Museumite, Pb5AuSbTe2S12, a new mineral from the gold

Eur. J. Mineral.
2004, 16, 835-838
Museumite, Pb5AuSbTe2S12, a new mineral from the gold-telluride
deposit of Sacarîmb, Metaliferi Mountains, western Romania
LUCA BINDI and CURZIO CIPRIANI
Museo di Storia Naturale, sezione di Mineralogia, Università degli Studi di Firenze,
Via La Pira 4, I-50121, Firenze, Italy
Abstract: Museumite, ideally Pb5AuSbTe2S12, is a newly identified mineral from the gold-telluride deposit of Sacarîmb,
Metaliferi Mountains, western Romania. The mineral occurs as anhedral to subhedral grains up to 300 µm in cavities and vugs of
large nagyágite crystals and it does not show any inclusion or intergrowth of other minerals. The associated minerals are nagyágite,
hessite, sylvanite, petzite and coloradoite, whereas the gangue minerals are calcite and quartz. Museumite is dark silver-grey in
colour and shows a grey-black streak. It has a perfect {001} cleavage, the fracture is hackly and the Vickers hardness (VHN15) is
42 kg/mm2. Museumite is greyish white in reflected light, with very low bireflectance and pleochroism. With crossed polars it
shows distinct anisotropism, similar to nagyágite, but slightly stronger. Internal reflections are absent. Reflectance percentages for
Rmin and Rmax are 38.4, 40.3 (471.1 nm), 38.1, 40.1 (548.3 nm), 37.5, 39.4 (586.6 nm), and 35.9, 38.0 (652.3 nm), respectively.
Museumite is monoclinic, space group P21 or P21/m, with the following unit-cell parameters: a = 4.361(2) Å, b = 6.618(3) Å,
c = 20.858(9) Å, β = 92.71(5)°, V = 601.3(5) Å3. The strongest five powder-diffraction lines [d in Å (I/I0) (hkl)] are: 4.80 (52)
(013); 3.56 (100) (111); 3.47 (58) ( 112); 2.99 (50) (023); 2.56 (41) ( 116). The average of 25 electron microprobe analyses gave
Pb 52.0(4), Au 10.7(1), Sb 6.2(2), Te 11.7(2), S 19.4(2), total 100.0 wt. %, corresponding, on the basis of total atoms = 21, to
Pb5.00Au1.08Sb1.01Te1.83S12.08.
Key-words: museumite, new mineral, Sacarîmb, Romania, nagyágite-buckhornite.
1. Introduction
The new mineral species described herein, museumite,
Pb5AuSbTe2S12, is from the gold-telluride deposit of
Sacarîmb (the former Nagyág), Metaliferi Mountains,
western Romania, a well-known source of rare telluride
minerals (i.e., petzite, stützite, krennerite, nagyágite and
muthmannite). Although the morphology and chemistry of
this mineral resemble those of nagyágite, [Pb2(Pb,Sb)2S4]
[(Te,Au)]2, museumite was initially recognised as being a
new species, due to its sulphur content, higher than that
required by the crystal-chemical formula of nagyágite.
The rock-sample containing the new mineral was not
found in situ but it is an old sample of the mineralogical
collection of the Natural History Museum of the
University of Florence, bought by the former Director of
our Museum, Giuseppe Grattarola, in 1890’s from the
Geol. Company of Geneva, and labelled “nagyágite,
Nagyág – Transylvanie”. In order to give the proper credit
to all museums in the world preserving their old samples
with care and accuracy we have named the new mineral
museumite. Type-material is housed in the mineralogical
collection of the Natural History Museum of the
University of Florence under the catalogue number 899/G.
The new mineral and mineral name have been approved by
the IMA Commission on New Minerals and Mineral
Names (2003-39).
2. Occurrence
The Sacarîmb gold-telluride deposit is located in the
southeastern part of the Metaliferi Mountains, western
Romania. Although now close to exhaustion, it is one of the
most famous Neogene epithermal deposits in the world. As
reported by Simon et al. (1994), the telluride-bearing veins
are located in a volcanic body consisting of hornblendeand pyroxene-bearing quartz-andesites of Neogene age.
