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Jakobssonite, CaAlF5, a new mineral from fumaroles at the Eldfell and Hekla volcanoes, Iceland

Published online by Cambridge University Press:  05 July 2018

T. Balić-Žunić ,
A. Garavelli ,
D. Mitolo ,
P. Acquafredda  and
E. Leonardsen
T. Balić-Žunić*
Affiliation:
Natural History Museum of Denmark, University of Copenhagen, Øster Voldgade 5-7, DK-1350, Copenhagen K, Denmark
A. Garavelli
Affiliation:
Dipartimento di Scienze della Terra e Geoambientali, Universitá di Bari, via E. Orabona 4, I-70125 Bari, Italy
D. Mitolo
Affiliation:
Dipartimento di Scienze della Terra e Geoambientali, Universitá di Bari, via E. Orabona 4, I-70125 Bari, Italy
P. Acquafredda
Affiliation:
Dipartimento di Scienze della Terra e Geoambientali, Universitá di Bari, via E. Orabona 4, I-70125 Bari, Italy
E. Leonardsen
Affiliation:
St Karlsmindevej 46, DK-3390 Hundested, Denmark
*
E-mail: toncib@snm.ku.dk
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Abstract

The new mineral jakobssonite, ideally CaAlF5, was first found in crusts collected in 1988 from a fumarole on the Eldfell volcano, Heimaey Island, Iceland. It was subsequently found in similar crusts collected in 1991 from a fumarole on the Hekla volcano, Iceland. It is associated with leonardsenite (IMA2011-059), ralstonite, heklaite, anhydrite, gypsum, jarosite, hematite, opal and several fluoride minerals that have not been fully characterized. Jakobssonite occurs as soft white fragile crusts of acicular crystals <50 μm long. Its calculated density is 2.89 g cm–3. Chemical analyses by energy-dispersive spectrometry on a scanning electron microscope produced a mean elemental composition as follows: Ca, 18.99; Al, 18.55; Mg, 1.33; Na, 0.33; F, 50.20; O, 10.39; total 99.79 wt.%. The empirical chemical formula, calculated on the basis of 7 atoms per formula unit with all of the oxygen as OH, is (Ca0.73Mg0.09Na0.02)Σ0.84Al1.06F4.09(OH)1.01. Jakobssonite is monoclinic, space group C2/c, with a = 8.601(1), b = 6.2903(6), c = 7.2190(7) Å, β = 114.61(1)o, V = 355.09(8) Å3and Z = 4. The crystal structure contains chains of [AlF6] octahedra which run parallel to the c axis. These chains are interconnected by chains of [CaF7] pentagonal bipyramids. Jakobssonite is isostructural with several other CaMIIIF5 compounds. The eight strongest lines in the powder diffraction diagram [d in Å (I) (hkl)] are as follows: 4.91 (18) (110), 3.92 (76) (200), 3.15 (68) (020), 3.13 (100) (1), 2.27 (22) (2), 1.957 (21) (400), 1.814 (20) (1), 1.805 (22) (20). The chemical and crystal-structure analyses of jakobssonite are similar to synthetic CaAlF5 with minor substitutions of light elements (e.g. Na) or vacancies for Ca, and OH for F.

Keywords

jakobssonitenew mineralfumarolessublimatesfluorideEldfellHeklaIceland
Type
Research Article
Information
Mineralogical Magazine , Volume 76 , Issue 3 , June 2012 , pp. 751 - 760
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2016

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Rietveld Analysis results

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Powder XRD data

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