Poppiite, the V3+ end-member of the pumpellyite group: Description and crystal structure

Poppiite, a new mineral from Gambatesa mine (Val Graveglia, Genova, Northern Italy), is the V3+ end-member of the pumpellyite group [([VII])(Ca7.68Na0.27K0.03Rb0.02)(Sigma 8.00)([VI])(V1.263+Fe1.023+Mg0.78Mn0. 2+59Al0.31Cu0.042+)(Sigma 4.00)([VI])(V6.893+Al1.07Ti0.04)(Sigma 8.00)([IV])(Si11.69Al0 (.31))(Sigma 12.00)O-42(OH)(14); C2/m, a = 19.2889(6), b = 6.0444(2), c = 8.8783(3) angstrom, beta = 97.328(2)degrees, V = 1026.66(6) angstrom(3), D-meas = 3.36(2) g/cm(3), and D-calc = 3.44 g/cm(3)]. Poppiite crystals, with size varying from 0.1 to 0.6 mm, are minute, greenish-brown, and prismatic, and are associated with roscoelite, ganophyllite, manganaxinite, goldmanite, and calcite. The strongest lines in the X-ray powder diffraction pattern [d(abs) (angstrom), I-rel, (hkl)] are: 2.930, 100, (511); 3.817, 70, (202); 2.548, 65, ((3) over bar 13); 2.551, 62, (420); 1.612, 57, ((7) over bar 31,424); and 2.367, 51, (222, 403). Poppiite is optically negative, with 2V(calc) = 44 degrees, n(alpha) = 1.768(9), n(beta) 1.804(8), n(gamma) 1.810(9). The pleochroic scheme is a = light yellowish brown, = deep greenish brown, and gamma = brown to reddish brown. The crystal structure was refined using 1918 unique reflections to R = 0.0307. Like the other pumpellyite-group minerals the crystal structure of poppiite consists of chains of edge-sharing octahedra linked by SiO4, Si2O7, and CaO7 polyhedra.