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

Author(s)

Andrade, Marcelo B. ^[1] ; Atencio, Daniel ^[2] ; Chukanov, Nikita V. ^[3] ; Ellena, Javier ^[1]

Affiliation

^[1] Univ Sao Paulo, Inst Fis Sao Carlos, Dept Fis & Informat, BR-13560970 Sao Carlos, SP - Brazil

^[2] Univ Sao Paulo, Inst Geociencias, Dept Mineral & Geotecton, BR-05508080 Sao Paulo - Brazil

^[3] Russian Acad Sci, Inst Problems Chem Phys, Moscow 142432 - Russia

• Document type: Journal article
• Source: AMERICAN MINERALOGIST; v. 98, n. 2-3, p. 292-296, FEB-MAR 2013.
• Web of Science Citations: 8

Abstract

Hydrokenomicrolite, (square,H2O)(2)Ta-2(O,OH)(6)(H2O) or ideally square Ta-2(2){[}O-4(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 41/2-5, 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 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 {[}square(0.71)(H2O)(0.48)Ba0.33Sr0.27U0.10Mn0.02Nd0.02Ce0.02La0.02Ca0.0 1 Bi0.01Pb0.01](Sigma 2.00) (Ta1.75Nb0.10Sn0.10Si0.04Ti0.001)(Sigma 2.00){[}(O-5.77(OH)(Sigma 6.00){[}(H2O)(0.97)Cs-0.03](Sigma 1.00). The strongest eight X-ray powder-diffraction lines {[}din angstrom(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, Fd((3) over bar)m, a = 10.454(1) angstrom, V = 1142.5(2) angstrom(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 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. (AU)

• FAPESP's process: 11/22407-0 - New and rare Brazilian mineral species characterization
• Grantee: Daniel Atencio
• Support Opportunities: Regular Research Grants
• FAPESP's process: 09/09125-5 - New minerals from Brazil: crystal chemistry characterization and synthesis
• Grantee: Daniel Atencio
• Support Opportunities: Regular Research Grants
• FAPESP's process: 08/04984-7 - Determination and refinement of crystal structure of pyrochlore group minerals
• Grantee: Marcelo Barbosa de Andrade
• Support Opportunities: Scholarships in Brazil - Post-Doctoral