2019/02/21 by I. Campostrini, Francesco Demartin, M. Russo · 7 citations
Chemistry · Materials Science · #Chemistry #Crystal Structures and Properties #Earth science #Geochemistry #Geology #Inorganic Fluorides and Related Compounds #Mineral #Mineralogy #Seismology #Volcano #X-ray Diffraction in Crystallography
paper · open access · doi:10.1127/ejm/2018/0030-2799
published in European Journal of Mineralogy 31(1), 153-158 (Elsevier BV)
openalex publication_date 2019/02/21 · openalex created_date 2025/10/10 · openalex updated_date 2025/11/06
The new mineral sbacchiite (IMA 2017-097), Ca 2 AlF 7 , 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 μm in length.On the basis of PXRD measurements and chemical analysis, the mineral was recognized to be identical to the corresponding synthetic phase.Crystals are transparent or translucent and colorless, with vitreous lustre and white streak.The tenacity is brittle.The measured density is 3.08(2) g/cm 3 , the calculated density is 3.116 g/cm 3 .The empirical formula, (based on 10 apfu) is Ca 2.02 Mg 0.03 Al 0 .99 F 6.97 .Sbacchiite is orthorhombic, space group Pnma, with a = 7.665( 2), b = 6.993(1), c = 9.566(2) Å, V = 512.2(2)Å 3 and Z = 4.The eight strongest X-ray powder diffraction lines are [d obs 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 >2σ(I).The asymmetric unit contains an Al 3+ and two independent Ca 2+ cations and five fluorine anions.Al 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 (C.N. 7+1).All the fluorine atoms have C.N. 3. The structure framework shows "isolated" [AlF 6 ] 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.