Mineralogical Magazine, February 2016, Vol. 80(1), pp. 175–186

Ferrostalderite, CuFe2TlAs2S6, a new mineral from
Lengenbach, Switzerland: occurrence, crystal structure, andemphasis on the role of iron in sulfosalts
CRISTIAN BIAGIONI1,*, LUCA BINDI2, FABRIZIO NESTOLA3, RALPH CANNON4, PHILIPPE ROTH5 AND THOMAS RABER6
1
2
3
4
5
6

Dipartimento di Scienze della Terra, Università di Pisa, Via Santa Maria, 53, I-56126 Pisa, Italy
Dipartimento di Scienze della Terra, Università degli Studi di Firenze, Via G. La Pira, 4, I-50121 Firenze, Italy
Dipartimento di Geoscienze, Università di Padova, Via Gradenigo, 6, I-35131 Padova, Italy
FGL (Forschungsgemeinschaft Lengenbach), Breite Steine, CH-3996 Binn, Switzerland
FGL (Forschungsgemeinschaft Lengenbach), Ilanzhofweg 2, CH-8057 Zurich, Switzerland
FGL (Forschungsgemeinschaft Lengenbach), Edith-Stein-Str. 9, D-79110 Freiburg, Germany[Received 25 February 2015; Accepted 30 March 2015; Associate Editor: Ed Grew]
ABSTR ACTThe new mineral species ferrostalderite, CuFe2TlAs2S6, was discovered in the Lengenbach quarry, Binn
Valley, Wallis, Switzerland. It occurs as minute, metallic, black, equant to prismatic crystals, up to 50 μm,
associated with dolomite, realgar, baumhauerite (?) and pyrite. Minimum and maximum reflectance data for
COM wavelengths in air are [λ (nm): R (%)]: 471.1: 24.2/25.4; 548.3: 23.7/24.7; 586.6: 22.9/23.8; 652.3:
21.0/22.0. Electron microprobe analyses give (wt.%): Cu 6.24(25), Ag 4.18(9), Fe 9.95(83), Zn 4.46(91),
Hg 1.22(26), Tl 26.86(62), As 19.05(18), Sb 0.63(6), S 25.39(47), total 97.98(72). On the basis of 12 atoms
per formula unit, the chemical formula of ferrostalderite is Cu0.75(2)Ag0.30(1)Fe1.36(10)Zn0.52(11)Hg0.05(1)Tl1.00(1)[As1.94(4)Sb0.04(1)]Σ1.98(4)S6.04(4). The new mineral is tetragonal, space group I42 m, with a = 9.8786(5),c = 10.8489(8) Å, V = 1058.71(11) Å3, Z = 4. The main diffraction lines of the calculated powder diagram
are [d (in Å), intensity, hkl]: 4.092, 70, 211; 3.493, 23, 220; 3.396, 35, 103; 3.124, 17, 310; 2.937, 100, 222;
2.656, 19, 321; 2.470, 19, 400; 2.435, 33, 303. The crystal structure of ferrostalderite has been refined by Xray single-crystal data to a final R1 = 0.050, on the basis of 1169 reflections with F0 > 4σ(F0). It shows a three
dimensional framework of (Cu,Fe)-centred tetrahedra (1 M1 + 2 M2), with channels parallel to [001] hosting
disymmetric TlS6 and (As,Sb)S3 polyhedra. Ferrostalderite is derived from its isotype stalderite
M1
CuM2Zn2TlAs2S6 through the homovalent substitution M2Zn2+ → M2Fe2+. The ideal crystal-chemical
formula of ferrostalderite is M1CuM2Fe2TlAs2S6.
K E Y WO R D S : ferrostalderite, new mineral, sulfosalt, thallium, copper, iron, arsenic, crystal structure,

