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Fluorcalcioroméite, (Ca,Na)2Sb25+(O,OH)6F, a new roméite-group mineral from Starlera mine, Ferrera, Grischun, Switzerland: description and crystal structure

Published online by Cambridge University Press:  05 July 2018

D. Atencio ,
M. E. Ciriotti  and
M. B. Andrade
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
M. E. Ciriotti
Affiliation:
Associazione Micromineralogica Italiana, via San Pietro 55, I-10073 Devesi-Ciriè, Italy
M. B. Andrade
Affiliation:
Department of Geosciences, University of Arizona, 1040 East 4th Street, Tucson, Arizona 85721, USA
*
* E-mail: datencio@usp.br
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Abstract

Fluorcalcioroméite, (Ca,Na)2Sb25+(O,OH)6F, is a new roméite-group, pyrochlore-supergroup mineral (IMA 2012-093), from Starlera mine, Ferrera, Hinterrhein district, Grischun, Switzerland. The intimately associated minerals are: braunite, hematite, calcite, quartz and, rarely, wallkilldellite-(Mn). It occurs as euhedral octahedra, untwinned, from 0.1 to 1 mm in size. The crystals are yellow to orange and translucent; the streak is white, and the lustre is vitreous to resinous. It is non-fluorescent under ultraviolet light. Mohs' hardness is ∼5½, tenacity is brittle. Cleavage is not observed; fracture is conchoidal. The calculated density is 5.113 g/cm3. The mineral is isotropic, ncalc. = 1.826. The Raman spectrum is dominated by bands of Sb–O octahedral bond stretching and O–Sb–O bending modes. The chemical composition (n = 13) is (by wavelength-dispersive spectroscopy (WDS), H2O calculated by difference, wt.%): Na2O 4.11, CaO 15.41, MnO 0.54, CuO 0.01, ZnO 0.01, PbO 0.02, Al2O3 0.10, FeO 0.50, Y2O3 0.07, SiO2 0.04, TiO2 0.01, UO2 0.01, Sb2O5 76.18, WO3 0.78, F 2.79, H2O 0.59, O = F–1.17, total 100.00. The empirical formula, based on 2 cations at the B site, is (Ca1.16Na0.56☐0.22Fe0.032+ Mn0.032+)Σ2.00(Sb5+1.98Al0.01W0.01)Σ2.00O6[F0.62(OH)0.28O0.06☐0.04]Σ1.00. The strongest eight X-ray powder-diffraction lines [d in Å (I)(hkl)] are: 5.934(81)(111), 3.102(20)(311), 2.969(100)(222), 2.572(6)(400), 1.979(7)(333), 1.818(8)(440), 1.551(15)(622), and 1.484(5)(444). The crystal structure refinement (R1 = 0.0106) gave the following data: cubic, Fdm, a = 10.2987(8) Å, V = 1092.31(15) Å3, Z = 8. Unit-cell parameters refined from the powder data are: a = 10.284(2), V = 1087.7(7) Å3, Z = 8.

Keywords

fluorcalcioroméitenew mineralStarlera mineFerreraGrischunSwitzerlandpyrochlore supergrouproméite groupcrystal structure
Type
Research Article
Information
Mineralogical Magazine , Volume 77 , Issue 4 , June 2013 , pp. 467 - 473
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2013

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