Mineralogical Magazine, December 2009, Vol. 73(6), pp. 1027–1032

Dalnegroite, Tl5 xPb2x(As,Sb)21 xS34, a new thallium sulphosalt
from Lengenbach quarry, Binntal, Switzerland
F. NESTOLA1,*, A. GUASTONI1, L. BINDI2 AND L. SECCO1
1

Dipartimento di Geoscienze, Università degli Studi di Padova, Via Giotto 1, I-35137 Padova, Italy
Museo di Storia Naturale, sezione di Mineralogia, Università degli Studi di Firenze, Via La Pira, 4, I-50121
Firenze, Italy

2[Received 16 October 2009; Accepted 14 December 2009]

ABSTR ACT

Dalnegroite, ideally Tl4Pb2(As12Sb8)S20S34, is a new mineral from Lengenbach, Binntal, Switzerland. Itoccurs as anhedral to subhedral grains up to 200 mm across, closely associated with realgar, pyrite, Sbrich seligmanite in a gangue of dolomite. Dalnegroite is opaque with a submetallic lustre and shows abrownish-red streak. It is brittle; the Vickers hardness (VHN25) is 87 kg mm 2 (range: 69 101) (Mohshardness ~3 3A). In reflected light, dalnegroite is highly bireflectant and weakly pleochroic, from whiteto 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 withchabournéite, 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, a = 95.72(4)º, b = 90.25(4)º, g = 96.78(4)º, V = 5832(4) Å3, Z = 4.
The nine strongest powder-diffraction lines [d (Å) (I/I0) (hkl)] are: 3.927 (100) (2̄ 10 0); 3.775 (45)(22̄2); 3.685 (45) (4̄60); 3.620 (50) (440); 3.124 (50) (2̄8̄2); 2.929 (60) (42̄2); 2.850 (70) (4̄42); 2.579(45) (0 14 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.

K EY WORDS : dalnegroite, new mineral, thallium, arsenic, sulphosalt, Lengenbach, Switzerland.

Introduction
SINCE the late eighteenth century the Lengenbach
quarry, in Binntal, Switzerland, has been a uniquesource of a number of exclusive arsenic
sulphosalts with additional elements including
Tl, Pb, Ag and Cu. The quarry is the type locality
for 27 new mineral species (Graeser et al., 2008)
and, to date, 15 Tl-sulphosalts have been
discovered at this locality, of which twelve are
new species including: imhoﬁte Tl3As7.67S13(Burri et al., 1965); edenharterite TlPbAs3S6(Graeser and Schwander, 1992); jentschite
TlPbAs2SbS6 (Graeser and Edenharter, 1997);* E-mail: fabrizio.nestola@unipd.it
DOI: 10.1180/minmag.2009.073.6.1027# 2009 The Mineralogical Societysicherite TlAg2As3S6 (Graeser et al., 2001); andgabrielite Tl2AgCu2As3S7 (Graeser et al., 2006).
Here, we report the description and characterization of a new Tl-sulphosalt from theLengenbach quarry. Dalnegroite, ideally
Tl5 xPb2x(As,Sb)21 xS34 (x ~1) shows strong
chemical and structural afﬁnities with themineral chabournéite, Tl4Pb2(Sb,As)20S34 (Johanet al., 1981), and of which it is the As-dominantanalogue.
The name of the new mineral honors Alberto
Dal Negro (b. 1941), Professor of Mineralogy andCrystallography at the University of Padova since
1976, for his contribution to the discovery andcrystal-structure determination of several newmineral species. The new mineral and its namehave been approved by the Commission of New

F. NESTOLA ET AL.

Minerals, Nomenclature and Classiﬁcation,
I.M.A. (IMA 2009-058). The holotype is deposited in the collections of the Museum ofMineralogy of the Department of Geosciences at
the University of Padova under the catalogue
number MMP M7620.