Lengenbach, Binn Valley, Switzerland.
Introduction
MINERALOGICAL studies carried out on thalliumbearing mineralizations from Lengenbach, Binn
Valley, Switzerland, and Monte Arsiccio mine,
Apuan Alps, Tuscany, Italy, allowed the revision of
the crystal chemistry of the routhierite isotypic*E-mail: biagioni@dst.unipi.it
DOI: 10.1180/minmag.2015.079.7.10© 2016 The Mineralogical Society

series, as defined by Biagioni et al. (2014b), and thedescription of two new mineral species, arsiccioite,
AgHg2TlAs2S6 (Biagioni et al., 2014b), and its Zn
isotype ralphcannonite, AgZn2TlAs2S6 (Bindi
et al., 2015b). During energy dispersive spectrometry (EDS) screening of a suite of specimens
collected in the Lengenbach quarry, Ag-rich andFe-rich stalderite-like phases were identified in
2013. The former were later identified as the newmineral ralphcannonite (Bindi et al., 2015b),
whereas crystallographic studies and chemical

CRISTIAN BIAGIONI ET AL.

analyses allowed us to describe the Fe-rich phase as
the new mineral ferrostalderite. The mineral and its
name have been approved by the Commission on
New Minerals, Nomenclature and Classification of
the International Mineralogical Association(CNMNC-IMA) (2014-090, Bindi et al., 2015a).
The holotype specimen of ferrostalderite is deposited in the mineralogical collection of the Museo diStoria Naturale, Università degli Studi di Firenze,
Via G. La Pira 4, Florence, Italy, under catalogue
number 3148/I. The name indicates that this newphase is the Fe isotype of stalderite, CuZn2TlAs2S6.
This paper presents the definition, occurrence, andcrystal structure of the new mineral species
ferrostalderite, and discusses its relationships withthe other members of the routhierite isotypic series.

discordant sulfosalt and sulfide veins; and (4)idiomorphic crystals within druses and open
fissures. Ferrostalderite was discovered in thesecond type of mineralization, roughly corresponding to the so-called ‘Zone 1’ of Graeser et al.
(2008). The specimen studied was mined in 1999
by the Interessengemeinschaft Lengenbach (thethen active Lengenbach syndicate) on the thirdlevel of the quarry.

Physical and optical properties

Occurrence and mineral description
The Lengenbach quarry (latitude 46°21′54″N,
longitude 8°13′15″E) exploits a Triassic dolostone
stratigraphically overlying the gneiss basement at
the northern front of the Monte Leone Nappe,
belonging to the Penninic Domain of the Alps.
These rocks have undergone upper greenschist –lower amphibolite metamorphism. The dolostone
formation is 240 m thick at Lengenbach; mineralization occurs in the uppermost part of thisdolostone sequence, close to the contact with thestratigraphically overlying Jurassic to Lower
Cretaceous Bündnerschiefer. Hofmann and Knill(1996) recognized four major types of mineralization: (1) stratiform layers of pyrite with minor
galena, sphalerite and xenomorphic sulfosalts; (2)massive to interstitial sulfosalt accumulations; (3)Ferrostalderite was observed as black, equant toprismatic euhedral crystals; on the basis of scanningelectron microscopy images (Fig. 1) the dominant
forms could be the prism {110} and the bipyramid{101}, in agreement with the crystal forms given by
Graeser et al. (1995) for stalderite. Ferrostalderite isbrittle, with an irregular fracture. Its streak is blackand the lustre is metallic. Owing to the rarity and thesmall size of the available crystals, the microhardness was not measured. In plane-polarized incident
light, ferrostalderite is dark grey in colour. Undercrossed polars, it is weakly anisotropic, withyellowish to bluish rotation tints. Internal reflections are very weak and there is no optical evidenceof growth zonation. Reflectance measurements
were performed in air using an MPM-200 Zeiss
microphotometer equipped with an MSP-20 systemprocessor on a Zeiss Axioplan ore microscope. The
filament temperature was ∼3350 K. An interference
filter was adjusted, in turn, to select four wavelengths (approximating those recommended by theCommission on Ore Mineralogy of the IMA) for
measurement (471.1, 548.3, 586.6 and 652.3 nm).

FIG. 1. Scanning electron microscope image of ferrostalderite (light grey), as equant crystals on dolomite (dark grey).