The host volcanic rocks exhibit a pervasive propylitic alteration, while argillic alteration is dominant adjacent to the
veins. Ghitulescu & Soculescu (1941) and Udubasa et al.
(1992) summarised the dominant mineral assemblages in
the three main vein groups of Sacarîmb. The Magdalena
vein group, in the southeastern part of the mine, consists of
quartz, rhodochrosite, nagyágite and abundant base-metal
sulphides. The Longin vein group, in the northeastern part
of the mine, consists of quartz, baryte, rhodochrosite,
sylvanite, krennerite, gold and subordinate base-metal
sulphides. The Nepomuc vein group, in the southwestern
part of the mine, consists of calcite, petzite and alabandite.
*E-mail: [email protected]
0935-1221/04/0016-0835 $ 1.80
DOI: 10.1127/0935-1221/2004/0016-0835
© 2004 E. Schweizerbart’sche Verlagsbuchhandlung. D-70176 Stuttgart
836
L. Bindi, C. Cipriani
Fig. 1. BSE microphotographs of a cavity in nagyágite (nag) filled by museumite (mus).
As described in the former section, the sample
containing the new mineral museumite was not found in
situ but came from the mineralogical collection of the
Natural History Museum of the University of Florence,
where it was simply labelled “nagyágite, Nagyág –
Transilvania, Romania”. However, the mineral assemblage
suggests that this sample comes from the pyroxene quartzandesites of the Magdalena vein group. The associated
minerals are nagyágite, hessite, sylvanite, petzite and
coloradoite whereas the gangue minerals are calcite and
quartz. The mineral occurs as anhedral to subhedral grains
up to 300 µm in cavities and vugs of large nagyagite crystals (Fig. 1), and it does not show any inclusion or intergrowth of other minerals. The contacts between the “vug”
mineral (museumite) and the nagyágite “host” are usually
sharp with evidence of replacement.
and shows a grey colour with a slightly greenish tint. With
crossed polars, museumite shows distinct anisotropism,
similar to nagyágite, but slightly stronger. Internal reflections
are absent. No evidence of growth zonation is observed.
Reflectance measurements were performed in air by
means of a MPM-200 Zeiss microphotometer equipped
with a MSP-20 system processor on a Zeiss Axioplan ore
microscope. Filament temperature was approximately 3350
K. An interference filter was adjusted, in turn, to select four
wavelengths for measurement (471.1, 548.3, 586.6, and
652.3 nm). 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 for Rmin and Rmax are 38.4, 40.3
(471.1 nm), 38.1, 40.1 (548.3 nm), 37.5, 39.4 (586.6 nm),
and 35.9, 38.0 (652.3 nm), respectively. These data are
consistent with the visual impression of very low bireflection.
3. Physical properties
Museumite is dark silver-grey in colour and shows a
grey-black streak. The mineral is opaque with a metallic
luster. It has a perfect {001} cleavage and the fracture is
hackly. As a common feature for several telluride minerals
[i.e., tetradymite group (Bayliss, 1991; Clarke, 1997;
Spiridinov et al., 1989); nagyágite-buckhornite series
(Effenberger et al., 1999, 2000)], some crystal fragments
exhibit a platy to flaky morphology. The dominant form is
{001} and twinning is not observed. Unfortunately the
density could not be measured because of the small grain
size. The micro-indentation measurements carried out with
a VHN load of 15g gave the mean value of 42 kg/mm2
(range: 41-44) corresponding to a Mohs hardness of about
1-11/2.
4. Optical properties
In plane-polarized incident light museumite is greyish
white in colour, with very low bireflectance and
pleochroism. When observed near nagyágite it is darker
Fig. 2. Reflectivity curves for museumite, nagyágite, and arsenian
nagyágite. Upward-triangles refer to nagyágite (Simon et al.,
1994), downward-triangles refer to arsenian nagyágite (Simon et
al., 1994), circles refer to museumite (this study). Filled and open
symbols refer to Rmax and Rmin values, respectively.
