| Popis |
Oxybismutomicrolite, ideally [(Bi3+,#)(2)]Sigma 4+Ta2O6O, where # = subordinate substituents, such as Na+, Ca2+ and vacancy OD, is a microlitegroup, pyrochlore-supergroup mineral discovered at the Solnechnaya ('Sunny') pegmatite vein, Malkhan pegmatite field, Zabaykalskiy Kray, Central Transbaikalia, Russia. It forms rough octahedral crystals up to 1 mm across and equant grains up to 2 mm across embedded in an albite-lepidolite-elbaite complex. Other associated minerals are Bi-rich fluornatromicrolite, bismutotantalite and stibiotantalite. The new mineral is black, with resinous lustre; the streak is greyish white. It is non-fluorescent under ultraviolet light. Oxybismutomicrolite is brittle, with Mohs' hardness of similar to 5. Cleavage is not observed, fracture is uneven. D-meas. = 6.98(2) g/cm(3) and D-calc. = 7.056 g/cm(3). The mineral is optically isotropic. The mean refractive index calculated from the Gladstone-Dale equation is 2.184. The infrared spectrum shows the absence of H2O molecules and OH groups. The chemical composition is (electron microprobe, wt.%): Na2O 3.45, CaO 2.88, MnO 0.31, PbO 0.76, Bi2O3 29.81, ThO2 0.18, TiO2 3.89, SnO2 1.77, Nb2O5 4.50, Ta2O5 51.08, F 1.17, O = F -0.49, total 99.31. The empirical formula, on the basis of 2 cations at the B site, is (Bi0.79Na0.68Ca0.32Mn0.03Pb0.02 square(0.)(16))Sigma(2.00)(Ta1.42Ti0.30Nb0.23Sn0.07)(Sigma 2.00) O-6.00(O0.52F0.38 square(0.10))(Sigma 1.00). The crystal structure refinement (R = 0.019) gave the following data: cubic, Fd (3) over barm, a = 10.4746(11) angstrom, V = 1149.2(4) angstrom(3) and Z= 8. The eight strongest lines of the powder X-ray diffraction pattern [d, angstrom(I, %)(hkl)] are: 6.051(12)(111), 3.160(10)(311), 3.026(100)(222), 2.621(32)(400), 1.854(33)(440), 1.581(27)(622), 1.514(7)(444) and 1.203(7)(662). Type material is deposited in the collections of the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow, Russia, registration number 5409/1.
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