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Lead-antimony sulfosalts from Tuscany (Italy). XXI. Bernarlottiite, Pb12(As10Sb6)Σ16S36, a new N = 3.5 member of the sartorite homologous series from the Ceragiola marble quarry: Occurrence and crystal structure

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Title: Lead-antimony sulfosalts from Tuscany (Italy). XXI. Bernarlottiite, Pb12(As10Sb6)Σ16S36, a

new N = 3.5 member of the sartorite homologous series from the Ceragiola marble quarry, Apuan Alps: occurrence and crystal structure

Running title: Bernarlottiite, a new member of the sartorite series

Plan of the article:

Abstract 1. Introduction

2. Occurrence and mineral description

2.1. Occurrence and physical properties of bernarlottiite 2.2. Chemical data

2.3. Crystallography 3. Crystal structure description

3.1. General organization

3.2. Cation coordinations and site occupancies 3.3. Polymerization of (Sb/As) sites

4. Discussion

4.1. Crystal-chemistry of bernarlottiite

4.2. Chemical variability of “baumhauerites” and the occurrence of superstructures 4.3. Building operators in baumhauerite homeotypes and other homologues

5. Conclusion References

Corresponding author: Cristian Biagioni Computer: PC

OS: Windows Software: Word

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Lead-antimony sulfosalts from Tuscany (Italy). XXI.

Bernarlottiite, Pb

12

(As

10

Sb

6

)

Σ16

S

36

, a new N = 3.5 member

of the sartorite homologous series from the Ceragiola

marble quarry: occurrence and crystal structure

PAOLO ORLANDI1, CRISTIAN BIAGIONI1,*, ELENA BONACCORSI1, YVES MOËLO2 and

WERNER H. PAAR3

1Dipartimento di Scienze della Terra, Università di Pisa, Via S. Maria 53, I-56126 Pisa, Italy

2Institut des Matériaux Jean Rouxel, UMR 6502, CNRS, Université de Nantes, 2, rue de la

Houssinière, 44322 Nantes Cedex 3, France

3Department of Chemistry and Physics of Materials, University, Hellbrunnerstr. 34, A-5020

Salzburg, Austria

*Corresponding author, e-mail: biagioni@dst.unipi.it

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Abstract: The new mineral species bernarlottiite, Pb12(As10Sb6)Σ16S36, has been discovered in

cavities of the Early Jurassic marbles from the Ceragiola quarry, Seravezza, Apuan Alps, Tuscany, Italy. Its name honours Bernardino Lotti (1847–1933) for his significant contribution to the knowledge of the geology of Tuscany and to the development of the Tuscan mining industry. It occurs as lead-grey acicular crystals up to 1 mm in length and few μm in width, with a metallic luster, associated with Sb-rich sartorite. Under the ore microscope, bernarlottiite is white with abundant red internal reflections; pleochroism is weak in air, with shades of grey-blue. Anisotropism is distinct to strong, with greyish-bluish rotation tints. Reflectance percentages for the four COM wavelengths are [Rmin, Rmax (%), (λ)]: 30.0, 37.5 (470 nm); 30.3, 37.3 (546 nm); 29.7,

36.8 (589 nm); and 29.3, 36.2 (650 nm). Electron-microprobe analyses, collected on two different grains, gave (in wt%): Cu 0.09(16), Pb 48.89(1.26), As 17.48(22), Sb 11.36(10), S 23.11(32), total 100.93(1.38) (sample # 2987) and Cu 0.02(3), Pb 47.43(26), As 14.56(24), Sb 13.92(18), S 22.64(17), total 98.58(46) (sample # 3819). On the basis of ΣMe = 28 atoms per formula unit, the chemical formulae are Cu0.07(12)Pb11.71(18)As11.59(21)Sb4.63(9)S35.78(48) and

Cu0.02(2)Pb11.92(6)As10.12(14)Sb5.95(8)S36.76(32) for samples # 2987 and # 3819, respectively. The main

diffraction lines, corresponding to multiple hkl indices, are [d in Å (relative visual intensity)]: 3.851 (s), 3.794 (s), 3.278 (s), 3.075 (s), 2.748 (vs), 2.363 (s), 2.221 (vs). The crystal structure study gives a triclinic unit cell, space group P

1

, with a = 23.704(8), b = 8.386(2), c = 23.501(8) Å, α = 89.91(1), β = 102.93(1), γ = 89.88(1)°, V = 4553(2) Å3, Z = 3. The crystal structure has been solved

and refined to R1 = 0.088 on the basis of 7317 reflections with Fo > 4σ(Fo). Bernarlottiite is a new N

