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

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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 mm long. Its calculated density is 2.89 g cm3. 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)S0.84Al1.06F4.09(OH)1.01. Jakobssonite is monoclinic, space group C2/c, with a = 8.601(1), b = 6.2903(6), c = 7.2190(7) Å , b = 114.61(1)o, V = 355.09(8) Å3 and 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) (1-1-2 ), 2.27 (22) (2-2-2 ), 1.957 (21) (400), 1.814 (20) (1-3-2), 1.805 (22) (20-4 ). 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.
Original languageEnglish
JournalMineralogical Magazine
Issue number3
Pages (from-to)751–760
Number of pages10
Publication statusPublished - 2012

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