Datolite: a new occurrence in volcanic ejecta (Pitigliano, Tuscany, Italy) and crystal-structure refinement

This paper describes the first occurrence of datolite in a volcanic ejectum collected at Pitigliano, Vulsini volcanic complex, Toscana, Italy. The studied specimen was sampled within the pyroclastic levels erupted during one of the several eruptive phases of the Latera caldera in the Roman Comagmatic region (253 - 166 ka). The host rock is massive, with a syenitic appearance, and consists of predominant sanidine, both as fine-grained groundmass and phenocrystals up to 1 cm long. Associated minerals are andraditic garnet, clinopyroxene (augite), biotite, iron oxide and vishnevite. Datolite occurs within vugs in the ejectum as well-formed, transparent, colourless crystals, with stubby prismatic shape; the crystals are about 0.5 mm in maximum diameter. The crystal structure of datolite from Pitigliano has been refined in the P21/c space group to R = 2.18%. Cell dimensions are: a (Å) = 4.8318(4), b (Å) = 7.6116(3), c (Å) = 9.6380(2), b (°) = 90.141(4), V (Å3) = 354.46(3). The chemical composition is (Ca2.006K0.001)Σ2.007(Fe0.007Mn0.004Mg0.002)Σ0.013(B1.986Si1.988Al0.026)Σ4.000O8(OH1.962O0.026F0.012)Σ2.000 and the density calculated on the basis of composition and cell dimensions is Dc = 3.00 g/cm3.