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Hydroxycalciomicrolite, Ca1.5Ta2O6(OH), a new member of the microlite group from Volta Grande pegmatite, Nazareno, Minas Gerais, Brazil

Published online by Cambridge University Press:  02 January 2018

M. B. Andrade ,
H. Yang ,
D. Atencio ,
R. T. Downs ,
N. V. Chukanov ,
M. H. Lemée-Cailleau ,
A. I. C. Persiano ,
A. E. Goeta  and
J. Ellena
M. B. Andrade*
Affiliation:
Department of Physics and Interdisciplinary Science, São Carlos Institute of Physics, University of São Paulo, Caixa Postal 369, 13560-970 São Carlos, SP, Brazil Department of Geosciences, University of Arizona, 1040 East 4th Street, Tucson, Arizona 85721, USA
H. Yang
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
R. T. Downs
Affiliation:
Department of Geosciences, University of Arizona, 1040 East 4th Street, Tucson, Arizona 85721, USA
N. V. Chukanov
Affiliation:
Institute of Problems of Chemical Physics, Russian Academy of Sciences, Chernogolovka, Moscow Region 142432, Russia
M. H. Lemée-Cailleau
Affiliation:
Institut Laue Langevin (ILL), 6 Rue Jules Horowitz, BP 156 38042, Grenoble, France
A. I. C. Persiano
Affiliation:
Departamento de Física, Universidade Federal de Minas Gerais, Avenida Presidente Antonio Carlos 6627, 30123-970, Belo Horizonte, MG, Brazil
A. E. Goeta
Affiliation:
Department of Chemistry, Durham University, Durham DH1 3LE, UK
J. Ellena
Affiliation:
Department of Physics and Interdisciplinary Science, São Carlos Institute of Physics, University of São Paulo, Caixa Postal 369, 13560-970 São Carlos, SP, Brazil
*
*E-mail: [email protected]
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Abstract

Hydroxycalciomicrolite, Ca1.5Ta2O6(OH) is a new microlite-group mineral found in the Volta Grande pegmatite, Nazareno, Minas Gerais, Brazil. It occurs as isolated octahedral and as a combination of octahedral and rhombic dodecahedral crystals, up to1.5 mm in size. The crystals are yellow and translucent, with a white streak and vitreous to resinous lustre. The mineral is brittle, with a Mohs hardness of 5–6. Cleavage is not observed and fracture is conchoidal. The calculated density is 6.176 g cm–3. Hydroxycalciomicroliteis isotropic, ncalc. = 2.010. The infrared and Raman spectra exhibit bands due to O–H stretching vibrations. The chemical composition determined from electron microprobe analysis (n = 13) is (wt.%): Na2O 0.36(8), CaO 15.64(13), SnO2 0.26(3),Nb2O5 2.82(30), Ta2O5 78.39(22), MnO 0.12(2), F 0.72(12) and H2O 1.30 (from the crystal structure data), O = F –0.30, total 99.31(32), yielding an empirical formula, (Ca1.48Na0.06Mn0.01)∑1.55(Ta1.88Nb0.11Sn0.01)∑2.00O6.00[(OH)0.76F0.20O0.04].Hydroxycalciomicrolite is cubic, with unit-cell parameters a = 10.4205(1) Å, V = 1131.53(2) Å3 and Z = 8. It represents a pyrochlore supergroup, microlite-group mineral exhibiting P4332 symmetry, instead of Fd3m. Thereduction in symmetry is due to long-range ordering of Ca and vacancies on the A sites. This is the first example of such ordering in a natural pyrochlore, although it is known from synthetic compounds. This result is promising because it suggests that other species with P4332or lower-symmetry space group can be discovered and characterized.

Keywords

hydroxycalciomicroliteCa1.5Ta2O6(OH)pyrochlore supergroupmicrolite groupcrystal structureRaman spectrainfrared spectrum
Type
Research Article
Information
Mineralogical Magazine , Volume 81 , Issue 3 , June 2017 , pp. 555 - 564
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2017

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Footnotes

†

Deceased

References

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