Physical properties

Occurrence
The Lengenbach Pb-Zn-As-Tl-Ba mineralization
occurs in Triassic dolomitic rocks hosted in
Penninic nappes which reached lower amphibolites facies during Alpine metamorphism.
Sulphide melts, rich in As, Pb and Tl, coexistingwith hydrothermal ﬂuids, line cavities with anumber of As-Ag-Pb-Tl sulphosalt/sulphideassemblages. Geological and metallogenic dataconcerning the deposit are summarized by
Hofmann and Knill (1996) and Knill (1996).
The sample containing dalnegroite wasprovided by Luca Debattisti, an amateur collector
who collected the sample at the Lengebach quarry
in the summer of 2006. Dalnegroite, along with
abundant realgar and pyrite, is hosted incentimetre-scale irregular cavities in the marble,
where it forms anhedral to subhedral crystals(200 300 mm long) dark grey in colour, or occurs
as tiny parallel aggregates (Fig. 1). It shows no
obvious twinning, inclusions or intergrowths withother minerals. The mineral assemblage also
includes Sb-rich hutchinsonite, jordanite, Sb-richseligmanite and sinnerite.

Dalnegroite is opaque with a submetallic lustre
and possesses a brownish-red streak. It is brittle
with an uneven fracture and no cleavage.
Unfortunately, the density could not be measured
because of the small grain-size; the calculated
density is 4.82 g cm 3 (on the basis of theempirical formula based on 34 S atoms). Microindentation measurements carried out with a VHN
load of 25 g gave a mean value of 87 kg mm 2(range 69 101), corresponding to a Mohs hardness of 3 3A.
In plane-polarized reﬂected light, dalnegroite ishighly bireﬂectant and weakly pleochroic fromwhite to a slightly greenish-grey. In crosspolarized light, the mineral is highly anisotropic
with bluish to green rotation tints and red internal
reﬂections. Reﬂectance measurements were madein air with a MPM-200 Zeiss microphotometerequipped with a MSP-20 system processor on aZeiss Axioplan ore microscope. Filament temperature was approximately 3350 K. Readings
were taken for specimen and standard (SiC)
maintained under the same focus conditions.
The diameter of the circular measuring area was
0.1 mm. Reﬂectance percentages (Rmin and Rmax)
for dalnegroite for the four standard COM
wavelengths are: 45.6, 46.1 (471.1 nm); 46.1,46.6 (548.3 nm); 46.3, 47.0 (586.6 nm); 46.4, 47.1(652.3 nm) respectively. Figure 2 shows thereﬂectance percentages obtained for dalnegroite

FIG. 1. BSE image of dalnegroite associated with jordanite and corroded crystals of realgar. Scale bar is indicated.

1028

DALNEGROITE, A NEW THALLIUM SULPHOSALT

wavelength dispersive analysis (WDS) by meansof a JEOL JXA-8200 electron microprobe(accelerating potential 20 kV, probe current
20 nA, ﬁnal beam diameter 2 mm, with counting
times of 20 s on peaks and 10 s on backgrounds.
Element standards used for WDS analysis were as
follows: Pb metal (Pb-Ma line); synthetic TlI andTlBr (Tl-Ma line); Sb metal (Sb-Lb1 line);synthetic InAs (As-La line); natural FeS2 (S-Kaline). Analytical data are given in Table 1. The
crystal fragment was found to be quite homogeneous within analytical error (11 analyses ondifferent spots). The concentration ranges(in wt.%) for the elements measured are:Pb 8.5 12.4; Tl 19.6 21.5; Sb 22.2 25.6; As
21.0 21.8; S 26.0 26.4. The empirical formula,
based on a total of 34 sulphur atoms isTl4.15 Pb 2.03(As11.86Sb 8.20)S34. The simpliﬁed
formula is Tl5 xPb2x(As,Sb)21 xS34 (x ~1).

FIG. 2. Reﬂectivity curves for dalnegroite in air. Opensymbols refer to chabournéite, Tl5 xPb2x(Sb,As)21 xS34
(Johan et al., 1981), ﬁlled symbols refer to dalnegroite(this study). Circles and squares refer to Rmax and Rminvalues respectively.

and those measured by Johan et al. (1981) for
chabournéite, Tl5 x Pb 2x (Sb,As) 21 x S 34 , withwhich it is isotypic, as we discuss below.

