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

Published online by Cambridge University Press:  05 July 2018

M. B. Andrade ,
D. Atencio ,
A. I. C. Persiano  and
J. Ellena
M. B. Andrade
Affiliation:
Department of Geosciences, University of Arizona, 1040 East 4th Street, Tucson, Arizona 85721, USA
D. Atencio*
Affiliation:
Departamento de Mineralogia e Geotectônica, Instituto de Geociências, Universidade de São Paulo, Rua do Lago, 562, 05508-080 São Paulo, SP, Brazil
A. I. C. Persiano
Affiliation:
Departamento de Física, Universidade Federal de Minas Gerais, Av. Presidente Antonio Carlos, 6627, 30123-970, Belo Horizonte, MG, Brazil
J. Ellena
Affiliation:
Departamento de Física e Informática, Instituto de Física de São Carlos, Universidade de São Paulo, Caixa Postal 369, 13560-970 São Carlos, SP, Brazil
*
* E-mail: datencio@usp.br
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Abstract

Fluorcalciomicrolite, (Ca, Na, ☐)2Ta2O6F, 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 4½–5, tenacity is brittle. Cleavage is not observed; fracture is conchoidal. The calculated density is 6.160 g/cm3. The mineral is isotropic, ncalc. = 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, SnO20.63, UO20.02, Nb2O53.47, Ta2O576.02, 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☐0.12)Σ2.00(Ta1.84Nb0.14Sn0.02)Σ2.00[O5.93(OH)0.07]6.00[F0.79(OH)0.21]. The strongest eight X-ray powder-diffraction lines [d in Å (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 (R1 = 0.0132) gave the following data: cubic, Fdm, a = 10.4191(6) Å, V = 1131.07(11) Å3, Z = 8.

Keywords

fluorcalciomicrolitenew mineralVolta Grande pegmatiteNazarenoMinas GeraisBrazilpyrochlore supergroupmicrolite groupcrystal structure
Type
Research Article
Information
Mineralogical Magazine , Volume 77 , Issue 7 , October 2013 , pp. 2989 - 2996
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2013

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