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

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 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)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) A ˚ , b = 114.61(1)o, V = 355.09(8) A ˚3 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 A ˚ (I) (hkl)] are as follows: 4.91 (18) (110), 3.92 (76) (200), 3.15 (68) (020), 3.13 (100) (11¯2¯ ), 2.27 (22) (22¯2¯ ), 1.957 (21) (400), 1.814 (20) (13¯2¯ ), 1.805 (22) (204¯ ). 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

Eldfellite, NaFe(SO4)2, a new fumarolic mineral from Eldfell volcano, Iceland

A new mineral, eldfellite, was found among fumarolic encrustations collected in 1990 on the Eldfell volcano, Heimaey Island, Iceland. Associated minerals are ralstonite, anhydrite, gypsum, bassanite, hematite, opal and tamarugite, as well as a presumably new mineral with the composition Na3Fe(SO4)3. Along with opal and tamarugite, eldfellite forms soft and fragile aggregates built of thin, platy crystals of micrometre size. The mineral is yellowish-green to greenish-white, with a white streak. The calculated density is 3.062 g/cm3. Eldfellite is monoclinic, C2/m, a 8.043(4) A ˚ , b 5.139(2) A ˚ , c 7.115(4) A ˚ , b 92.13(2)º, Vuc 293.9(2) A ˚ 3, Z = 2 and is isostructural with yavapaiite[KFe (SO4)2]. The strongest lines in the powder diffraction diagram are [d (A˚ ), I (relative to 10)]: 3.72, 8; 3.64, 5; 3.43, 5; 2.77, 10; 2.72, 6; 2.57, 3; 2.370, 6; 1.650, 3. Theche mical analysis and theX-ray diffraction data of eldfellite correspond to those of the synthetic compound NaFe(SO4)2