176

FERROSTALDERITE, A NEW MINERAL FROM LENGENBACH

crystal X-ray diffraction experiment did not indicate
the presence of elements (Z > 9) other than Fe, Cu,
Zn, As, Ag, Hg, Tl and S. The same fragment wasthen analysed with a CAMECA SX 50 electron
microprobe (wavelength dispersive spectrometrymode, accelerating voltage = 20 kV, beam current =20 nA, beam size = 1–2 μm) available at the IGGCNR Padova, Italy. Counting times were 20 s forpeak and 15 s for background. The following
standards (element, emission line) were used: Ag
metal (AgLα), Cu metal (CuKα), sphalerite (ZnKα,SKα), Fe metal (FeKα), HgS (HgMα), Tl2Se(TlMα), AsGa (AsLα), and Sb2S3 (SbLα).
Cadmium, Se and Pb were sought but not foundabove detection limits (0.01 wt.%). Chemical data
are given in Table 1. The relatively low total could
be related to the small size of the polished grain.
The only significant chemical variation (taking
into account that only three spot analyses were
performed, owing to the very small size of thestudied grain) involves an inverse correlation
between iron and zinc. On the basis of 12 atoms
per formula unit (a.p.f.u.), the chemical formula offerrostalderite is Cu0.75(2)Ag0.30(1)Fe1.36(10)Zn0.52(11)Hg0.05(1)Tl1.00(1)[As1.94(4)Sb0.04(1)]Σ1.98(4)S6.04(4).

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 24.2, 25.4 (471.1 nm), 23.7, 24.7 (548.3 nm),
22.9, 23.8 (586.6 nm) and 21.0, 22.0 (652.3 nm),respectively. Owing to the small amount of purematerial available, the density was not measured.
The calculated density, based on the empiricalformula (see below), is 4.691 g cm−3; the ideal
chemical formula gives a calculated density of
4.528 g cm−3. In the specimen studied, ferrostalderite has grown directly on dolomite and isassociated with realgar, baumhauerite (?) andpyrite. Its crystallization is related to the activityof late-stage Tl-As-Cu-Fe-rich hydrothermal fluids
during tectono-metamorphic Alpine events.
Chemical analysis
A preliminary chemical analysis using EDS performed on the crystal fragment used for the singleTABLE 1. Microprobe analysis of ferrostalderite:
chemical composition as wt.% and number ofatoms on the basis of 12 atoms per formula unit
(a.p.f.u.).
Element

wt.%

range

e.s.d.

Cu
Ag
Fe
Zn
Hg
Tl
As
Sb
S
Total

6.24
4.18
9.95
4.46
1.22
26.86
19.05
0.63
25.39
97.98
a.p.f.u.
0.749
0.296
0.521
1.359
0.046
1.003
1.941
0.040
6.044
–2.0

5.98–6.47
4.09–4.26
8.99–10.48
3.88–5.50
0.99–1.50
26.24–27.48
18.88–19.24
0.56–0.68
24.85–25.72
97.18–98.59
range
0.728–0.773
0.287–0.301
0.449–0.651
1.245–1.421
0.037–0.058
0.993–1.018
1.914–1.986
0.035–0.043
5.995–6.076–3.2–0.2

0.25
0.09
0.83
0.91
0.26
0.62
0.18
0.06
0.47
0.72
e.s.d.
0.023
0.008
0.113
0.099
0.011
0.013
0.039
0.004
0.043
1.9

Cu
Ag
Zn
Fe
Hg
Tl
As
Sb
S
Ev

Crystallography

Valence equilibrium: Ev (%) = [Σ(val+) – Σ(val–)] × 100/Σ(val–).

177

For the X-ray single-crystal studies, intensity data
were collected using a Supernova (Agilent
Technologies) diffractometer equipped with a
Mova micro-source (spot size = 0.120 mm,
MoKα radiation) combined with a Pilatus 200 K
detector (Dectris), at the Department ofGeosciences, University of Padova, Italy. The
detector-to-crystal distance was 68 mm and 2592
frames were collected with the ω scan mode, in 1°
slices, with an exposure time of 10 s per frame.
Intensity integration and standard Lorentzpolarization corrections were performed with theCrysAlis RED (Oxford Diffraction, 2006) software
package. The program ABSPACK in CrysAlis RED(Oxford Diffraction, 2006) was used for theabsorption correction. Statistical tests on the
distribution of |E| values (|E 2 – 1| = 0.584) andthe systematic absences suggested the space group
I42 m. The refined unit-cell parameters are a =9.8786(5), c = 10.8489(8) Å, V = 1058.71(11) Å3.
The crystal structure of ferrostalderite wasrefined starting from the atom coordinates given
by Bindi (2008) for routhierite, using Shelxl-97(Sheldrick, 2008). Scattering curves for neutral
atoms were taken from the International Tables for

CRISTIAN BIAGIONI ET AL.

