The crystal structure and composition of pottsite, (Pb3xBi4-2x)(VO4)4•H2O(0.8 < x < 1.0)

Pottsite from the Linka mine (Nevada, USA) was studied by scanning electron microscopy, energy- and wavelength-dispersive analyses, and single-crystal X-ray diffraction. The formula, calculated on the basis of (V + As) = 4, shows that the Pb/Bi atomic ratio in pottsite varies between 0.86 and 1.48, and a plot of Pb vs. Bi (atoms per formula unit) suggests that the 3Pb(2+) <-> 2Bi(3+) substitution is operative defining two compositional groups: one with low Pb/Bi atomic ratio (0.86-1.01), the other with high Pb/Bi ratio (1.22-1.48). Structure refinement shows that the crystal studied by X-ray diffraction has a low Pb/Bi ratio. Pottsite crystallizes in the tetragonal space group I4(1)/a, a = 11.0839(5), c = 12.6516(6) angstrom(3), V = 1554.29(16) angstrom(3), Z = 1. The crystal structure was solved by direct methods and refined to R-1 = 0.040 for 1167 unique reflections with F > 4 sigma(F). The structure contains one A site split into partially occupied A(1) and A(2) sites. The A(1) site is populated by Pb0.500Bi0.380Ca0.030, whereas A(2) is occupied by Pb0.068Ca0.032, with a site occupation factor (s.o.f.) of 0.07. The Bi(1) site has a s.o.f. of 0.84 and forms a distorted but symmetrical square antiprismatic coordination environment, which indicates a low degree of stereochemical activity of the lone electron pair on Bi3+. The symmetrically independent V(1) site has a V0.983As0.017 population and is coordinated by four oxygen atoms to form a distorted VO4 tetrahedron. The minor amount of chlorine in pottsite is restricted to the O-w site (O0.928Cl0.072) which belongs to the H2O molecule. The atomic arrangement in the structure of pottsite can be described as the packing of isolated V5+O4 tetrahedra, linked with heavy cations via common oxygen atoms to form 2D blocks or double-layered (DL) structural units that are parallel to (001). The DL units are formed by pairs of vanadate tetrahedra arranged opposite one to each other and oriented in opposite directions. Water molecules are located in between vanadate tetrahedral groups to form [(VO)(4)(H2O)](3-) chains which are separated from adjacent chains in the (001) plane by rows of Pb2+ and Bi3+ cations extending in the [120] direction. Two adjacent DL units are shifted one relative to the other by a translation of +/- 1/2 unit along the b axis, with an interlayer spacing of about 1.13 angstrom, measured as the shortest distance between adjacent O-O edges of the VO4 tetrahedra. The structure of pottsite is compared to that of related vanadate minerals based on isolated V5+O4 tetrahedra with formation of DL units. The observed variations in Pb and Bi contents and the single-crystal X-ray diffraction data suggest a general formula of pottsite as (Pb3xBi4-2x)(VO4)(4)center dot H2O, 0.8 < x < 1.0, and an ideal mineral formula of (Pb3Bi)Bi(VO4)(4)center dot H2O.