Fluorcalciomicrolite, (Ca,Na,□)2Ta2O6F, a new microlite-group mineral from Volta Grande pegmatite, Nazareno, Minas Gerais, Brazil

Fluorcalciomicrolite, (Ca,Na,square)(2)Ta2O6F, is a new microlite-group, pyrochlore supergroup mineral approved by the CNMNC (IMA 2012-036). It occurs as an accessory mineral in the Volta Grande pegmatite, Nazareno, Minas Gerais, Brazil. Associated minerals include: microcline, albite, quartz, muscovite, spodumene, "lepidolite'', cassiterite, tantalite-(Mn), monazite-(Ce), fluorite, "apatite'', beryl, "garnet'', epidote, magnetite, gahnite, zircon, "tourmaline'', bityite, hydrokenomicrolite, and other microlite-group minerals under study. Fluorcalciomicrolite occurs as euhedral, untwinned, octahedral crystals 0.1 1.5 mm in size, occasionally modified by rhombododecahedral faces. The crystals are colourless and translucent; the streak is white, and the lustre is adamantine to resinous. It does not fluoresce under ultraviolet light. Mohs' hardness is 41/2-5, tenacity is brittle. Cleavage is not observed; fracture is conchoidal. The calculated density is 6.160 g/cm(3). The mineral is isotropic, n(calc). - 1.992. The Raman spectrum is dominated by bands of B-X octahedral bond stretching and X B-X bending modes. The chemical composition (n = 6) is (by wavelength dispersive spectroscopy, H2O calculated to obtain charge balance, wt.%): Na2O 4.68, CaO 11.24, MnO 0.01, SrO 0.04, BaO 0.02, SnO2 0.63, UO2 0.02, Nb2O5 3.47, Ta2O5 76.02, F 2.80, H2O 0.48, O=F -1.18, total 98.23. The empirical formula, based on 2 cations at the B site, is (Ca1.07Na0.81 square(0.12))(Sigma 2.00)(Ta1.84Nb0.14Sn0.02)(Sigma 2.00) [O-5.93(OH)(0.07)](6.00)[F-0.79(OH)(0.21)]. The strongest eight X-ray powder-diffraction lines [d in angstrom(I)(hkl)] are: 5.997(59)(111), 3.138(83)(311), 3.005(100)(222), 2.602(29)(400), 2.004(23)(511), 1.841(23)(440), 1.589(25)(533), and 1.504(24)(444). The crystal structure refinement (R-1 = 0.0132) gave the following data: cubic, Fd (3) over barm, a = 10.4191(6) angstrom, V = 1131.07(11) angstrom(3), Z = 8.