TABLE 2. Crystal and experimental data for ferrostalderite.
Crystal data
Crystal size (mm)Cell setting, space groupa, c (Å);
V (Å3)Z
Data collection and refinement
Radiation, wavelength (Å)
Temperature (K)2θmax (°)
Measured reflections
Unique reflections
Reflections with Fo > 4σ(Fo)RintRσRange of h, k, l
R [Fo > 4σ(Fo)]R (all data)wR (on F 2)
Gof
Number of least-squares parameters
Maximum and minimum residual peak (e Å–3)

0.010 × 0.020 × 0.040
Tetragonal, I42m
9.8786(5), 10.8489(8)1058.71(11)4MoKα, λ = 0.71073
293
69.92
28,4631258
1169
0.09510.0324−15 ≤ h ≤ 15, −15 ≤ k ≤ 15, −17 ≤ l ≤ 17
0.0496
0.05450.11061.172
38
3.04 (at 0.91 Å from Tl)–3.03 (at 0.36 Å from Tl)

Crystallography (Wilson, 1992). Crystal data anddetails of intensity data collection and refinement
are reported in Table 2. After several cycles ofisotropic refinement, R1 converged to 0.115,
suggesting the validity of the structural model.
Four independent cation sites occur in the crystal
structure of ferrostalderite, labelled Tl, M1, M2 andAs. M1 and M2 sites correspond to Cu and Hg sites
in the crystal structure of routhierite (Bindi, 2008;
Biagioni et al., 2014a). The occupancies of thesefour independent cation sites were refined using thefollowing scattering curves: Tl site: Tl vs. □(vacancy); M1 site: Cu vs. □; M2 site: Ag vs. □; As
site: As vs. □. The Tl site was found to be fully
occupied by Tl and its site occupancy was fixed to
1. At the M1 and M2 sites the refined site-scatteringvalues are 27.2 and 31.9 electrons, whereas at theAs site a near-pure As occupancy was observed(33.3 electrons). Final atom coordinates anddisplacement parameters are given in Table 3,
whereas selected bond distances are reported inTable 4. Table 5 gives the refined and calculated site
scattering (in electrons per formula unit, e.p.f.u.) on
the basis of the proposed site populations (seebelow). Iron has been considered as divalent, as in
the other members of the routhierite isotypic series;
this is consistent with the observation of

Makovicky et al. (1990) that the presence of Zn
and Hg favours Fe2+ with respect to Fe3+. Table 6
shows the results of the bond valence calculations.
Owing to the very small size and the few
available crystals, obtaining a powder X-ray diffraction pattern was not attempted. Table 7 reports thecalculated powder X-ray diffraction data of ferrostalderite on the basis of the crystallographic datacollected through single-crystal X-ray diffraction.

Crystal structure description

178

The crystal structure of ferrostalderite is isotypicwith those of the other members of the routhierite
isotypic series (Graeser et al., 1995; Bindi, 2008;
Biagioni et al., 2014a,b; Bindi et al., 2015b),
showing a framework formed by two independent
MeS4 tetrahedra sharing corners, hosting channels
parallel to [001]. These channels contain TlS6 and(As,Sb)S3 polyhedra, sharing corners and edges
with the tetrahedron framework (Fig. 2).
As stated above, four independent cation sites
occur. The two independent tetrahedral sites, M1
and M2, have average bond distances of 2.332 and2.385 Å, respectively. The <M1–S> bond distanceis a little shorter than that observed by Graeser et al.

