Sbacchiite, Ca2AlF7, a new fumarolic mineral from the Vesuvius volcano, Napoli, Italy

Abstract

The new mineral sbacchiite (IMA 2017-097), Ca2AlF7, was found in a fossil fumarole (1944 eruption, T ! 80 !C) at the rim of the crater of the Vesuvius volcano, Napoli, Italy, associated with gearsksutite, usovite, creedite and opal. It forms elongated crystals up to about 60 lm in length. On the basis of powder X-ray diffraction data and chemical analysis, the mineral was recognized to be identical to the corresponding synthetic phase. Crystals are transparent or translucent and colourless, with vitreous lustre and white streak. The tenacity is brittle. The measured density is 3.08(2) g/cm3, the calculated density is 3.116 g/cm3. The empirical formula, (based on 10 atoms per formula unit, apfu) is Ca2.02Mg0.03Al0.99F6.97. Sbacchiite is orthorhombic, space group Pnma, with a = 7.665(2), b = 6.993(1), c = 9.566(2) Å, V = 512.2(2) Å3 and Z = 4. The eight strongest X-ray powder diffraction lines are [dobs Å(I)(hkl)]: 3.840(45)(200), 3.563(85)(201), 3.499(100)(020), 2.899(55)(013), 2.750(30)(212), 2.281(20)(104), 2.255(52)(302) and 2.173(36)(131). The structure was refined to R = 0.0479 for 457 reflections with I > 2r(I). The asymmetric unit contains one Al3+ and two independent Ca2+ cations and five fluorine anions. Aluminium is octahedrally coordinated by six fluorine atoms; the arrangement of F around the 7-coordinated Ca(1) conforms to a distorted pentagonal bipyramid and that around Ca(2) to a very distorted polyhedron (in 7 + 1 coordination). All the fluorine atoms are threefold coordinated. The structure framework shows ‘‘isolated’’ [AlF6] octahedra, whereas the coordination polyhedra around Ca are linked by common edges [sequence: Ca(1)–Ca(2)– Ca(1). . .] along [010] and the same holds for the connection along [001]. Along [100], however, only the pentagonal bipyramids around Ca(1) are connected by bridging corners.