Museumite: a new mineral from Romania
As clearly made evident in Fig. 2, the reflectance
percentages obtained for museumite are very similar with
the values measured by Simon et al. (1994) for both
nagyágite and arsenian nagyágite from Sacarîmb. The
sulphur enrichment in the mineral studied here accounts for
the slightly lower reflectivity values observed.
5. Chemical composition
A crystal fragment of museumite was analysed by
means of a Jeol JXA-8600 electron microprobe. Major and
minor elements were determined at 20 kV accelerating
voltage and 40 nA beam current, with variable counting
times: 30 s were used for Pb, Sb, Au, Te and S and 60 s for
the minor elements Fe, As, Cu, Bi, and Ag. For the WDS
analyses the following lines were used: PbMα, SbLα,
AuMα, TeLα, SKα, FeKα, AsLα, CuKα, BiMα, AgLα. The
estimated analytical precision (in wt. %) is: ± 0.40 for Pb;
± 0.20 for Sb, Te and S; ± 0.10 for Au; ± 0.05 for Ag and
As; ± 0.04 for Bi; ± 0.02 for Cu; ± 0.01 for Fe. The standards employed were: galena (Pb, S), Au- pure element
(Au), Ag- pure element (Ag), synthetic GaAs (As), Cupure element (Cu), marcasite (Fe), synthetic Sb2Te3 (Sb,
Te), Bi- pure element (Bi). The crystal fragment was found
to be homogeneous within the analytical error. The average
chemical composition (25 analyses on different spots),
together with ranges of wt. % of elements, is reported in
Table 1. On the basis of 21 atoms, the empirical formula of
museumite is Pb5.00Au1.08Sb1.01Te1.83S12.08, ideally
Pb5AuSbTe2S12.
6. X-ray crystallography
For single-crystal X-ray investigation several crystal
fragments were checked by Weissenberg and precession
film techniques and with a Nonius CAD4 four-circle
diffractometer. Due to the limited diffraction quality of
the crystals available, only after many trials a fragment
of museumite (110 x 130 x 180 µm) was found to be
suitable for the structural study, which was performed
by means of the precession method employing
Zr-filtered Mo radiation. The fragment was oriented
such that a* was coincident with the dial axis and,
subsequently, with 011* coincident with the dial axis.
The levels collected were: hk0 → hk2, h0l → h2l, and
0kl → 2kl. No evidence of twinning was observed on the
precession films. Museumite is monoclinic with
measured cell-parameters derived from precession films
of: a = 4.355 Å, b = 6.620 Å, c = 20.87 Å, β = 92.56°.
The only observed extinction rule, 0k0 with k = 2n + 1,
indicates that P21 or P21/m are the permissible spacegroup choices. Fully indexed 114.6 mm Gandolfi
camera X-ray powder data (Ni-filtered CuKα) are
presented in Table 2. The intensities were measured with
an automated densitometer. The refined unit-cell parameters for museumite, based on 60 reflections between
20.85 and 1.261 Å, aided by visual inspection of
precession films, are: a = 4.361(2) Å, b = 6.618(3) Å,
837
Table 1. Chemical composition (means and ranges of elements in
wt. %) for museumite.
Pb
Au
Ag
As
Cu
Fe
Sb
Bi
Te
S
wt. %
52.00
10.68
0.00
0.00
0.00
0.00
6.16
0.00
11.71
19.43
total
99.98
range
51.97 – 52.08
10.66 – 10.77
0.00 – 0.09
0.00 – 0.10
0.00 – 0.03
0.00 – 0.01
6.08 – 6.19
0.00 – 0.05
11.61 – 11.74
19.40 – 19.56
c = 20.858(9) Å, β = 92.71(5)°, V = 601.3(5) Å3, and a :
b : c = 0.6590 : 1 : 3.1517.