–0.0178(5)00–0.0003(3)0.0054(6)–0.0019(8)0000.0009(3)0.0021(7)–0.0018(6)00–0.0009(5)0.0009(3)–0.0016(6)–0.0018(6)TABLE 4. Selected bond distances (Å) in ferrostalderite.
Tl – S2– S1
average
As – S1– S2
average

0.0593(4)0.0149(5)0.0222(5)0.0172(3)0.0201(8)0.0230(8)0.0593(4)0.0149(5)0.0325(6)0.0172(3)0.0200(8)0.0230(8)0.0479(3)0.0210(5)0.0256(3)0.0180(3)0.0207(3)0.0213(5)0.34889(6)¼½0.2486(2)0.3790(2)0.1300(2)0½½0.26343(7)0.3410(2)0.1287(2)000.2241(1)0.26343(7)0.1017(2)0.1287(2)
4e
4d
8f
8i
16j
8i
Tl
M1
M2
As
S1
S2

2.979(3) × 2
3.530(2) × 4
3.346
2.268(2) × 2
2.280(3)2.272

M1– S12.332(2) × 4

M2– S1– S2
average

2.377(2) × 2
2.392(2) × 2
2.385(1995) for the Cu site of stalderite and by Bindi
et al. (2015b) for ralphcannonite, i.e. 2.357(4) and2.358(7) Å, respectively. Similarly, the <M2–S>
bond distance is shorter than those reported at theM2 site of ralphcannonite (2.468 Å) and at the Znsite of stalderite (2.406 Å). Whereas in othermembers of the routhierite isotypic series the M2
tetrahedron is distorted, with two relatively shortand two relatively long bonds, in ferrostalderite thecoordination is quite regular, with bond distances of
2.377(2) and 2.392(2) Å. The As site forms a
trigonal pyramid with three S atoms, with anaverage bond distance of 2.272 Å. The refinementof the site occupancy points to the dominantoccupancy of this site by arsenic, with only anegligible amount of Sb, in agreement withchemical data showing only 0.03 Sb a.p.f.u. The
coordination of the Tl site can be described as an
orthorhombic pyramid with a split apex, as in theother members of the routhierite isotypic series. On
the other side of the split apex, a relatively short
Tl–Tl distance occurs, i.e. 3.279(1) Å. This value
perfectly agrees with that reported for stalderite(3.28 Å) by Graeser et al. (1995), being in therange of Tl–Tl distances observed in othermembers of the routhierite group. Such a shortdistance is most likely indicative of some type ofTl–Tl interaction, as observed in other Tl compounds, e.g. christite (Brown and Dickson, 1976)
and gabrielite (Balić-Žunić et al., 2006).

0.0252(3)0.0333(9)0.0220(5)0.0196(4)0.0214(8)0.0176(11)

U 13
Ueq
z/c
y/b
x/a
Wyckoff site
Site

TABLE 3. Atom coordinates and displacement parameters (Å2) for ferrostalderite.

U 11

U 22

U 33

U 23

U 12

FERROSTALDERITE, A NEW MINERAL FROM LENGENBACH

Site population and bond valence balance
The similar scattering factors of Fe (Z = 26), Cu(Z = 29) and Zn (Z = 30) do not permit accurate
knowledge of their partitioning between the twotetrahedral sites M1 and M2 by using conventional
MoKα radiation only. Consequently, some additional considerations based on average bonddistances and bond-valence sums are required.
Since the description of arsiccioite (Biagioni et al.,2014b), it has become clear that the steric effect has
179

CRISTIAN BIAGIONI ET AL.

TABLE 5. Refined site-scattering values (e.p.f.u.), assigned site population (a.p.f.u.) with corresponding calculatedsite-scattering (e.p.f.u.), and comparison between observed and calculated bond distances (in Å) at M1, M2 andAs sites in ferrostalderite.
Site Refined site-scattering values Assigned site population Calculated site-scattering <Me–S>obs <Me–S>calc
M1
M2
As

27.0
31.8
33.1

Cu0.75Zn0.25
Fe0.68Ag0.15Zn0.14Hg0.03
As0.98Sb0.02

29.2
31.3
33.4

2.332
2.385
2.272

2.364
2.448
2.264

TABLE 6. Bond-valence sums (BVS, in valence units, vu) calculated using the parameters given by Brese andO’Keeffe (1991).*
M1
S1
S2
Σ cations
Theor.

