Colchesterite, Bi ³⁺ ₂ O[Mo ⁶⁺ ₂ O ₈ ], a new Bi molybdate mineral from the Kingsgate mines, New South Wales, Australia

Colchesterite, Bi 3+2 OMo 6+ 2 O 8 , is a new mineral from the Kingsgate mines, Gough Co., New South Wales, Australia.It forms greenish-yellow to brownish-yellow, equant to prismatic crystals to 1.1 mm in length.The mineral is transparent, with a vitreous luster.The crystals show excellent cleavage on {001}.The hardness is 2½ based on scratch tests and the tenacity is brittle with an uneven, stepped fracture.The calculated density is 6.522 g/cm -3 for the empirical formula.Colchesterite is nonpleochroic, biaxial (+), with calculated indices of refraction of α = 2.18, β = 2.20 and γ = 2.23.The measured 2V is 80(5)° and the optical orientation is Z = b, X ^ c = 20° in obtuse angle β.Dispersion is r < v, strong and crossed.Colchesterite is monoclinic, space group P2 1 /n, with the unit-cell parameters: a = 8.1030( 16), b = 8.5396(17), c = 11.209(2)Å, β = 93.58(3)º,V = 774.1(3)Å 3 and Z = 4.The crystal structure was refined to an R 1 index of 2.39% for 1719 observed reflections with I > 2σ I .The structure consists of layers of Bi 4 O 22 tetrameric clusters comprised of edge-sharing Bi1O 8 and Bi2O 7 polyhedra that share edges with Mo 2 O 8 dimers to form a dense framework.