Hydrokenomicrolite, (□,H2O)2Ta2(O,OH)6(H2O), a new microlite-group mineral from Volta Grande pegmatite, Nazareno, Minas Gerais, Brazil

Hydrokenomicrolite, (□,H2O)2Ta2(O,OH)6(H2O) or ideally □2Ta2[O4(OH)2](H2O), is a new microlite-group mineral approved by the CNMNC (IMA 2011-103). It occurs as an accessory mineral in the Volta Grande pegmatite, Nazareno, Minas Gerais, Brazil. Associated minerals are: microcline, albite, quartz, muscovite, spodumene, “lepidolite”, cassiterite, tantalite-(Mn), monazite-(Ce), fluorite, “apatite”, beryl, “garnet”, epidote, magnetite, gahnite, zircon, “tourmaline”, bityite, and other microlite-group minerals under study. Hydrokenomicrolite occurs as euhedral octahedral crystals, occasionally modified by rhombododecahedra, untwinned, from 0.2 to 1.5 mm in size. The crystals are pinkish brown and translucent; the streak is white, and the luster is adamantine to resinous. It is non-fluorescent under ultraviolet light. Mohs hardness is 4½–5, tenacity is brittle. Cleavage is not observed; fracture is conchoidal. The calculated density is 6.666 g/cm3. The mineral is isotropic, ncalc = 2.055. The infrared spectrum contains bands of O-H stretching vibrations and H-O-H bending vibrations of H2O molecules. The chemical composition (n = 3) is [by wavelength-dispersive spectroscopy (WDS), H2O calculated from crystal-structure analysis, wt%]: CaO 0.12, MnO 0.27, SrO 4.88, BaO 8.63, PbO 0.52, La2O3 0.52, Ce2O3 0.49, Nd2O3 0.55, Bi2O3 0.57, UO2 4.54, TiO2 0.18, SnO2 2.60, Nb2O5 2.18, Ta2O5 66.33, SiO2 0.46, Cs2O 0.67, H2O 4.84, total 98.35. The empirical formula, based on 2 cations at the B site, is [□0.71(H2O)0.48Ba0.33Sr0.27U0.10Mn0.02Nd0.02Ce0.02La0.02Ca0.01 Bi0.01Pb0.01]Σ2.00 (Ta1.75Nb0.10Sn0.10Si0.04Ti0.01)Σ2.00[(O5.77(OH)0.23]Σ6.00[(H2O)0.97Cs0.03]Σ1.00. The strongest eight X‑ray powder-diffraction lines [d in Å(I)(hkl)] are: 6.112(86)(111), 3.191(52)(311), 3.052(100)(222), 2.642(28)(400), 2.035(11)(511)(333), 1.869(29)(440), 1.788(10)(531), and 1.594(24)(622). The crystal structure refinement (R1 = 0.0363) gave the following data: cubic, Fd3m, a = 10.454(1) Å, V = 1142.5(2) Å3, Z = 8. The Ta(O,OH)6 octahedra are linked through all vertices. The refinement results and the approximate empirical bond-valences sums for the positions A (1.0 v.u.) and Y′ (0.5 v.u.), compared to valence calculations from electron microprobe analysis (EMPA) and ranges expected for H2O molecules, confirm the presence of H2O at the A(16d) site and displaced from the Y(8b) to the Y′(32e) position. The mineral is characterized by H2O dominance at the Y site, vacancy dominance at the A site, and Ta dominance at the B site.FAPESP (08/04984-7, 09/09125-5, 11/22407-0)CNPqRFBR (11-05-12001-ofi-m-2011)