M2
×4

0.34

2×

1.36
1.25

Tl
×2

0.55
0.53×2
2.16
1.85

Σ anions

As
×4

×2

0.09
0.39×2
1.13
1.00

0.99
0.96
2.94
3.00

1.97
2.41

*In mixed sites, the bond-valence contribution of each cation has been weighted according to its site-population (seeTable 5). Left and right superscripts indicates the number of bonds involving cations and anions, respectively.

priority over the valence state in governing the metal
distribution between M1 and M2 sites in routhierite
isotypes. This observation has been confirmed by
the structure refinement of ralphcannonite (Bindi
et al., 2015b). As a consequence, Ag has been

attributed to the M2 site in the crystal structure offerrostalderite and the substitution M1Cu+ + M2(Fe,Zn,Hg)2+ → M1Zn2+ + M2Ag+ is assumed to takeplace. In addition, and notwithstanding the very low
number of spot analyses (only three, as discussed

TABLE 7. Calculated powder X-ray diffraction data for ferrostalderite.*
Icalc

dcalc

hkl

Icalc

dcalc

hkl

12
10
8
70
23
35
17
100
9
19
13
19
33
12

6.985
5.424
4.939
4.092
3.493
3.396
3.124
2.937
2.798
2.656
2.528
2.470
2.435
2.377

110
002
200
211
220
103
310
222
213
321
114
400
303
204

7
10
14
17
10
13
13
16
8
9
7
7
5
13

2.340
2.184
2.048
1.997
1.845
1.826
1.809
1.746
1.734
1.713
1.606
1.576
1.565
1.501

411
323
314
413
404
404
521
440
433
424
611
514
316
622

*Intensity and dhkl were calculated using the software Powdercell 2.3 (Kraus and Nolze, 1996) on the basis of thestructural model given in Table 3; only reflections with Icalc > 5 are listed. The seven strongest reflections are given in bold.

180

FERROSTALDERITE, A NEW MINERAL FROM LENGENBACH

FIG. 2. Crystal structure of ferrostalderite, as seen down [001] (a) and [010] (b). Polyhedra are: blue: M1 tetrahedra;orange: M2 tetrahedra. Circles are grey: Tl site; violet: As site; yellow: S1 site; red: S2 site.

above), a negative correlation between Fe2+ and(Zn2++Hg2+) was observed (Fig. 3), in agreementwith the data for stalderite given by Graeser et al.
(1995). This suggests that the homovalent substitution (Zn,Hg)2+ → Fe2+ takes place at the M2
site, which gives the site population in Table 5. The
calculated site-scattering values (e.p.f.u.) arecompared with refined site scattering, whereas
observed average <Me–S> bond distances arecompared with those calculated. According to theparameters given by Brese and O’Keeffe (1991), theideal Me–S distance (in Å) decreases according
181

to the sequence 2.663/Ag+, 2.576/Hg2+, 2.416/Fe2+,
2.370/Cu+ and 2.346/Zn2+.
The bond-valence sums (BVS) were calculated
by using the parameters given by Brese andO’Keeffe (1991) and are reported in Table 6. Aswith some other members of the routhierite isotypicseries (e.g. arsiccioite – Biagioni et al., 2014b;
ralphcannonite – Bindi et al., 2015b), significant
deviations from the expected values were observed(up to +17%), in particular at the M2 site, whichappears smaller than expected, resulting in an
overbonding of the hosted metals. Overbonding at

CRISTIAN BIAGIONI ET AL.

FIG. 3. (Zn + Hg) content (in a.p.f.u.) vs. Fe content in the pair stalderite (blue circles) – ferrostalderite (green diamonds).
Data for stalderite are after Graeser et al. (1995).

the metal sites influences the BVS of the S atoms,
particularly that occupying the S2 position, having
a strong BVS excess (corresponding to +20.5%
with respect to the expected value, i.e. 2 valence
units, vu). A BVS excess similar to those reported
in other routhierite isotypes occurs also for the Tl
atom. On the contrary, quite good agreement
between calculated and expected BVS at the As
site was obtained. The reason for the BVS excess at
the metal sites could possibly be related to theoverestimation of bond parameters for some (Me,S)
pairs by Brese and O’Keeffe (1991), as discussed
briefly for the (Tl,S) pair by Biagioni et al. (2014b).
For example, by using these parameters, an ideal<Ag–S> tetrahedral bond distance of 2.663 Å canbe calculated. It should be noted that in somecompounds having Ag in tetrahedral coordination,
the bond distance of the pure Ag site is shorter thanideal. For example, the synthetic compound
Ag2HgGeS4 (Parasyuk et al., 2002) has two Ag
tetrahedral sites. The first has a pure Ag populationand shows an average bond distance of 2.56 Å; theother site has a mixed (Ag0.5Hg0.5) site occupancy,with an average bond distance of 2.55 Å.
Consequently, the pure Ag site does not appear
significantly larger than the mixed (Ag,Hg) one,
contrasting with the assumption that <Ag–S> has asignificantly larger bond distance than <Hg–S>.
The crystal chemical formula of ferrostalderite canbe written as M1(Cu0.75Zn0.25)M2(Fe0.68Ag0.15Zn0.14
Hg0.03)2Tl(As0.98Sb0.02)2S6, with the relative error of
the valence equilibrium Ev (%) = –0.42.