X-ray Diffraction
Three fragments of dalnegroite were extracted
from a polished section and checked for
diffraction quality using Oxford Diffraction
Xcalibur 3 and STOE STADI4 (with CCDdetector) single-crystal diffractometers. The fragments gave extremely broad X-ray diffraction
proﬁles, thus indicating the powder study as theonly possible means of X-ray investigation. Fully
indexed 114.6 mm Gandolﬁ camera X-ray powder
data (Ni-ﬁltered Cu-Ka) are presented in Table 2.
The intensities were measured with an automatedChemical data
A preliminary chemical analysis using a JEOL
5610 LV equipped with an EDS detector(Bayerisches Geoinstitut, Bayreuth, Germany)
did not indicate the presence of any elementswith Z >9 other than Tl, Pb, As, Sb and S. The
chemical composition was then determined using

TABLE 1. Electron microprobe analyses and atomic ratios for the selected dalnegroite crystal.
1PbTl
Sb
As
STotal2345678

910

11

mean

11.15
9.96
9.93
8.60 11.23
8.46
9.49 10.77 12.38
8.80 10.18 10.09
20.33 20.07 20.06 20.39 19.76 21.50 21.15 20.65 19.56 20.76 19.77 20.36
23.73 24.66 25.23 25.57 23.85 24.03 24.10 22.56 22.18 23.63 23.94 23.95
21.51 21.56 21.13 21.09 21.16 21.45 21.05 21.50 21.84 21.21 21.11 21.33
26.13 26.37 26.20 26.06 26.04 26.30 26.15 26.17 26.17 25.99 26.22 26.16
102.85 102.62 102.55 101.71 102.04 101.74 101.94 101.65 102.13 100.39 101.22 101.89

Atomic ratios based on 34 S atoms
Pb
2.24
1.99
1.99
Tl
4.15
4.06
4.08
Sb
8.13
8.37
8.62
As
11.98 11.90 11.73

1.74
4.17
8.78
11.77

2.27
4.05
8.20
11.82

1.69
4.36
8.18
11.87

1.91
4.31
8.25
11.71

2.17
4.21
7.72
11.95

2.49
3.99
7.59
12.14

1.78
4.26
8.14
11.87

Probe standards: Pb (metal); TlI, TlBr (synthetic); Sb (metal); InAs (synthetic);FeS2 (natural)
Average errors: Pb = 1.5%, Tl = 1.0%, Sb = 1.2%, As = 0.5%, S = 0.8%

1029

2.04
4.02
8.17
11.71

2.03
4.15
8.20
11.86

F. NESTOLA ET AL.

densitometer. The unit-cell parameters were
derived by means of the program TREOR(Werner et al., 1985) and by comparison witht h e i s o t y p i c c h a b o u r n é i t e , T l 5 x P b 2 x(Sb,As)21 xS34 (Nagl, 1979; Johan et al., 1981).
Dalnegroite is triclinic with reﬁned unit-cellparameters, based on 63 reﬂections between
7.20 and 1.785 Å, of: a = 16.217(7) Å, b =42.544(9) Å, c = 8.557(4) Å, a = 95.72(4)º, b =90.25(4)º, g = 96.78(4)º, V = 5832(4) Å3, Z = 4.
Relationship to chabourne¤ite
According to Moëlo et al. (2008), the structural
formula proposed for chabournéite by Nagl(1979) for a b/2 sub-cell is Tl8Pb4Sb21As19S68.
This formula was questioned by Johan et al.

(1981), who proposed the formula
Tl21(Sb,As)91S147 for the unit cell of the Pb-freemember, but this formula shows clearly a sulphurexcess (when normalized to the same number ofatoms as in the structural formula obtained byNagl (1979) that is incompatible with the modular
topology of the crystal structure. According to thesubstitution Tl + + (Sb, As) 3+ ? 2 Pb 2+ ,demonstrated by Johan et al. (1981), the general
simpliﬁed formula should be written as
Tl5 xPb2x(Sb,As)21 xS34. The Pb-free pole (x =0) corresponds to Tl5(Sb,As)21S34, while the Pbrich composition studied by Nagl (x ~ 1) is close
to Tl4Pb2(Sb,As)20S34. Hence, dalnegroite is theAs-dominant analogue of the chabournéite studiedby Nagl (1979) having x ~ 1. The two minerals
have very similar X-ray powder-diffraction

TABLE 2. X-ray powder data for dalnegroite.
Id(meas.)d(calc.)hkl

Id(meas.)d(calc.)

hkl

20
10
35
25
15
5
540
100
25
525
45
20
45
50
25
40
35
20
25
30
25
20
10
50
20
530
20
60
30

