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

Paganoite, ideally NiBi3+As5+O5, triclinic space group P (1) over bar, a = 6.7127(8), b = 6.8293(8), c = 5.2345(6) Angstrom, alpha = 107.625(2)degrees beta = 95.409(2)degrees gamma = 111.158(2)degrees, V= 207.62 Angstrom (3), a:b:c: = 0.9829:1:0.7665, Z = 2, is a new mineral found on a single nickel ine-veined quartz specimen from Johanngeorgenstadt, Saxony, Germany. The st rongest seven lines of the X-ray powder-diffraction pattern [d in Angstrom (I) (hkl)] are: 5.943 (100) (010); 3.233 (100) (011); 3.067 (60) (0 (2) ove r bar1): 3.047 (50) (200); 2.116(50) ((1) over bar 12, 0 (3) over bar1, (3) over bar 11, (12) over bar2, 2 (3) over bar1); 2.095 (40) ((2) over bar 30 , 102); 1.659 (40) ((4) over bar 20). It occurs as isolated orange-brown to deep-golden-brown crystals and crystal aggregates which are always intimat ely associated with aerugite: additional associations include bunsenite, xa nthiosite, rooseveltite, native bismuth and two undefined arsenates. Indivi dual prismatic crystals are subhedral to euhedral, elongate along [010] wit h 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 {h0l} minor. Crystals possess a very pale orange-brown streak, are transparent (c rystals) to translucent (aggregates), brittle, adamantine (almost gemmy), a nd do not fluoresce under ultraviolet light. The mineral shows neither twin ning nor cleavage, has an uneven fracture, and the calculated density (for the empirical formula) is 6.715 g/cm(3). Electron-microprobe analyses yield ed NiO 15.37, CoO 2.05, Bi2O3 55.06, As2O5 28.0, total 100.48 wt. %. The em pirical formula, derived from the crystal-structure analysis and electron-m icroprobe analyses, is (Ni0.862+Co0.112+)(Sigma0.97)Bi0.993+As1.025+O5 base d on O = 5. In reflected plane-polarized light in air, it is grey with no o bvious internal reflections. bireflectance or pleochroism. Measured reflect ance values, in air and in oil, are tabulated: indicts of refraction calcul ated from these at 589 nm are 2.07 and 2.09. The name honours Renato and Ad riana Pagano for their long-standing service to the European mineralogical community. The crystal structure of paganoite has been solved by direct methods and re fined on the basis of F-2 using 977 unique reflections measured with MoK al pha X-radiation on a diffractometer equipped with a CCD-based detector. The final R1 was 4.4%. calculated for the 926 observed reflections. The struct ure contains AsO4 tetrahedra and distorted Ni2+O6 octahedra. as well as one -sided Bi3+O5 polyhedra due to the presence of an s(2) lone pair of electro ns on the Bi3+ cation. The structure is an open framework composed of dimer s of edge-sharing NiO6 octahedra that are linked by vertex-sharing with AsO 4 tetrahedra. Bi3+ cations occur within voids in the framework, and bond on ly to framework elements. The structure of paganoite is very closely relate d to that of jagowerite, BaAl2P2O8(OH)(2), which possesses an identical fra mework of octahedra and tetrahedra.