BELKOVITE - A NEW BARIUM - NIOBIUM SILICATE FROM CARBONATITES OF THE VUORIYARVI MASSIF (KOLA-PENINSULA, USSR)статья

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[1] Belkovite - a new barium - niobium silicate from carbonatites of the vuoriyarvi massif (kola-peninsula, ussr) / A. VOLOSHIN, V. SUBBOTIN, Y. PAKHOMOVSKII et al. // NEUES JAHRBUCH FUR MINERALOGIE-MONATSHEFTE. — 1991. — no. 1. — P. 23–31. Belkovite, Ba3(Nb,Ti)6(Si2O7)2O12, is a new mineral associated with magnetite, pyrochlore, phlogopite, chlorite, pyrite, pyrrothite, apatite, barite, alstonite and nenadkevichite in dolomite-calcite carbonatites of the Vuoriyarvi massif (Kola Peninsula, USSR). It is hexagonal, P62m, with a 8.966(3), c 7.799(3) angstrom and Z = 1. The strongest lines in the X-ray powder diffraction pattern are (angstrom): 7.81 (35)(001,010), 3.888 (51)(002,111,020), 3.481 (24)(012,021), 2.937 (100)(112,120), 2.750 (25)(022,121), 2.154 (22)(023,032,221,130), 1.948 (26)(123,222,040). Belkovite occurres as brown crystals which are 0.5-1.0 mm and show the forms {112BAR0} and {0001}. There is no cleavage; the refractive indexes are n(o) 1.928(2), n(e) 2.002(5) and the observed and calculated densities are 4.16(3) and 4.25 g/cm3, respectively. DTA shows endothermic effect at 790-degrees due to phase transformation. IR-spectrum are absorption bands (the strongest bands are underlined): 1160, 1020, 930, 760, 557, 522, 392, 340 and 260 cm-1. The crystal structure (R = 0.040) is identical to that of synthetic Ba3Si4Nb6O26 and Ba3Si4Ta6O26. (Nb,Ti)-octahedra and (Si2O7) diorthogroups (the Si-O-Si angle is 180-degrees) form a mixed framework. The Ba-cations are in a channel with pentagonal section. The name is after I. V. BELKOV (1917-1989) the soviet mineralogist.

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