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Magnesio-riebeckite from the Varenche mine (Aosta Valley, Italy): crystal-chemical characterization of a grandfathered end-member

Published online by Cambridge University Press:  26 January 2018

Roberta Oberti ,
Massimo Boiocchi ,
Frank C. Hawthorne  and
Marco E. Ciriotti
Roberta Oberti*
Affiliation:
CNR-Istituto di Geoscienze e Georisorse, Sede secondaria di Pavia, via Ferrata 1, I-27100 Pavia, Italy
Massimo Boiocchi
Affiliation:
Centro Grandi Strumenti, Università di Pavia, via Bassi 21, I-27100 Pavia, Italy
Frank C. Hawthorne
Affiliation:
Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, R3T 2N2, Canada
Marco E. Ciriotti
Affiliation:
Associazione Micromineralogica Italiana, via San Pietro 55, I-10073 Devesi-Cirié, Italy
*
* E-mail: oberti@crystal.unipv.it
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Abstract

Magnesio-riebeckite from the dumps of the abandoned mine of Varenche (45°47’22’’ N, 7°29’17’’ E), Saint-Barthélemy, Nus, Aosta Valley (Italy), was studied to provide the complete mineral description (including crystal structure) and insights into the crystal-chemistry of riebeckite. The empirical formula derived from electron microprobe analysis and single-crystal structure refinement is A(Na0.09K0.01)Σ=0.10B(Na1.77Ca0.11Mg0.08Mn2+ 0:04)Σ=2.00C(Mg2.93Mn2+0:13Fe2+0:07Zn0.01Ni0.12Fe3+1:25Al0.48Ti0.01)Σ=5.00T(Si7.92Al0.08)Σ=8.00 O22W(OH1.88F0.12)Σ=2.00. Magnesio-riebeckite is biaxial (+), with α = 1.678(2), β = 1.682(2), γ = 1.688(2) and 2V (meas.) = 80.2(1.7)°, 2V (calc.) = 78.7°. The unit-cell parameters are a = 9.6481(14), b = 17.873(3), c = 5.3013(7) Å, β = 103.630(2)°, V = 888.4 (2)Å3, Z = 2, space group C2/m. The strongest ten reflections in the powder X-ray pattern [d values (in Å), I, (hkl)] are: 2.701, 100, (151); 8.303, 83, (110); 3.079, 62, (310); 3.391, 53, (131); 4.467, 50, (040,021); 2.522, 50, (̅202); 2.578, 35, (061); 2.155, 30, (261), 4.855, 30, (̅111), 2.300, 29, (̅351).

Keywords

magnesio-riebeckiteelectron microprobe analysisoptical propertiespowder-diffraction patterncrystal-structure refinementVarenche mineItaly
Type
Research Article
Information
Mineralogical Magazine , Volume 81 , Issue 6 , December 2017 , pp. 1431 - 1437
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2017

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