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 theVico 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/cm<sup>3</sup>. Crystals are biaxial negative, non pleochroic, with alfa = 1.495(1), beta = 1.543(1), gamma = 1.544(1), 2Vobs = 7.3(2)º, 2Vcalc = 16.0º. Capranicaiteis monoclinic P21/n, with a = 4.8507(2), b = 16.6156(6), c = 20.5445(7) Å , beta = 90.245(1)º, 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) (1¯21; 121), 3.424 (8) (006), 2.184 (4) (048; 1¯64), 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%; theformula, based on 18 oxygens, is: (K0.69Cs0.10)S0.79(Ca1.19Na0.80)S1.99Al3.91B3.99Si2.09O18, 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.
Callegari, A.m., Boiocchi, M., Bellatreccia, F., Caprilli, E., Medenbach, O., AND CAVALLO, A. (2011). Capranicaite, (K,)(Ca,Na)Al<sub>4</sub>B<sub>4</sub>Si<sub>2</sub>O<sub>18</sub>: a new inosilicate from Capranica, Italy, with a peculiar topology of the periodic single chain [Si<sub>2</sub>O<sub>6</sub>]. MINERALOGICAL MAGAZINE, 75, 33-43 [10.1180/minmag.2011.075.1.33].
Capranicaite, (K,)(Ca,Na)Al4B4Si2O18: a new inosilicate from Capranica, Italy, with a peculiar topology of the periodic single chain [Si2O6]
BELLATRECCIA, FABIO;
2011-01-01
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 theVico 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 alfa = 1.495(1), beta = 1.543(1), gamma = 1.544(1), 2Vobs = 7.3(2)º, 2Vcalc = 16.0º. Capranicaiteis monoclinic P21/n, with a = 4.8507(2), b = 16.6156(6), c = 20.5445(7) Å , beta = 90.245(1)º, 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) (1¯21; 121), 3.424 (8) (006), 2.184 (4) (048; 1¯64), 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%; theformula, based on 18 oxygens, is: (K0.69Cs0.10)S0.79(Ca1.19Na0.80)S1.99Al3.91B3.99Si2.09O18, 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.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
