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Potassic-magnesio-arfvedsonite, KNa2(MgFe2+Fe3+)5Si8O22(OH)2: mineral description and crystal chemistry

Published online by Cambridge University Press:  02 July 2018

Momchil Dyulgerov*
Affiliation:
Faculty of Geology and Geography, Sofia University ‘St. Kliment Ohridski’, Tsar Osvoboditel Blvd. 15, Sofia 1504, Bulgaria
Roberta Oberti
Affiliation:
CNR-Istituto di Geoscienze e Georisorse, UOS Pavia, via Ferrata 1, I-27100 Pavia, Italy
Bernard Platevoet
Affiliation:
Universite Paris-Sud, Bat. 504, Orsay 91405, France
Milen Kadiyski
Affiliation:
Institute for Mineralogy and Crystallography, Bulgarian Academy of Science, Georgi Bonchev Str., bl. 107, Sofia 1113, Bulgaria
Ventzislav Rusanov
Affiliation:
Faculty of Physics, Sofia University ‘St. Kliment Ohridski’ James Bourchier Blvd 5, Sofia 1164, Bulgaria
*
*Author for correspondence: Momchil Dyulgerov, Email: [email protected]

Abstract

The complete mineral description of potassic-magnesio-arfvedsonite, a recently approved (IMA2016-083) new species of the amphibole supergroup is provided using electron microprobe analysis (EMPA), laser ablation inductively coupled plasma mass spectrometry, single-crystal structure refinement, Mössbauer and Raman spectroscopy, as well as measurement of optical and physical properties. The holotype material was found in syenitic and granitic dyke rocks in association with quartz, potassium feldspar and aegirine–augite from the Buhovo–Seslavtsi pluton, Bulgaria. Potassic-magnesio-arfvedsonite is monoclinic C2/m, with unit-cell parameters: a = 9.9804(11), b = 18.0127(19), c = 5.2971(6) Å, β = 104.341(2)° and V = 922.61 Å3. In transmitted plane-polarised light (λ = 590 cm–1), potassic-magnesio-arfvedsonite is pleochroic: X = yellow pale-green, Y = green and Z = dark-violet brown. It is biaxial (–), α = 1.645(2), β = 1.655(2), γ = 1.660(2) and 2Vmeas. = 60° and 2Vcalc. = 70°. The empirical unit formula obtained from EMPA and structure refinement is A(K0.86Na0.0.08)0.94B(Na1.74Ca0.25 Mn2+0.01)2.00C(Mg2.67Fe2+1.42Fe3+0.76Ti0.12Mn2+0.03)5.00TSi8O22W(OH1.58F0.22O0.20)2.00. The Fe3+/Fetot ratio (0.35) is consistent with both the Mössbauer spectra and the single-crystal structure refinement. The 10 strongest X-ray powder reflections [d values (in A°), I, (hkl)] are: 8.519, 80.5, (110); 3.402, 67.3, (131); 3.295, 41.0, (240); 3.173, 65.0, (310); 2.752, 35.6, ($\bar{3}$31); 2.715, 100.0 (151); 2.591, 44.1, (061); 2.542, 73.2, ($\bar{2}$02); 2.348, 38.5, ($\bar{3}$51); 2.174, 42.0, (261). Potassic-magnesio-arfvedsonite is the product of strongly peralkaline and potassic (perpotassic) magma compositions. Trace-element analysis shows that this amphibole did not exert significant control on trace-element distribution in the crystallising peralkaline magma.

Type
Article
Copyright
Copyright © Mineralogical Society of Great Britain and Ireland 2018 

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Footnotes

Associate Editor: Oleg I Siidra

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