KNASIBFITE, K3Na4[SiF6]3[BF4], A NEW HEXAFLUOROSILICATE-TETRAFLUOROBORATE FROM LA FOSSA CRATER, VULCANO, AEOLIAN ISLANDS, ITALY

Abstract

Knasibfite, ideally K3Na4[SiF6]3[BF4], is a newly discovered mineral species that can be considered a potassium and sodium hexafluorosilicate–tetrafluoroborate. It occurs in an altered pyroclastic breccia associated with hieratite, avogadrite and demartinite in a low-temperature fumarole at the rim of La Fossa crater, Vulcano Island, Aeolian Archipelago, Sicily, Italy. The mineral is orthorhombic, space group Imm 2, with a 5.522(1), b 17.106(3), c 9.175(2) A, V 866.7(3) A3, Z = 2. The crystals are colorless, prismatic up to 0.1 mm, most commonly showing the forms {001}, {100}, {010}, {110}, {011} and {101}. The mean analytical results are: K 16.78, Na 13.14, Si 11.36, F 57.83, B 1.50, for a total of 100.61 wt%. The boron content has been estimated from results of the crystal-structure analysis. The empirical formula (based on 33 apfu ) is: K3.09Na4.11Si2.91B1.00F21.89; the simplified formula is K3Na4[SiF6]3[BF4], which requires: K 16.24, Na 12.73, Si 11.67, F 57.87, B 1.50, for a total of 100.00 wt%. The crystal structure has been refined to a final R index of 0.0272 for 660 observed reflections. The asymmetric unit contains two twelve-coordinated independent K+ ions, located on m .. and on mm 2 sites, respectively [K–F distances in the range 2.756(3)–3.332(2) A], two independent Na+ ions, of which one is nine-coordinated [Na–F distances in the range 2.254(5)–2.949(3) A], and the other one displays instead octahedral coordination [Na–F distances in the range 2.224(3)–2.387(3) A]. There are also two crystallographically independent octahedral SiF62− anions with Si–F distances in the range 1.640(6)–1.684(4) A, and a tetrahedral BF4− anion, with B–F distances ranging from 1.385(6) to 1.386(7) A. The calculated density is 2.77 g/cm3, the measured density is 2.75(1) g/cm3. The mineral is optically nearly isotropic, with n = 1.32(1). The strongest six lines in the X-ray powder-diffraction pattern [dobs in A( I ) hkl ] are: 4.044(100)(022), 8.558(50)(020), 2.280(50)(222), 3.175(30)(141), 8.107(25)(011) and 2.094(25)(251). The name reflects its chemical composition.