QUADRIDAVYNE, [(NA,K)6CL2][CA2CL2][SI6AL6O24], A NEW FELDSPATHOID MINERAL FROM VESUVIUS AREA

A new feldspathoid mineral, quadridavyne, belonging to the davyne grou p, is described. It was found in the locality Ottaviano, near Napoli, southern Italy, within products of the 1906 volcanic activity of Vesuv ius. It occurs as transparent, colorless, prismatic crystals, elongate d [001] with hexagonal outline. Twinning (1100BAR) is frequent. The mi neral is brittle, with hardness ca. 5, d(meas) = 2.335(5), d(calc) = 2 .354 g/cm3. Optically it is uniaxial (+) with omega = 1.529(1) and eps ilon = 1.532(1). Its ideal chemical formula, derived from electron mic roprobe data, is [(Na,K)6Cl2][Ca2Cl2][(Si6Al6O24)]. Quadridavyne is he xagonal, space group P6(3)/M, a = 25.771(6), c = 5.371(1) angstrom, Z = 4. The structure of quadridavyne is topologically identical to that of davyne. The major difference lies in the long-range ordering of alk aline cations (Na+ and K+) and chlorine within the open channels runni ng along [001], resulting in the doubling of the a parameter.