BRIZZIITE, NASBO3, A NEW MINERAL FROM THE CETINE MINE (TUSCANY, ITALY) - DESCRIPTION AND CRYSTAL-STRUCTURE

Brizziite is a new sodium antimonate mineral from the Cetine mine, Tuscany, Italy. It occurs as dense aggregates of platy {001} hexagonal crystals. Associated minerals are stibiconite and mopungite. The crystals, which are polysynthetically twinned, are colourless and transparent, with pearly lustre. Electron microprobe analyses give Na2O 15.98 and Sb2O5 83.28 wt.%, corresponding to the formula NaSbO3. Optically, brizziite is uniaxial (-) with epsilon = 1.631(1), omega = 1.84, birefringence 0.21(1); VHN15 microhardness 57; D-meas = 4.8(2) g/cm(3); D-calc = 4.95 g/cm(3). The unit cell is rhombohedral (hexagonal) a = 5.301(1), c = 15.932(4) Angstrom, space group R ($) over bar 3 and Z = 6. The strongest lines in the X-ray Gandolfi pattern are (d(obs), I, hkl)5.30(53)(003), 3.00(55)(104), 2.650(67)(006,110), 2.365(69)(113), 1.874(100)(116), 1.471(69)(119,303). The IR spectrum is given also. The mineral is named for the mineral collector G. Brizzi (1936-1992). The crystal structure was refined to an R index of 0.025, using a set of 334 observed (6 sigma) reflections (R = 0.043 for an 385 collected reflections). The structure consists of the ilmenite-type octahedral arrangement; layers of distorted edge sharing Na octahedra alternate with layers of edge-sharing Sb+5 octahedra along [001]. The layers are connected to one another by octahedral face-sharing.