7. Relations with the nagyágite-buckhornite
homologous series
The strong chemical similarity between museumite and
the minerals of the nagyágite – buckhornite series
(Effenberger et al., 1999, 2000) led initially the authors to
suppose that museumite was the third member of the same
homologous series. The formula of museumite, indeed, can
also be written (based on 14 atoms) as [Pb2(Pb1.33
Sb0.67)Σ=2.00S8.05][Au1.22Te0.72]Σ=1.94, that
compares
favourably with the simplified formula of nagyagite,
[Pb2(Pb,Sb)2S4] [(Te,Au)]2, reported by Effenberger et al.
(1999). In addition, nagyágite and museumite are strongly
similar in morphology and physical properties. However, a
careful examination of the strict relationships between a
and b unit-cell parameters, and consequently of the Au-Te
layer, in the crystal structure of nagyágite (Effenberger et
al., 1999) and buckhornite (Effenberger et al., 2000) with
respect to museumite, showed that the latter has not to be
considered a homologue of nagyágite-buckhornite.
Moreover, if museumite belonged to nagyágite-buckhornite series the most probable chemical formula would be
{M9S9}{(Au,Te)3}, or more detailed {M9S9}{AuTe2}, with
M = Pb, Sb. The smallest unit-cell for an ordered structure
would have a ~ 4.3 Å, b ~ 12.9 Å, c ~ 20.9 Å, V ~ 1159 Å3,
α ~ β ~ γ ~ 90°, Z = 2, whereas we do not observed any
evidence for a doubling of the b axis in the precession
films.
Another hint distinguishing museumite from
nagyágite-buckhornite is given by the chemical composition. In both nagyágite and buckhornite the formal
charge balance is assured by considering the following
valence states of the elements: Pb2+, Sb3+, Bi3+, Au3+,
Te2- and S2-. If we apply these valence states to museumite we find a negatively charged sulphide layer, i.e.
[Pb2+2(Pb2+1.33 Sb3+0.67)Σ=2.00 S2-8.05]-7.43. The charge
balance in museumite could be assured if all the sulphur
atoms were bounded into covalent pairs or into other
838
L. Bindi, C. Cipriani
Table 2. X-ray powder diffraction pattern for museumite.
I
dmeas
dcalc
h
k
l
I
dmeas
dcalc
h
k
l
20
38
11
13
52
40
7
20
15
7
100 3.
58
5
40
20
5
4
50
30
18
22
5
41
11
6
3
3
9
6
28
20.85
6.93
6.31
5.21
4.80
4.10
3.95
3.76
3.65
3.62
56
3.47
3.42
3.31
3.27
3.08
3.03
2.99
2.98
2.80
2.69
2.60
2.56
2.48
2.46
2.44
2.40
2.32
2.26
2.21
20.8351
6.9450
6.3074
5.2088
4.7911
4.0931
3.9530
3.7713
3.6386
3.6143
3.5606
3.4781
3.4220
3.3090
3.2665
3.0848
3.0397
2.9872
2.9764
2.7930
2.6888
2.6049
2.5631
2.4873
2.4640
2.4406
2.3955
2.3246
2.2606
2.2129
0
0
0
0
0
0
1
1
1
1
1
1
1
0
1
1
1
0
0
0
1
1
1
1
1
1
0
1
1
0
0
0
1
0
1
1
0
0
1
0
1
1
0
2
0
0
1
2
0
2
1
2
1
2
1
2
2
2
1
2
1
3
1
4
3
4
2
3
0
3
1
2
4
0
4
5
4
3
7
4
5
1
6
3
6
3
6
4
7
7
11
7
3
6
22
3
7
5
15
18
12
6
8
4
7
8
11
5
4
8
8
7
6
3
11
3
4
3
3
3
2.170
2.120
2.110
2.100
2.080
2.050
2.030
2.010
1.982
1.961
1.940
1.889
1.821
1.810
1.775
1.767
1.728
1.712
1.695
1.653
1.557
1.520
1.517
1.487
1.449
1.415
1.359
1.318
1.280
1.261
2.1769
2.1285
2.1121
2.1025
2.0793
2.0469
2.0313
2.0076
1.9890
1.9554
1.9414
1.8938
1.8186
1.8071
1.7803
1.7651
1.7280
1.7110
1.6920
1.6545
1.5543
1.5198
1.5155
1.4885
1.4461
1.4141
1.3573
1.3172
1.2809
1.2630
2
1
0
2
0
3
1
1
3
1
1
3
3
1
1
0
2
3
3
0
2
4
3
2
3
1
4
1
2
4
1
5
1
6
2
3
8
2
4
3
9
1
2
4
11
6
2
5
6
8
5
0
8
8
9
13
7
2
14
1
15
1
more complex groups (e.g., [S3]). On the other hand, an
alternative hypothesis of layers formed by only positively charge cations (Au+3 and Te+4) seems to be
extremely unlikely.
