Capranicaite: a new mineral from Capranica, Viterbo, Italy

Abstract

Capranicaite, ideally KCaNaAl4B4Si2O18, is a new mineral recently approved by the IMA CNMNC (case no. 2009-086) found in the Vico volcanic complex at Capranica, Viterbo, Italy. Capranicaite occurs in miarolitic cavities of a feldspathoid-bearing syenite ejectum probably formed during late-stage metasomatic processes related to volcanic activity. Capranicaite forms small, tabular crystals no longer than 0.1 mm. Crystals are colourless, with a white streak and a vitreous lustre; they are not fluorescent and have a good {001} cleavage. Capranicaite is biaxial (-) with α = 1.495(1), β = 1.543(1), γ = 1.544(1); 2V(obs) = 7.3(2)°. Capranicaite is monoclinic, space group P21/n, a = 4.8507(2), b = 16.6156(6), c = 20.5445(7) Å, β = 90.245(1)°, V = 1655.82(17) Å3, Z = 4. Electron microprobe 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%. Calculated density is 2.41 gcm-3. The formula unit, calculated on the basis of 18 O, is (K0.69Cs0.10)Ʃ0.79(Ca1.19Na0.80)Ʃ1.99Al3.91B3.99Si2.09O18. Capranicaite is a Cs bearing inosilicate with a new topological arrangement of the 2-periodic Si2O6 single chains. The crystal structure of capranicaite consists of three overlapping layers of polyhedra (A, B, and C) normal to c. The A-layer contains the Si2O6 chains that extend along a. Respect than pyroxene, in capranicaite the Si-tetrahedra are rotated and the Si2O6 groups do not have two apical oxygens pointing towards the same direction. The B-layer is built by isolated AlO4 tetrahedra and BO3 triangles connected to form a continuous sheet of six-fold rings (3Al + 3B). Two B-layers are connected to a single sandwiched A-layer to forms a bi-dimensional B-A-B network. The B-A-B networks contain seven fold cavities not completely populated by K and minor Cs. Adjacent B-A-B bi-dimensional networks are linked together via the C-layer, constituted by isolated octahedra with a mixed (Ca,Na) population. Along c the sequence of layer results C-B-A-B-C-B-A-B-C.