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Oskarssonite, AlF3, a new fumarolic mineral from Eldfell volcano, Heimaey, Iceland

Published online by Cambridge University Press:  05 July 2018

M. J. Jacobsen ,
T. Balić -Žunić ,
D. Mitolo ,
A. Katerinopoulou ,
A. Garavelli  and
S. P. Jakobsson
M. J. Jacobsen
Affiliation:
Natural History Museum of Denmark, University of Copenhagen, Øster Voldgade 5-7, DK-1350, Copenhagen K, Denmark
T. Balić -Žunić*
Affiliation:
Natural History Museum of Denmark, University of Copenhagen, Øster Voldgade 5-7, DK-1350, Copenhagen K, Denmark
D. Mitolo
Affiliation:
Dipartimento di Scienze della Terra e Geoambientali, Università di Bari, via E. Orabona 4, I-70125 Bari, Italy
A. Katerinopoulou
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, Università di Bari, via E. Orabona 4, I-70125 Bari, Italy
S. P. Jakobsson
Affiliation:
Icelandic Institute of Natural History, Urridaholtsstraeti 6-8, IS-212 Gardabaer, Iceland
*
*E-mail: toncib@snm.ku.dk
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Abstract

The new mineral oskarssonite (IMA2012-088), with ideal formula AlF3, was found in August 2009 at the surface of fumaroles on the Eldfell volcano, Heimaey Island, Iceland (GPS coordinates 63°25′58.9″N 20°14′50.3″W). It occurs as sub-micron-sized crystals forming a white powder in association with anhydrite, bassanite, gypsum, jarosite, anatase, hematite, opal, ralstonite, jakobssonite and meniaylovite. Chemical analyses by energy-dispersive spectrometry with a scanning electronmicroscope produced the following mean elemental composition: Al, 31.70; F, 58.41; O, 9.22; total 99.33 wt.%. The empirical chemical formula is AlF2.6(OH)0.5 which suggests partial substitution of F by OH. Oskarssonite is rhombohedral, space group Rc, with ah = 4.9817(4) Å, c = 12.387(1) Å, Vuc = 266.23(5) Å3, Z = 6. The five strongest lines in the powder diffraction diagram [d in Å(I) (hkl)] are as follows: 3.54 (100) (012), 2.131 (13) (113), 1.771 (20) (024), 1.59 (15) (116), 1.574 (10) (122). Rietveld refinement confirms the identity of oskarssonite with the synthetic rhombohedral form of AlF3. Its structure can be described as a rhombohedral deformation of the idealized cubic perovskitetype octahedral framework of corner-sharing AlF6 groups. Oskarssonite appears in the surface part of the fumaroles where fluorides are abundant. At greater depths (below 10 cm) sulfates dominate among the fumarolic minerals. In accordance with its occurrence, we surmise that oskarssonite forms in the later stages of the fumarolic activity in an environment poor in alkalies and Mg. Ralstonite (NaxMgxAl1−xF3(H2O)y), which, unlike oskarssonite, contains Na and Mg as important constituents, dominated in the first-formed fumaroles, but now, 41 years after the eruption of Eldfell, is only a minor phase. The new mineral is named after the Icelandic volcanologist Niels Oskarsson.

Keywords

oskarssonitenew mineralfumarolessublimatesfluorideEldfellIceland
Type
Research Article
Information
Mineralogical Magazine , Volume 78 , Issue 1 , February 2014 , pp. 215 - 222
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2014

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Footnotes

†

present address: FL Smidth R & D Center, Daniavej 49, Assens, Denmark

‡

present address: Haldor Topsøe A/S, Nymøllevej 55, DK-2800 Lyngby, Denmark

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