Abstract

Capranicaite, ideally (K,□)(Ca,Na)Al4B4Si2O18, is a new inosilicate mineral from the Vico volcanic complex collected at Capranica, Viterbo Province, Latium, Italy. It occurs in miarolitic cavities of a feldspathoid-bearing syenite ejectum and formed by late-stage metasomatic processes related to the activity of the Vico volcano.

Capranicaite occurs as thin, tabular crystals no larger than 0.1 mm. Crystals are colourless, with a white streak and a vitreous lustre; they are brittle and their Mohs hardness is certainly <6. Capranicaite is non-fluorescent with good {001} cleavage and no observable parting. The calculated density is 2.41 g/cm3. Crystals are biaxial negative, non pleochroic, with α = 1.495(1), β = 1.543(1), γ = 1.544(1), 2Vobs = 7.3(2)°, 2Vcalc = 16.0°.

Capranicaite is monoclinic P21/n, with a = 4.8507(2), b = 16.6156(6), c = 20.5445(7) Å, β = 90.245(1)°, V = 1655.82(17) Å3, Z = 4. The strongest six X-ray diffraction lines in the simulated powder pattern are [d in Å (I) (hkl)]: 3.234 (10) (124; 044), 4.104 (9) (graphic21; 121), 3.424 (8) (006), 2.184 (4) (048; graphic64), 2.405 (4) (160), 2.425 (3) (200). EMP-WDS analysis gives: SiO2 20.70, Al2O3 32.91, B2O3 22.90, K2O 5.36, CaO 11.04, Na2O 4.08, Cs2O 2.20, sum 99.19%; the formula, based on 18 oxygens, is: (K0.69Cs0.10)Σ0.79(Ca1.19Na0.80)Σ1.99Al3.91B3.99Si2.09O18, corresponding to the ideal formula: (K,□)(Ca,Na)Al4B4Si2O18. The crystal structure shows three overlapping layers of polyhedra parallel to (001): (1) the A layer contains periodic single chains formed by Si2O6 units with a topology not previously observed; (2) the B layer contains isolated AlO4 tetrahedra and BO3 triangles forming a sheet of six-fold rings (3Al + 3B); (3) the C layer contains two octahedral sites: M(1) and M(2), with a mixed (Ca, Na) population. Two B layers and an intermediate A layer are vertex-connected to form a bi-dimensional B-A-B network characterized by large channels not completely populated and accommodating K and minor Cs. Along c from the origin the following layer sequence results: C-[B-A-B]-C-[B-A-B]-C.

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