• journal_title：Mineralogical Magazine
• Contributor：F. Nestola ; A. Guastoni ; L. Bindi ; L. Secco
• Publisher：Mineralogical Society of Great Britain and Ireland
• Date：2009-
• Format：text/html
• Language：en
• Identifier：10.1180/minmag.2009.073.6.1027
• journal_abbrev：Mineralogical Magazine
• issn：0026-461X
• volume：73
• issue：6
• firstpage：1027
• section：Articles

Dalnegroite, ideally Tl₄Pb₂(As₁₂Sb₈)_Σ20S₃₄, is a new mineral from Lengenbach, Binntal, Switzerland. It occurs as anhedral to subhedral grains up to 200 μm across, closely associated with realgar, pyrite, Sb-rich seligmanite in a gangue of dolomite. Dalnegroite is opaque with a submetallic lustre and shows a brownish-red streak. It is brittle; the Vickers hardness (VHN₂₅) is 87 kg mm^-2 (range: 69-101) (Mohs hardness ∼3-3½). In reflected light, dalnegroite is highly bireflectant and weakly pleochroic, from white to a slightly greenish-grey. In cross-polarized light, it is highly anisotropic with bluish to green rotation tints and red internal reflections.

According to chemical and X-ray diffraction data, dalnegroite appears to be isotypic with chabournéite, Tl_5-xPb_2x(Sb,As)_21-xS₃₄. It is triclinic, probable space group P1, with a = 16.217(7) Å, b = 42.544(9) Å, c = 8.557(4) Å, α = 95.72(4)°, β = 90.25(4)°, γ = 96.78(4)°, V = 5832(4) ų, Z = 4.

The nine strongest powder-diffraction lines [d (Å) (I/I₀) (hkl)] are: 3.927 (100) ( 10 0); 3.775 (45) (22); 3.685 (45) (60); 3.620 (50) (440); 3.124 (50) (2); 2.929 (60) (42); 2.850 (70) (42); 2.579 (45) (0 2); 2.097 (60) (024). The mean of 11 electron microprobe analyses gave elemental concentrations as follows: Pb 10.09(1) wt.%, Tl 20.36(1), Sb 23.95(1), As 21.33(8), S 26.16(8), totalling 101.95 wt.%, corresponding to Tl_4.15Pb_2.03(As_11.86Sb_8.20)S₃₄. The new mineral is named for Alberto Dal Negro, Professor in Mineralogy and Crystallography at the University of Padova since 1976.