• journal_title：American Mineralogist
• Contributor：Marcelo B. Andrade ; Daniel Atencio ; Nikita V. Chukanov ; Javier Ellena
• Publisher：Mineralogical Society of America
• Date：2013-02-01
• Format：text/html
• Language：en
• Identifier：10.2138/am.2013.4186
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：98
• issue：2-3
• firstpage：292
• section：Article

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, n_calc = 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)_0.48Ba_0.33Sr_0.27U_0.10Mn_0.02Nd_0.02Ce_0.02La_0.02Ca_0.01 Bi_0.01Pb_0.01]_Σ2.00 (Ta_1.75Nb_0.10Sn_0.10Si_0.04Ti_0.01)_Σ2.00[(O_5.77(OH)_0.23]_Σ6.00[(H₂O)_0.97Cs_0.03]_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, Fd m, a = 10.454(1) Å, V = 1142.5(2) ų, 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.