7.20
6.78
4.67
4.27
4.21
4.10
4.09
4.02
3.927
3.921
3.813
3.810
3.775
3.690
3.685
3.620
3.600
3.551
3.524
3.392
3.379
3.363
3.287
3.277
3.151
3.124
3.113
3.097
3.084
2.949
2.929
2.916

7.20656.79334.66144.25644.2031
4.09244.09164.02533.92263.91823.81363.81263.77503.69353.68503.61773.60333.5561
3.52653.39673.38073.36173.28653.2766
3.15533.1235
3.1167
3.09743.0803
2.94902.93602.9145

1̄
2̄
2̄
00
0042̄
4̄
002
24̄
42̄
22̄
4̄
2̄2̄2̄
202̄
24̄
24̄
442̄
482̄
1024̄
0
10446̄
2̄
4̄
644̄
10486̄
1268̄
88̄
610
120
2̄
4̄

100
202
200
0
22220
0
20
20
20
2222
200
222107030
20520252540
20
1025
4530
525
520
1530
1520
60
10
51015
15
515
5

2.853
2.850
2.827
2.813
2.768
2.760
2.735
2.710
2.702
2.618
2.613
2.590
2.579
2.547
2.493
2.390
2.315
2.198
2.194
2.141
2.133
2.127
2.097
2.050
2.027
2.000
1.998
1.958
1.910
1.893
1.785

2.85762.85222.82652.81542.76812.76192.73502.70932.70122.61262.61212.59002.57912.54482.49902.38812.3153
2.19682.19382.14102.13322.12662.09642.05012.02662.0001
1.9975
1.95791.9105
1.89191.7845

24̄
4̄
4
2̄
24̄
2
4̄
26̄
2̄
0
4
4
4
2̄
6
2̄
0
4̄
6
0
6̄
8̄
6̄
26̄
0
68̄

8
44̄
2
10
126
146̄10616146128148̄161618102102168̄1222144̄2222222022002202222202420042002

The strongest intensities are reported in bold.

1030

DALNEGROITE, A NEW THALLIUM SULPHOSALTTABLE 3. Comparison between dalnegroite and chabournéite.

Reference
Chemical formula
Unit-cell parameters

Density (calc.)
VHN25
Strongest diffractionlines (Å)

Dalnegroite

ChabournéiteThis study
Tl5-xPb2x(As,Sb)21-xS34 (x ~1)
a = 16.217(Å)
b = 42.544(Å)
c = 8.557(Å)
a = 95.72(º)
b = 90.25(º)
g = 96.78(º)
V = 5832(4) Å34.819 g cm 369 101 Kg mm 23.927, 2.850, 2.929, 2.097, 3.620,3.124, 2.775, 3.685(Nagl, 1979; Johan et al., 1981)
Tl4Pb2(Sb,As)21S34
a = 16.320(Å)
b = 42.636(Å)
c = 8.543(Å)
a = 83.98(º)
b = 89.06(º)
g = 83.20(º)
V = 5870 Å34.88 g cm 378 124 Kg mm 23.573, 2.135, 2.808, 3.928, 3.358,2.853, 2.709

