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

Dukeite, Bi243+Cr86+O57(OH)(6)(H2O)(3), space group P3 Ic, a = 15.067(3), c = 15.293(4) Angstrom, V = 3007(1) Angstrom (3), Z = 2, is a new mineral fo und on a museum specimen labeled as originating from the Sao Jose Mine, Bre jauba, Minas Gerais, Brazil. The strongest seven lines of the X-ray powder- diffraction pattern [d in Angstrom (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 (4 0) (226), 1.701 (50) (228). It occurs as groupings of tightly bound Z x 0.3 mm(2) sized sheaves that are associated with pucherite, schumacherite, bis mutite, and hechtsbergite. Individual acicular crystals do not exceed 100 m um in length by 1-2 mum in width. Crystals are yellow inclining to a dirty yellow-brown, possess a bright yellow streak, are transparent, brittle, res inous, and do not fluoresce under ultraviolet light. The estimated Mohs har dness is between 3 and 4, the calculated density (for the empirical formula ) is 7.171 g/cm(3), and the mineral is slowly soluble in concentrated HCl. Electron-microprobe analyses yielded Bi2O3 85.06, CrO3 11.65, V2O5 0.59, H2 O (calc.) [1.67], total [98.97] wt%. The empirical formula, derived from th e crystal structure analysis and electron-microprobe analyses, is Bi-23.95( 3+)(Cr7.645+V0.435+)Sigma O-8.07(56.84)(OH)(6.16) .3.01H(2)O, based on O = 66. In reflected plane-polarized light in air it is gray to purplish gray w ith strong yellow internal reflections. Bireflectance is very weak. Measure d reflectance values, in air and in oil. are tabulated. indices of refracti on 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 fou nd and also recognizes the contribution of the Duke family to the advanceme nt of scientific knowledge. The crystal structure of dukeite was solved by direct methods and refined on the basis of FZ using all unique reflections measured with MoK alpha X-radiation on a CCD-equipped diffractometer. The f inal RI index was 8.7%, calculated using 1033 observed reflections. It cont ains irregular layers of Bi phi (n),, polyhedra (phi: O, OH-, H2O) parallel to (001), separated and connected by CrO4 tetrahedra to form a framework s tructure. One CrO4 tetrahedron shares all of its vertices with Bi phi (n) p olyhedra, whereas the other three CrO4 tetrahedra share only three vertices each with the Bi phi (n) polyhedra on either side.