PARANIITE-(Y), A NEW TUNGSTATE ARSENATE MINERAL FROM ALPINE FISSURES

Paraniite-(Y) has been found in the Alpine fissures of the Italian sid e of Scherbadung (Pizzo Cervandone, Ossola). as a unique specimen bear ing only a few creamy-yellow crystals up to 3 mm, having an elongated bipyramidal tetragonal habit. From microprobe analyses and from a sing le crystal X-ray structure determination the following chemical formul a has been derived: [Ca1.64(Y,REE)0.36]SIGMA = 2.00(Y,REE)[(As0.96,W0. 04)SIGMA = 1.00O4][(W0.89,As0.11)SIGMA = 1.00O4]2. The mineral is tetr agonal, space group I4(1)/a with a = 5.135(1) angstrom, c = 33.882(5) angstrom, V = 893.4(4) angstrom3, Z = 4. The structure was refined to R = 0.024 for 840 observed reflections. The unit-cell parameters and t he symmetry of the mineral are strictly related to those of scheelite, the c parameter being approximately three times the corresponding val ue for scheelite. The structure conforms to the scheelite type and con sists of layers with composition near to CaWO4 and (Y,REE)AsO4, respec tively, stacked along [001] with a molar ratio of 2:1.