustre vitreous. Brittle, cleavage not observed, fracture rough. Mohs' hardness approx 5. Measured density is 2.88, calculated - 2.89 g/cm~3. Optically biaxial, positive; alpha = 1.683, beta = 1.688, gamma = 1.785; 2V_(means) = 45(15) deg. Beta = b. Chemical composition (electron probe, H_2O content determined thermogravimetrically), wt percent: Na_2O 0.67, K_2O 3.77, CaO 1.14, SrO 0.19, BaO 2.24, MgO 0.10, MnO 0.70, FeO 0.24, ZnO 5.39, Al_2O_3 0.45, SiO_2 39.27, TiO_2 10.49, Nb_2O_5 26.35, H_2O 10.20, total 101.20. Empirical formula calculated on [(Si,Al)_(16)O_(48)](O,OH)_8 is: (K_(1.93)Na_(0.52)Ca_(0.49)Ba_(0.35)Sr_(0.04))_(SIGMA3.33)(Zn_(1.60)Mn_(0.24)Fe_(0.08)Mg_(0.06))_(SIGMA1.98)(Nb_(4.79)Ti_(3.17))_(SIGMA7.96) [Si_(15.79)Al_(0.21)O_(48)][O_(4.59)(OH)_(3.41)]_(SIGMA8) centre dot 11.97H_2O. Simplified formula is: K_2Zn(Nb,Ti)_4(Si_4O_(12))_2(O,OH)_4 centre dot 6H_2O (Z = 4). Crystal structure is studied; R = 0.063. Monoclinic, space group C2/m. unit cell parameters: a = 14.535, b = 13.927, c = 15.655 A, beta = 117.6 deg, V = 2808 A~3. Isostructural with organovaite-Mn. Strongest reflexes of X-ray powder pattern (d, A-I[hkl]): 6.96-100[020, 002]; 6.43-24[200, 20-2]; 4.924-30[022]; 3.222-84[42-2, 400, 40-4]; 3.114-66[042, 024]; 2.514-30[44-2, 402]. IR spectrum is given. Organovaite-Zn is named honoring Russian crystallographter N.I. Organova and for Zn prevailing in D-site. Type specimen is deposited in Fersman Mineralogical Museum of Russian Academy of Sciences, Moscow.