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) ų, 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 Pb_1.91C_1.05O_4.00. 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.