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Hydrokenomicrolite, (□,H 2 O) 2 Ta 2 (O,OH) 6 (H 2 O) or ideally □ 2 Ta 2 [O 4 (OH) 2 ](H 2 O), 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/cm 3 . The mineral is isotropic, n calc  = 2.055. The infrared spectrum contains bands of O-H stretching vibrations and H-O-H bending vibrations of H 2 O molecules. The chemical composition (n = 3) is [by wavelength-dispersive spectroscopy (WDS), H 2 O calculated from crystal-structure analysis, wt%]: CaO 0.12, MnO 0.27, SrO 4.88, BaO 8.63, PbO 0.52, La 2 O 3  0.52, Ce 2 O 3  0.49, Nd 2 O 3  0.55, Bi 2 O 3  0.57, UO 2  4.54, TiO 2  0.18, SnO 2  2.60, Nb 2 O 5  2.18, Ta 2 O 5  66.33, SiO 2  0.46, Cs 2 O 0.67, H 2 O 4.84, total 98.35. The empirical formula, based on 2 cations at the B site, is [□ 0.71 (H 2 O) 0.48 Ba 0.33 Sr 0.27 U 0.10 Mn 0.02 Nd 0.02 Ce 0.02 La 0.02 Ca 0.01  Bi 0.01 Pb 0.01 ] Σ2.00  (Ta 1.75 Nb 0.10 Sn 0.10 Si 0.04 Ti 0.01 ) Σ2.00 [(O 5.77 (OH) 0.23 ] Σ6.00 [(H 2 O) 0.97 Cs 0.03 ] Σ1.00 . The strongest eight X-ray powderdiffraction 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 (R 1  = 0.0363) gave the following data: cubic, Fd3̄m, a = 10.454(1) Å, V = 1142.5(2) Å 3 , 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 H 2 O molecules, confirm the presence of H 2 O at the A(16d) site and displaced from the Y(8b) to the Y′(32e) position. The mineral is characterized by H 2 O dominance at the Y site, vacancy dominance at the A site, and Ta dominance at the B site.

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