Paganoite, NiBi³⁺As⁵⁺O₅, a new mineral from Johanngeorgenstadt, Saxony, Germany : description and crystal structure

Paganoite, ideally NiBi³⁺As⁵⁺O⁵, triclinic space group P1, a = 6.7127(8), b = 6.8293(8), c = 5.2345(6) Å, α = 107.625(2)°, β = 95.409(2)°, γ = 111.158(2)°, V = 207.62 ų, a:b:c: = 0.9829:1:0.7665, Z = 2, is a new mineral found on a single nickeline-veined quartz specimen from Johanngeorgenstadt, Saxony, Germany. The strongest seven lines of the X-ray powder-diffraction pattern [d in Å (hkl)] are: 5.943 (100) (010); 3.233 (100) (011); 3.067 (60) (021); 3.047 (50) (200); 2.116 (50) (112,031, 311,122,231); 2.095 (40) (230,102); 1.659 (40) (420). It occurs as isolated orange-brown to deep-golden-brown crystals and crystal aggregates which are always intimately associated with aerugite; additional associations include bunsenite, xanthiosite, rooseveltite, native bismuth and two undefined arsenates. Individual prismatic crystals are subhedral to euhedral, elongate along [010] with a length-to-width ratio of 3:1, and average 0.3 mm in longest dimension. Forms observed are {100} major, {010} minor, {001} minor and perhaps {hOl} minor. Crystals possess a very pale orange-brown streak, are transparent (crystals) to translucent (aggregates), brittle, adamantine (almost gemmy), and do not fluoresce under ultraviolet light. The mineral shows neither twinning nor cleavage, has an uneven fracture, and the calculated density (for the empirical formula) is 6.715 g/cm³. Electron-microprobe analyses yielded NiO 15.37, CoO 2.05, Bi₂O₃ 55.06, As₂O₅ 28.0, total 100.48 wt.% The empirical formula, derived from the crystal-structure analysis and electron-microprobe analyses, is (Ni²⁺_0.86C²⁺_0.11) _Σ0.97 Bi³⁺_0.99As⁵⁺_1.02 O⁵, based on O = 5. In reflected plane-polarized light in air, it is grey with no obvious internal reflections, bireflectance or pleochroism. Measured reflectance values, in air and in oil, are tabulated: indices of refraction calculated from these at 589 nm are 2.07 and 2.09. The name honours Renato and Adriana Pagano for their long-standing service to the European mineralogical community.

The crystal structure of paganoite has been solved by direct methods and refined on the basis of F² using 977 unique reflections measured with MoKα X-radiation on a diffractometer equipped with a CCD-based detector. The final R1 was 4.4%, calculated for the 926 observed reflections. The structure contains AsO₄ tetrahedra and distorted Ni²⁺O₆ octahedra, as well as one-sided Bi³⁺O₅ polyhedra due to the presence of an s² lone pair of electrons on the Bi³⁺ cation. The structure is an open framework composed of dimers of edge-sharing NiO₆ octahedra that are linked by vertex-sharing with AsO₄ tetrahedra. Bi³⁺ cations occur within voids in the framework, and bond only to framework elements. The structure of paganoite is very closely related to that of jagowerite, BaAl₂P₂O₈(OH)₂, which possesses an identical framework of octahedra and tetrahedra.