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Ferri-obertiite from the Rothenberg quarry, Eifel volcanic complex, Germany: mineral data and crystal chemistry of a new amphibole end-member

Published online by Cambridge University Press:  02 January 2018

Roberta Oberti ,
Massimo Boiocchi ,
Frank C. Hawthorne ,
Neil A. Ball  and
Günter Blass
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
Neil A. Ball
Affiliation:
Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, R3T 2N2, Canada
Günter Blass
Affiliation:
Merzbachstrasse 6, 52246 Eschweiler-St-Joris, Germany
*
*E-mail: [email protected]
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Abstract

Pink-orange crystals of a composition within the ferri-obertiite compositional space were found in vesicles in a pale beige silicate vein found from a basalt quarry at Mount Rothenberg, Eifel district, Germany. Associated minerals are potassic feldspar, alpha quartz paramorphic afterbeta quartz, eifelite (the second occurrence after the Caspar quarry at Bellerberg volcano, Eifel region), tridymite, rutile, roedderite and other amphiboles. The ideal formula of ferri-obertiite is ANaBNa2C(Mg3Fe3+Ti)TSi8O22WO2; the empirical formula derived for the holotype specimen from Mount Rothenberg from the results of electron-microprobe analysis and single-crystal structure refinement is A(Na0.76K0.22)∑0.98B(Na1.61Ca0.35Mn0.042+)∑2.00C(Mg3.58Mn0.112+Fe0.623+Ti0.664+Cr0.013+Zn0.01Ni0.01)∑5.00T(Si7.82Ti0.124+Al0.06)∑8.00O22W[O1.26F0.55(OH)0.19]∑2.00. The unit-cell dimensions are a = 9.7901(7), b = 17.9354(13), c = 5.2892(4)Å, β= 104.142(2)°, V = 900.58 (11) Å3. The space group is C2/m, Z = 2. Ferri-obertiite is biaxial (+), with α = 1.664, β = 1.680, γ = 1.722, all ±0.002 and 2V (meas.) = 66.4(3)o, 2V (calc.) = 64.7o.The strongest eight reflections in the powder X-ray pattern [d values (in Å), I, (hkl)] are: 2.704, 100, (151); 3.116, 76, (310); 3.388, 72, (131); 8.931, 72, (110); 2.529, 67, (202); 2.583, 39, (061); 2.160, 38, (261); 3.260, 37, (240). Both the mineral and thename have been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA 2015-079); the rock specimen has been deposited at the Museo di Mineralogia, Dipartimento di Scienze della Terra e dell'Ambiente, Universitàdegli Studi di Pavia, under the code 2015-02.

Keywords

ferri-obertiiteamphiboleoxo componentelectron-microprobe analysiscrystal-structure refinementEifel districtGermany
Type
Research Article
Information
Mineralogical Magazine , Volume 81 , Issue 3 , June 2017 , pp. 641 - 651
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2017

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