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Dalnegroite, Tl5–xPb2x(As,Sb)2l–xS34, a new thallium sulphosalt from Lengenbach quarry, Binntal, Switzerland

Published online by Cambridge University Press:  05 July 2018

F. Nestola ,
A. Guastoni ,
L. Bindi  and
L. Secco
F. Nestola*
Affiliation:
Dipartimento di Geoscienze, Universitá degli Studi di Padova, Via Giotto 1, 1-35137 Padova, Italy
A. Guastoni
Affiliation:
Dipartimento di Geoscienze, Universitá degli Studi di Padova, Via Giotto 1, 1-35137 Padova, Italy
L. Bindi
Affiliation:
Museo di Storia Naturale, sezione di Mineralogia, Universitá degli Studi di Firenze, Via La Pira, 4, 1-50121 Firenze, Italy
L. Secco
Affiliation:
Dipartimento di Geoscienze, Universitá degli Studi di Padova, Via Giotto 1, 1-35137 Padova, Italy
*
* E-mail: fabrizio.nestola@unipd.it
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Abstract

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, Tl5-xPb2x(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) Å3, 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 Tl4.15Pb2.03(As11.86Sb8.20)S34. The new mineral is named for Alberto Dal Negro, Professor in Mineralogy and Crystallography at the University of Padova since 1976.

Keywords

dalnegroitenew mineralthalliumarsenicsulphosaltLengenbachSwitzerland
Type
Research Article
Information
Mineralogical Magazine , Volume 73 , Issue 6 , December 2009 , pp. 1027 - 1032
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2009

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