SHANNONITE, PB2OCO3, A NEW MINERAL FROM THE GRAND-REEF-MINE, GRAHAM COUNTY, ARIZONA, USA

Shannonite, ideally Pb2OCO3, is a new mineral species that occurs as m m-sized white porcellanous crusts, associated with fluorite, at the Gr and Reef mine, Graham County, Arizona, USA. Other associated minerals are plumbojarosite, hematite, Mn-oxides, muscovite-2M1, quartz, lithar ge, massicot, hydrocerussite, minium, and unnamed PbCO3.2PbO. Shannoni te is orthorhombic, space group P2(1)22(1) or P2(1)2(1)2(1), with unit -cell parameters (refined from X-ray powder data): a 9.294(3), b 9.000 (3), c 5.133(2) angstrom, V 429.3(3) angstrom-3, a:b:c 1.0327:1:0.5703 , Z = 4. The strongest five lines in the X-ray powder pattern [d in an gstrom (I)(hkl)] are: 4.02(40)(111); 3.215(100)(211); 3.181(90)(121); 2.858(40)(130); 2.564(35)(002). The average of eight electron micropro be analyses is PbO 89.9(5), CO2 (by CHN elemental analyser) 9.70, tota l 99.60 wt.%. With O = 4, the empirical formula is Pb1.91C1.05O4.00. T he calculated density for the empirical formula is 7.31 and for the id ealized formula is 7.59 g/cm3. In reflected light, shannonite is colou rless-grey to white, with ubiquitous white internal reflections (x 16 objectives), weak anisotropy, barely detectable bireflectance, and no evidence of pleochroism. The calculated refractive index (at 590 nm) i s 2.09. Measured reflectance values in air and in oil (x 4 objectives) are tabulated. Transmission electron-microscopy studies reveal that i ndividual crystallites range in size from 10-400 nm, are platy, and ar e anhedral. Physical properties for cryptocrystalline crusts include: white streak; waxy lustre; opaque; nonfluorescent under both long- and short-wave ultraviolet light; uneven fracture; brittle; VHN100 97 (ra nge 93-100); calculated Mohs' hardness 3-3 1/2. Shannonite is soluble in concentrated HCl and in dilute HNO3 and H2SO4. The mineral name is for David M. Shannon, who helped collect the samples and who initiated this study.