Balliranoite, (Na,K)6Ca2(Si6Al6O24)Cl2(CO3), a new cancrinite-group mineral from Monte Somma - Vesuvio volcanic complex, Italy

The new cancrinite-group mineral balliranoite was found in a metasomatic rock from Monte Somma – Vesuvio volcanic complex, Campania, Italy. Associated minerals are orthoclase, phlogopite, clinohumite, calcite, diopside, pargasite, hau¨yne and apatite. The mineral is named for the Italian crystallographer Paolo Ballirano. Balliranoite is transparent, colourless. It occurs in the cavities of the rock as coarse prismatic crystals up to 1 1 2mmand as anhedral grains up to 1 cmin the groundmass. The mineral is brittle, with Mohs hardness 5 and perfect cleavage on (10-10). Dmeas is 2.48(1), Dcalc is 2.486(12) g/cm3. Optically, the new mineral is uniaxial (þ), o ¼ 1.523(2), e ¼ 1.525(2). IR spectrum is given. The chemical composition is (mean of 5 analyses, wt%): Na2O 13.05, K2O 3.08, CaO 12.70, Al2O3 27.28, SiO2 32.38, SO3 1.96, Cl 7.43, –O¼Cl2 1.68; CO2 (determined by selective sorption of ignition products) 3.24; H2O (determined by Penfield method) 0.19; total 99.63. The empirical formula based on 12 (Si þ Al) is: Na4.70Ca2.53K0.73(Si6.02Al5.98O23.995)Cl2.34(CO3)0.82(SO4)0.270.12H2O. The simplified formula is: Na5KCa2(Si6Al6O24)Cl2(CO3). The crystal structure was refined (R ¼ 0.0396). Balliranoite is hexagonal, P63; a ¼ 12.695(2) A ˚ , c ¼ 5.325(1) A ˚ , V ¼ 743.2(2) A ˚ 3, Z ¼ 1. Balliranoite is an analogue of cancrinite with . . .–Ca–Cl–Ca–Cl–. . . chains in narrow channels instead of . . .–Na–H2O–Na–H2O–. . . and an analogue of davyne with prevailing of (CO3) in the broad channels instead of (SO4). The strongest lines of the powder diffraction pattern [d, A˚ (I, %) (hkl)] are: 4.797 (100) (101), 3.669 (57) (300), 3.281 (73) (211), 2.754 (16) (400), 2.662 (58) (002), 2.446 (31) (401), 2.120 (18) (330). The holotype specimen is deposited in Fersman Mineralogical Museum of Russian Academy of Sciences, Moscow, Russia, with the registration number 3756/1