Abstract: Cannonite, ideally Bi₂O(OH)₂SO₄, is a new mineral from the Tunnel Extension mine, Ohio Mining District, Marysvale, Utah, USA. It occurs mostly as intergrown crystalline aggregates (<1 mm) of subhedral to euhedral equant to prismatic crystals (<200 µm) in cavities in quartz gangue. Other associated minerals are cuprobismutite, bismuthinite, and covelline. Cannonite is colourless and transparent with an adamantine lustre and white streak. It is brittle with an uneven to conchoidal fracture. In reflected light it is low reflecting, weakly to moderately bireflectant and weakly anisotropic. Internal reflections (colourless to white) are abundant. Measured reflectance values in air and oil are tabulated. Colour values relative to the CIE illuminant C for R₁ and R₂ in air respectively are: Y% 10.4, 11.7; Lambda_d 475,475; P_e% 2.6, 3.0. Calculated refractive indices at 589 nm: R₁ 1.91 and R₂ 1.99. VHN₁₀₀ 229 (range 183–280); calculated Mohs hardness is 4.

X-ray studies show that cannonite is monoclinic with space group P2₁/c and a 7.700(3), b 13.839(6), c 5.686(2) Å, β 109.11(3)°. It has a cell volume of 572.5(4) ų with Z = 4. D_calc. is 6.515 g/cm³. The strongest six lines of the X-ray powder pattern are [d in Å (I) (hkl)] 3.206 (100) (2¯21); 1.984 (90) (340, 1¯52); 2.924 (70) (131); 3.644 (60) (111); 3.466 (60) (040); 2.782 (50) (1¯12). Averaged probe analyses gave the empirical formula Bi_1.99O(OH_1.04)₂S_0.99O₄ on the basis of 7 oxygen atoms. The name is for Benjamin Bartlett Cannon of Seattle, Washington, United States of America.