patterns and, consequently, very similar unit-cellparameters (Table 3). The a and b parameters ofdalnegroite are slightly shorter than those of
chabournéite (Nagl, 1979) owing to the smaller
size of As relative to Sb.
One of the difﬁculties encountered during adescription of a complex metallic mineral forwhich a complete crystal-structure characterization is not possible because of small crystal size,
complex twinning and/or intergrowths, is thevalence state of the elements. On the basis of
the analysed formula Tl5 xPb2x(As,Sb)21 xS34 fordalnegroite, the formal charge balance is assumed
by considering the normal valence states of theelements: Tl + , Pb 2+ , As 3+ , Sb 3+ and S 2 .
Furthermore, it should be pointed out thatdalnegroite could be also described by a triclinic
unit cell with b’ = b/2 = 21.272 Å, as all themeasured reﬂections have k = 2n. Moreover, the aand c values of dalnegroite depend only on thepresence of one reﬂection (i.e. hkl: 1̄2̄1). A recent
single-crystal X-ray diffraction study of chabournéite (Bonaccorsi, pers. comm.) indicates that its
a and c parameters are halved and that thestructure is centrosymmetric (i.e. P1̄). As a fullstructural study of dalnegroite remains to be
carried out, discussion of charge balance, degreeof metallic bonding, unit-cell type and possible
structural models must await the availability ofbetter crystals.
Acknowledgements
The authors wish to thank Anke Potzel and Detlef
Krauße from Bayerisches Geoinstitut (Bayreuth,Germany) for access to the WDS electron
microprobe facility and Luca De Battisti forproviding the sample. The manuscript took
advantage from the revision of Anna Garavelli
and Yves Moëlo. Financial support was provided
by P.R.I.N 2006047943.
References
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(1965) Imhoﬁt, ein neues thallium-arsensulfosalz aus
dem Lengenbach (Binnatal, Kt. Wallis). Chimia, 19,499 500.
Graeser, S. and Edenharter, A. (1997) Jentschite(TlPbAs2SbS6); a new sulphosalt mineral from
Lengenbach, Binntal (Switzerland). Mineralogical
Magazine, 61, 131 137.
Graeser, S. and Schwander, H. (1992) Edenharterite(TlPbAs3S6): a new mineral from Lengenbach,Binntal (Switzerland). European Journal ofMineralogy, 4, 1265 1270.
Graeser, S., Berlepsch, P., Makovicky, E. and BalićŽunić, T. (2001) Sicherite, TlAg2(As,Sb)3S6, a new
sulfosalt mineral from Lengenbach (Binntal,Switzerland): description and structure determination. American Mineralogist, 86, 1087 1093.
Graeser, S., Topa, D., Balić-Žunić, T. and Makovicky,E. (2006) Gabrielite, Tl2AgCu2As3S7, a new speciesof thallium sulfosalt from Lengenbach, Binntal,
Switzerland. The Canadian Mineralogist, 44,
135 140.
Graeser, S., Cannon, R., Drechser, E., Raber, T. andRoth, P. (2008) Faszination Lengenbach abbau Forschung - Mineralien 1958 2008.
Kristallographik Verlag, Achberg, 192 pp.
Hofmann, B.A. and Knill, M.D. (1996) Geochemistry1031

F. NESTOLA ET AL.

and genesis of the Lengenbach Pb-Zn-As-Tl-Bamineralisation, Binn Valley, Switzerland.
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Johan, Z., Mantienne, J. and Picot, P. (1981) La
chabournéite, un nouveau minéral thallifère.
Bulletin de Minéralogie, 104, 10 15.
Knill, M. (1996) The Pb-Zn-As-Tl-Ba-deposit at
Lengenbach, Binn Valley, Switzerland. Beiträge zur
Geologie der Schweiz, Geotechnische Serie 90;
Hrsg.: Schweizerische Geotechnische Kommission,
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Moëlo, Y., Makovicky, E., Mozgova, N.N., Jambor,J.L., Cook, N., Pring, A., Paar, W., Nickel, E.H.,
Graeser, S., Karup-Møller, S., Balić-Žunić, T.,

Mumme, W.G., Vurro, F., Topa, D., Bindi, L.,
Bente, K. and Shimizu, M. (2008) Sulfosalt
systematics: a review. Report of the sulfosalt subcommittee of the IMA Commission on Ore
Mineralogy. European Journal of Mineralogy, 20,7 46.
Nagl, A. (1979) The crystal structure of a thallium
sulfosalt, Tl 8 Pb 4 Sb 21 As 19 S 68 . Zeitschrift für
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TREOR, a semi-exhaustive trial-and-error powderindexing program for all symmetries. Journal ofApplied Crystallography, 18, 367 370.

1032

ERRATUM

Dalnegroite, Tl5 xPb2x(As,Sb)21 xS34, a new thallium sulphosalt
from Lengenbach quarry, Binntal, Switzerland
In the above paper by Nestola et al., published in the December 2009 issue of Mineralogical Magazine, amineral was incorrectly named. All references, including that in Fig. 1, to jordanite (Pb14As6S23), should be
replaced by sartorite (PbAs2S4).
Under the heading of Occurrence, the authors also wish to point out that the sample containing dalnegroite
was provided by Luca De Battisti, a mineral collector, who collected the sample at the Lengenbach quarry inthe summer of 2006.

187