Publication Date:
2011-01-31
Description:
The crystal structure of the rare mineral dalnegroite, Tl5-xPb2x(As,Sb)21-xS34 with x {approx} 1, was determined for a crystal from Lengenbach, Binn Valley, Switzerland. The structure is triclinic, space group P1, with a = 16.218(3), b = 42.546(7), c = 8.558(1) A, {alpha} = 95.70(4), {beta} = 90.18(3), {gamma} = 96.38(4){degrees}, V = 5838.9(9) A3, Z = 4. Refinement of an isotropic model led to an R1 index of 0.0536 for 22226 observed reflections and 980 parameters, and R1 = 0.0590 for all 25266 independent reflections. Although dalnegroite cannot be considered a layered compound, its structure can be usefully described as a regular alternation of two kinds of layers stacked along the b axis, with four layers in the unit cell: (1) a layer 7.8 A thick, at y {approx} 0.15 and 0.65, can be considered as derived from the SnS archetype; (2) a layer 13.6 A thick, at y {approx} 0.35 and 0.85, derived from the PbS archetype. Different chemical compositions, such as Tl:Pb and Sb:As ratios, for different samples belonging to the chabourneite-dalnegroite family could play a central role in controlling different degrees of order, leading to different superstructures.
Print ISSN:
0026-461X
Electronic ISSN:
1471-8022
Topics:
Geosciences
Permalink
