Shannonite, Pb2OCO3, a New Mineral from the Grand Reef Mine, Graham County, Arizona, USA*

A. C. Roberts, J. A. R. Stirling, G. J. C. Carpenter, A. J. Criddle, G. C. Jones, T. C. Birkett and W. D. Birch
Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada
Materials Technology Laboratories, CANMET, 568 Booth Street, Ottawa, Ontario K1A 0G1, Canada
Department of Mineralogy, Natural History Museum, Cromwell Road, London SW7 5BD, UK
Québec Geoscience Centre, P.O. Box 7500, Ste-Foy, Quebec G1V 4C7, Canada
Department of Mineralogy and Petrology, Museum of Victoria, 285 Russell Street, Melbourne, Victoria 3000, Australia
*Geological Survey of Canada Contribution Number 20194

Abstract: Shannonite, ideally Pb2OCO3, is a new mineral species that occurs as mm-sized white porcellanous crusts, associated with fluorite, at the Grand Reef mine, Graham County, Arizona, USA. Other associated minerals are plumbojarosite, hematite, Mn-oxides, muscovite-2M1, quartz, litharge, massicot, hydrocerussite, minium, and unnamed PbCO3·2PbO. Shannonite is orthorhombic, space group P21221 or P212121, with unit-cell parameters (refined from X-ray powder data): a 9.294(3), b 9.000(3), c 5.133(2) Å, V 429.3(3) Å3, a:b:c 1.0327:1:0.5703, Z = 4. The strongest five lines in the X-ray powder pattern [d in Å (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 microprobe analyses is PbO 89.9(5), CO2 (by CHN elemental analyser) 9.70, total 99.60 wt.%. With O = 4, the empirical formula is Pb1.91C1.05O4.00. The calculated density for the empirical formula is 7.31 and for the idealized formula is 7.59 g/cm3. In reflected light, shannonite is colourless-grey to white, with ubiquitous white internal reflections (× 16 objectives), weak anisotropy, barely detectable bireflectance, and no evidence of pleochroism. The calculated refractive index (at 590 nm) is 2.09. Measured reflectance values in air and in oil (× 4 objectives) are tabulated. Transmission electron-microscopy studies reveal that individual crystallites range in size from 10–400 nm, are platy, and are 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 (range 93–100); calculated Mohs’ hardness 3–3½. 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.

Keywords: shannonite • new mineral • Grand Reef mine • Graham County • Arizona • lead oxycarbonate • reflectance data • X-ray data • electron-microprobe analyses • CHN elemental analysis • transmission electron-microscopy study

Mineralogical Magazine; June 1995 v. 59; no. 395; p. 305-310; DOI: 10.1180/minmag.1995.059.395.14
© 1995, The Mineralogical Society
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