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Dukeite, Bi₂₄³⁺Cr₈⁶⁺O₅₇(OH)₆(H₂O)₃, a new mineral from Brejaúba, Minas Gerais, Brazil: Description and crystal structure

¹ Department of Civil Engineering and Geological Sciences, 156 Fitzpatrick, University of Notre Dame, Notre Dame, Indiana 46556-0767, U.S.A.
² Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada
³ Department of Mineralogy, The Natural History Museum, Cromwell Road, London, SW7 5BD, U.K.
⁴ Box 3921, Duke University Medical Center, Durham, North Carolina 27710, U.S.A.

Correspondence: ^* E-mail: pburns{at}nd.edu

Dukeite, Bi₂₄³⁺Cr₈⁶⁺O₅₇(OH)₆(H₂O)₃, space group P31c, a = 15.067(3), c = 15.293(4) Å, V = 3007(1) ų, Z = 2, is a new mineral found on a museum specimen labeled as originating from the São José Mine, Brejaúba, Minas Gerais, Brazil. The strongest seven lines of the X-ray powder-diffraction pattern [d in Å (I) (hkl)] are: 7.650 (50) (002), 3.812 (40) (004), 3.382 (100) (222), 2.681 (70) (224), 2.175 (40) (600), 2.106 (40) (226), 1.701 (50) (228). It occurs as groupings of tightly bound 1 x 0.3 mm² sized sheaves that are associated with pucherite, schumacherite, bismutite, and hechtsbergite. Individual acicular crystals do not exceed 100 µm in length by 1–2 µm in width. Crystals are yellow inclining to a dirty yellow-brown, possess a bright yellow streak, are transparent, brittle, resinous, and do not fluoresce under ultraviolet light. The estimated Mohs hardness is between 3 and 4, the calculated density (for the empirical formula) is 7.171 g/cm³, and the mineral is slowly soluble in concentrated HCl. Electron-microprobe analyses yielded Bi₂O₃ 85.06, CrO₃ 11.65, V₂O₅ 0.59, H₂O (calc.) [1.67], total [98.97] wt%. The empirical formula, derived from the crystal-structure analysis and electron-microprobe analyses, is Bi_23.95³⁺(Cr_7.64⁶⁺V_0.43⁵⁺)_8.07O_56.84(OH)_6.16·3.01 H₂O, based on O = 66. In reflected plane-polarized light in air it is gray to purplish gray with strong yellow internal reflections. Bireflectance is very weak. Measured reflectance values, in air and in oil, are tabulated: indices of refraction calculated from these at 590 nm are 2.33 and 2.37. The name honors Duke University, Durham, North Carolina, in whose collection the mineral was found and also recognizes the contribution of the Duke family to the advancement of scientific knowledge.

The crystal structure of dukeite was solved by direct methods and refined on the basis of F² using all unique reflections measured with MoK X-radiation on a CCD-equipped diffractometer. The final R1 index was 8.7%, calculated using 1033 observed reflections. It contains irregular layers of Bi_n polyhedra (: O, OH^–, H₂O) parallel to (001), separated and connected by CrO₄ tetrahedra to form a framework structure. One CrO₄ tetrahedron shares all of its vertices with Bi_n polyhedra, whereas the other three CrO₄ tetrahedra share only three vertices each with the Bi_n polyhedra on either side.

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