Discussion
Crystal chemistry of ferrostalderite in theframework of the routhierite isotypic series

182

Ferrostalderite, ideally M1CuM2Fe2TlAs2S6, is the Fe
analogue of stalderite and routhierite (Fig. 4). So far,the divalent metal ions found in the routhierite
isotypic series are Fe2+, Hg2+ and Zn2+. The latter
seems to play a central role in this series, being
widely substituted by both Fe and by Hg, as occurs
in other compounds having a sphalerite-related
structure, e.g. the tetrahedrite isotypes (Moëlo
et al., 2008). Whereas the substitutions Zn2+ → Fe2+
and Zn2+ → Hg2+ are widely represented in minerals,
available data suggest a more limited Fe2+ → Hg2+
substitution (e.g. a new potential fettelite-likemineral; Bindi et al., 2012). These considerations
are in keeping with the finding of a relatively wide
compositional range between stalderite and ferrostalderite (taking into account the small amount ofavailable chemical data) and a very limited solid
solution between the latter and routhierite.
Interestingly, a higher Zn content seems to favour
the incorporation of Hg in the crystal structure ofthese compounds, in agreement with the crystal
chemistry of other sphalerite-related compounds.
The crystallization of the different members of
the routhierite isotypic series is probably mainly
controlled by the geochemistry of the mineralizing
fluids. In Hg-rich environments (e.g., Monte
Arsiccio mine, Tuscany, Italy – Biagioni et al.,

FERROSTALDERITE, A NEW MINERAL FROM LENGENBACH

FIG. 4. Chemical composition of the members of the routhierite isotypic series, M +M 2+
2 TlAs2S6. Symbols: light blue
triangles = arsiccioite; violet squares = routhierite from Monte Arsiccio mine; orange squares = routhierite from Jas
Roux; blue circles = stalderite; red circles = ralphcannonite; green diamonds = ferrostalderite. Chemical data are afterBiagioni et al. (2014a, b), Bindi (2008), Bindi et al. (2015b), Graeser et al. (1995) and Johan et al. (1974).

2013; Jas Roux, Haute-Alpes, France – Johan and
Mantienne, 2000), routhierite and arsiccioite crystallized, differing by the Ag-dominant nature of thelatter; the same relationship is displayed by the pairstalderite–ralphcannonite, crystallizing in Hg-poor
environments (e.g. Lengenbach, Binn Valley,
Switzerland – Graeser et al., 2008; Roth et al.,2014). An increasing amount of Ag promotes ashift in the crystal chemical formula fromM1
Me +M2Me 2+
2 TlAs2S6 (routhierite and stalderite)
to M1Me 2+M2(Me +0.5Me 2+
0.5)2TlAs2S6 (arsiccioite andralphcannonite). It is not clear if an Ag analogue offerrostalderite could exist. In fact, the atomic radiusof Fe2+ is larger than those of Zn2+ and Cu+ and so
it is not clear if the M1 site could host this metal as aconsequence of Ag incorporation at the M2
position.
Iron in sulfosalts
Among sulfosalts, few mineral species contain Fe
as an essential constituent (Table 8). Iron shows two
different coordination environments, being hosted
in tetrahedral as well as octahedral polyhedra, theformer generally occurring in sphalerite-related
structures. Tetrahedral coordination, similar to that183

