• journal_title：Mineralogical Magazine
• Contributor：L. Bindi ; F. Nestola ; A. Guastoni ; L. Secco
• Publisher：Mineralogical Society of Great Britain and Ireland
• Date：2010-
• Format：text/html
• Language：en
• Identifier：10.1180/minmag.2010.074.6.999
• journal_abbrev：Mineralogical Magazine
• issn：0026-461X
• volume：74
• issue：6
• firstpage：999
• section：Articles

The crystal structure of the rare mineral dalnegroite, Tl_5–xPb_2x(As,Sb)_21–xS₃₄ with x ≈ 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) Å, α = 95.70(4), β = 90.18(3), γ = 96.38(4)°, V = 5838.9(9) ų, 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 Å thick, at y ≈ 0.15 and 0.65, can be considered as derived from the SnS archetype; (2) a layer 13.6 Å thick, at y ≈ 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 chabournéite-dalnegroite family could play a central role in controlling different degrees of order, leading to different superstructures.