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Magnesio-hornblende from Lüderitz, Namibia: mineral description and crystal chemistry

Published online by Cambridge University Press:  16 May 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-hornblende (IMA2017-059) has been characterized in a specimen collected in the sand dunes of Lüderitz, Karas Region, Namibia. The empirical formula derived from electron microprobe analysis and single-crystal structure refinement is A(□0.73Na0.22K0.05)Σ1.00B(Ca1.79Fe2+0.10Mg0.04Mn2+0.03Na0.04)Σ2.00C(Mg3.48Fe2+0.97Al0.28Fe3+0.23Cr3+0.01Ti0.03)Σ5.00T(Si7.18Al0.82)Σ8.00O22W[(OH)1.93F0.05Cl0.02]Σ2.00. Magnesio-hornblende is biaxial (–), with α = 1.640(2), β = 1.654(2), γ = 1.666(2) (measured with gel-filtered Na light, λ = 589.9 nm), 2V (meas.) = 82(1)° and 2V (calc.) = 84.9°. The unit-cell parameters are a = 9.8308(7), b = 18.0659(11), c = 5.2968(4) Å, β = 104.771(6)° and V = 909.64 (11) Å3 with Z = 2 and space group C2/m. The strongest eight reflections in the X-ray powder pattern [d values (in Å), I, (hkl)] are: 2.709, 100, (151); 8.412, 74, (110); 3.121, 73, (310); 2.541, 58, ($\bar{2}$02); 3.386, 49, (131); 2.596, 45, (061); 2.338, 41, ($\bar{3}$51); and 2.164, 39, (261).

Keywords

magnesio-hornblendeelectron-microprobe analysisoptical propertiespowder-diffraction patterncrystal-structure refinementLüderitzNamibia
Type
Article
Information
Mineralogical Magazine , Volume 82 , Issue 6 , December 2018 , pp. 1253 - 1259
Copyright
Copyright © Mineralogical Society of Great Britain and Ireland 2019 

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Footnotes

Associate Editor: Anthony Kampf

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