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

Author(s)

Andrade, M. B. ^[1] ; Atencio, D. ^[2] ; Persiano, A. I. C. ^[3] ; Ellena, J. ^[4]

Affiliation

^[1] Univ Arizona, Dept Geosci, Tucson, AZ 85721 - USA

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

^[3] Univ Fed Minas Gerais, Dept Fis, BR-30123970 Belo Horizonte, MG - Brazil

^[4] Univ Sao Paulo, Inst Fis Sao Carlos, Dept Fis & Informat, BR-13560970 Sao Carlos, SP - Brazil

• Document type: Journal article
• Source: MINERALOGICAL MAGAZINE; v. 77, n. 7, p. 2989-2996, OCT 2013.
• Web of Science Citations: 7

Abstract

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. (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: 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