observed in ferrostalderite, was reported inmembers of the tetrahedrite isotypic series (e.g.
Andreasen et al., 2008). In ferrostalderite as well as
in the tetrahedrite isotypes, Fe occurs in tetrahedral
sites having mixed occupancy, usually represented
by zinc and, rarely, by mercury. An additional
species is miharaite, an unclassified Cu sulfosalt of
which the crystal structure was solved by Petrova
et al. (1988). Iron is tetrahedrally coordinated, withbond distances ranging between 2.30 and 2.37 Å,with an average value of 2.33 Å.
Octahedral coordination of iron has beenreported in sulfosalts belonging to the berthierite
isotypic series (berthierite and garavellite), in thewell-known lead-antimony sulfosalt jamesonite,
and in members of the cylindrite homologous
series. In berthierite and garavellite the average<Fe–S> bond distances are 2.53 and 2.54 Å,
respectively (Buerger and Hahn, 1955; Lemoine
et al., 1991; Lukaszewicz et al., 2001; Bindi and
Menchetti, 2005); a very similar <Fe–S> bonddistance has been observed in jamesonite, i.e.
2.56 Å (Léone et al., 2003). Finally, iron is an
essential constituent in coiraite, cylindrite andfranckeite, species belonging to the cylindrite
group of complex Pb sulfosalts, as proved by

CRISTIAN BIAGIONI ET AL.

TABLE 8. Iron coordination number (C.N.) and chemical formulae of sulfosalts having Fe as an essential
constituent.
C.N.

Group

Tetrahedrite isotypic series
IV
Routhierite isotypic series
Unclassified Cu sulfosalt

Mineral species

Chemical formula

Argentotetrahedrite
Argentotennantite
Freibergite
Giraudite
Tennantite
Tetrahedrite
Ferrostalderite
Miharaite

Ag6[Ag4(Fe,Zn)2]Sb4S13
Ag6[Cu4(Fe,Zn)2]As4S13
Ag6[Cu4Fe2]Sb4S13
Cu6[Cu4(Fe,Zn)2]As4Se13
Cu6[Cu4(Fe,Zn)2]As4S13
Cu6[Cu4(Fe,Zn)2]Sb4S13
CuFe2TlAs2S6
Cu4FePbBiS6

Berthierite
Garavellite
Cylindrite
CoiraiteFranckeite
Jamesonite

FeSb2S4FeSbBiS4
FePb3Sn4Sb2S14
Fe(Pb,Sn)12.5Sn5As3S28
Fe(Pb,Sn)6Sn2Sb2S14
FePb4Sb6S14

Berthierite isotypic series
VI

Cylindrite homologous series
Jamesonite isotypic series

some experimental syntheses (Sachdev and Chang,1975; Li, 1984). These minerals have an incommensurate composite-layered structure, formed by
the regular stacking of two kinds of layers, a
pseudotetragonal Q layer and a pseudohexagonal H
layer. Owing to the incommensurate layered
structure of these phases, structural data are poor.
Whereas the crystal structure of franckeite has beensolved recently by Makovicky et al. (2011), thoseof cylindrite and coiraite have not. However, thecrystal structure of a synthetic Sn-Se representativeof the cylindrite structure type has been describedby Makovicky et al. (2008). Consequently, it has
been observed that in franckeite and cylindrite, ironis partitioned within the H layers, in an octahedral
environment. Owing to the relationships with theother phases of the cylindrite homologous series,
Paar et al. (2008) assumed the same iron partitioning within the H layers of coiraite.
Conclusion
The complex crystal chemistry shown by themembers of the routhierite isotypic series, strongly
influenced by the geochemistry of the crystallization environment, has, in recent years, favoured thedescription of three new mineral species, arsiccioite, ralphcannonite and ferrostalderite. The latter
represents a new kind of Fe sulfosalt and increases
the number of thallium minerals described from theLengenbach dolostone. Indeed, ferrostalderite is
the twentieth thallium species described from thisSwiss locality and confirms its exceptional status
as a hotspot for the study of sulfosalt assemblages
in a metamorphic environment. Lengenbach
appears as the most productive field, today, forsulfosalt systematics.

Acknowledgements
This research was supported by “Progetto d’Ateneo
2012. Università di Firenze” to LB. Comments by
Chris J. Stanley, Peter Leverett and Stefan Graeser
helped to improve the paper. Pete Williams is thanked
for efficient handling of the manuscript and usefulcomments.

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