It is in our opinion that the data presented here
substantially characterise the new mineral museumite,
although a full structural study remains to be accomplished. Discussion on charge balance, degree of
metallic bonding in the mineral and ordered-disordered
structural model must await the availability of suitable
crystals, although the soft platy nature of the mineral
makes them unlikely to be easily found.
Acknowledgements: We are deeply indebted with
Herta Effenberger and Emil Makovicky for their careful
and helpful reviews. The authors wish to thank the late
Giuseppe Mazzetti (Museo di Storia Naturale
dell’Università di Firenze, sezione di Mineralogia) for
his help in the microhardness measurements, Daniele
Borrini (Dipartimento di Scienze della Terra, Università
di Firenze) for his help in reflectance measurements, and
Filippo Olmi (CNR – Istituto di Geoscienze e
Georisorse – sezione di Firenze) for his help during the
electron microprobe analyses. Financial support was
provided by the University of Florence (60 % grant) and
by M.I.U.R., cofinanziamento 2003, project “crystal
chemistry of metalliferous minerals” issued to Curzio
Cipriani.
2
0
1
2
0
2
1
1
1
2
0
2
2
1
1
2
2
0
1
2
1
1
0
3
0
2
1
1
References
Bayliss, P. (1991): Crystal chemistry and crystallography of some
minerals in the tetradymite group. Am. Mineral., 76, 257-265.
Clarke, R.M. (1997): Saddlebackite, Pb2Bi2Te2S3, a new mineral
species from the Boddington gold deposit, Western Australia.
Austral. J. Mineral., 3(2), 119-124.
Effenberger, H., Culetto, F.J., Topa, D., Paar, W.H. (2000): The
crystal structure of synthetic buckhornite, [Pb2BiS3][AuTe2]. Z.
Kristallogr., 215, 10-16.
Effenberger, H., Paar, W.H., Topa, D., Culetto, F.J., Giester, G.
(1999): Toward the crystal structure of nagyagite,
[Pb(Pb,Sb)S2][(Au,Te)]. Am. Mineral., 84, 669-676.
Ghitulescu, T.P. & Socolescu, M. (1941): Etude géologique et minière
des Monts Métallifères. An. Inst. Geol. Roum., XXI, 181-463.
Simon, G., Alderton, D.H.M., Bleser, T. (1994): Arsenian nagyagite
from Sacarîmb, Romania: a possible new mineral species.
Mineral. Mag., 58, 473-478.
Spiridinov, E.M., Ershova, N.A., Tananaeva, O.I. (1989):
Kochkarite PbBi4Te7 – A new mineral from contact metamorphosed ores. Geol. Rud. Mest., 31(4), 98-102 (in Russian).
Udubasa, G., Strusievicz, R.O., Dafin, E., Verdes, G. (1992):
Excursion guide to “The first National Symposium on
Mineralogy in Romania”. Rom. J. Mineral., 75(2), 19-31.
Received 13 October 2003
Modified version received 2 February 2004
Accepted 12 May 2004