Alloriite, Na5K1.5Ca(Si6Al6O24)(SO4)(OH)0.5 • H2O, a new mineral species of the cancrinite group

Alloriite, a new mineral species, has been found in volcanic ejecta at Mt. Cavalluccio (Campagnano municipality, Roma province, Latium region, Italy) together with sanidine, biotite, andradite, and apatite. The mineral is named in honor of Roberto Allori (b. 1933), an amateur mineralogist and prominent mineral collector who carried out extensive and detailed field mineralogical investigations of volcanoes in the Latium region. Alloriite occurs as short prismatic and tabular crystals up to 1.5 x 2 mm in size. The mineral is colorless, transparent, with a white streak and vitreous luster. Alloriite is not fluorescent and brittle; the Mohs' hardness is 5. The cleavage is imperfect parallel to {10 (1) over bar0}. The density measured with equilibration in heavy liquids is 2.35g/cm(3) and calculated density (D-calc) is 2.358 g/cm(3) (on the basis of X-ray single-crystal data) and 2.333 g/cm(3) (from X-ray powder data). Alloriite is optically uniaxial, positive, omega = 1.497(2), and epsilon = 1.499(2). The infrared spectrum is given. The chemical composition (electron microprobe, H2O determined using the Penfield method, CO2, with selective sorption, wt %) is: 13.55 Na2O, 6.67 K2O, 6.23 CaO, 26.45 Al2O3, 34.64 SiO2, 8.92 SO3, 0.37 Cl, 2.1 H2O, 0.7 CO2, 0.08-O = Cl-2, where the total is 99.55. The empirical formula (Z = 1) is Na19.16K6.21Ca4.87(Si25.26Al22.74O96)(SO4)(4.88)(CO3)(0.70)Cl-0.46(OH)(0.76) center dot 4.73H(2)O. The simplified formula (taking into account the structural data, Z = 4) is: [Na(H2O)][Na4K1.5(SO4)] center dot [Ca(OH,Cl)(0.5)](Si6Al6O24). The crystal structure has been studied (R = 0.052). Alloriite is trigonal, the space group is P31c; the unit-cell dimensions are a = 12.892(3), c = 21.340(5) angstrom, and V = 3071.6(15) angstrom(3)supercript stop. The crystal structure of alloriite is based on the same tetrahedral framework as that of afghanite. In contrast to afghanite containing clusters [Ca-Cl](+) and chains ...Ca-Cl-Ca-Cl..., the new mineral contains clusters [Na-H2O](+) and chains ...Na-H2O-Na-H2O.... The strongest reflections in the X-ray powder diffraction pattern [d, angstrom (I, %)(hkl)] are: 11.3(70)(100), 4.85(90)(104), 3.76(80)(300), 3.68(70)(301), 3.33(100)(214), and 2.694(70)(314, 008). The type material of alloriite is deposited in the Fersman Mineralogical Museum, Russian Academy of Sciences, Moscow. The registration number is 3459/1.