Quadridavyne, [(Na,K)6Cl2][Ca2Cl2][Si6Al6O24], a new feldspathoid mineral from Vesuvius area

A new feldspathoid mineral, quadridavyne, belonging to the davyne group, is described. It was found in the locality Ottaviano, near Napoli, southern Italy, within products of the 1906 volcanic activity of Vesuvius. It occurs as transparent, colorless, prismatic crystals, elongated [001] with hexagonal outline. Twinning (1‾100) is frequent. The mineral is brittle, with hardness ca. 5, dmeas = 2.335(5), dcalc = 2.354 g/cm3. Optically it is uniaxial (+), with ω = 1.529(1) and ε = 1.532(1). Its ideal chem. formula, derived from electron microprobe data, is [(Na,K)6Cl2][Ca2Cl2][(Si6Al6O24)]. Quadridavyne is hexagonal, space group P63/m, a = 25.771(6), c = 5.371(1) Å, Z = 4. The structure of quadridavyne is topol. identical to that of davyne. The major difference lies in the long-range ordering of alk. cations (Na+ and K+) and chlorine within the open channels running along [001], resulting in the doubling of the a parameter.