= 3.5 homeotype of the sartorite homologous series, with a 3a superstructure relatively to that of primitive baumhauerite. Its crystal structure can be described as being formed by 1:1 alternation of sartorite-type (N = 3) and dufrénoysite-type (N = 4) layers along c, connected by Pb atoms with tricapped trigonal prismatic coordination. Each layer results from the stacking of two types of ribbons along a, a centrosymmetric one alternating with two acentric ones. The three main building operators of the structure are 1) the interlayers As-versus-Pb crossed substitution, stabilizing the combined N = (3,4) baumhauerite homologue, 2) the inter-ribbon Sb partitioning in the sartorite layer, with “symmetrization” of the Sb-rich ribbon, that induces the 3a superstructure, and 3) the common (As,Sb) polymerization through short (As,Sb)–S bonds.

Key-words: bernarlottiite; sartorite homologous series; new mineral species; sulfosalt; lead;

antimony; arsenic; crystal structure; building operators; Apuan Alps.

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1. Introduction

Mineral species belonging to the sartorite homologous series (Table 1), defined in details by Makovicky (1985), after the basic comparative crystallographic study of Le Bihan (1962), are structurally characterized by the regular stacking of one or two types of slabs (N = 3 and 4). This mineral group is an interesting field of research owing to its complex crystal-chemistry (Moëlo et al., 2008), allowing the description of several new mineral species. In this respect, the most prolific deposit is the Lengenbach quarry, Binn Valley, Switzerland, being the type locality for nine species within the sartorite group. It is followed by the Apuan Alps hydrothermal ore deposits, representing the type localities for four new mineral species. Whereas sartorite group members have been known from Lengenbach since a long time (e.g., Rath, 1864), the presence of these minerals in the hydrothermal veins from Apuan Alps was identified only in the last thirty-five years. Two different kinds of occurrence are known, i.e. the baryte + pyrite ore deposits from southern Apuan Alps (i.e. the Pollone and Monte Arsiccio mines) and the cavities of the marble quarries near the town of Seravezza. The first kind of occurrence represents the type locality for boscardinite (Orlandi et al., 2012; Biagioni & Moëlo, 2016), carducciite (Biagioni et al., 2014), and polloneite (Topa et al., 2015b). In addition, twinnite (also in a thallium-rich variety) and veenite have been recently identified. The cavities of the marble quarries from the Seravezza area have been known since the description of guettardite by Bracci et al. (1980). In addition, Orlandi et al. (1996) and Orlandi & Criscuolo (2009) described the identification of “Sb-rich baumhauerite” and sartorite. Whereas the latter has not been completely characterized yet, the former has been accurately studied collecting single-crystal X-ray diffraction and electron-microprobe data.

Single-crystal X-ray diffraction data of this “Sb-rich baumhauerite” clearly indicated the presence of a 3 × 7.9 Å superstructure periodicity, indicating homeotypic relationships with true primitive baumhauerite and pointing to a distinction between baumhauerite itself and this Sb-rich derivative, which has been named bernarlottiite. The mineral and its name have been approved by the IMA-CNMNC, under the number 2013-133 (Orlandi et al., 2014). The holotype specimen is deposited in the mineralogical collection of the Museo di Storia Naturale, Università di Pisa, Via Roma 79, Calci, Pisa (Italy), under catalogue number 19687. The name is in honour of Bernardino Lotti (1847–1933) for his significant contribution to the knowledge of the geology of Tuscany and to the development of the Tuscan mining industry. Indeed, his studies contributed to the discovery and exploitation of the Niccioleta, Boccheggiano, and Gavorrano world-class pyrite ore deposits, as well as of other important Tuscan mines. He greatly contributed to the geological mapping of Tuscany, publishing four books on the geology of Elba Island, the Massa Marittima mining district, Tuscany, and Umbria. In addition, Bernardino Lotti was author of more than 200 papers. Together

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with Domenico Zaccagna (1851–1940), he contributed to the geological mapping of the Apuan Alps. Marinelli (1983) reported a list of works published by Lotti. In naming the new mineral, “bernarlottiite” was preferred to “lottiite”, in order to avoid any confusion with the member of the cancrinite group liottite.

This paper reports the description of the occurrence of bernarlottiite and its crystal structure, discussing its relationships with the other members of the sartorite homologous series.

2. Occurrence and mineral description

2.1. Occurrence and physical properties of bernarlottiite

Bernarlottiite was collected in cavities of the Early Jurassic marble outcropping in the Ceragiola area, Seravezza marble quarries, Apuan Alps, Tuscany, Italy. These marble outcrops belong to the Apuane Unit (e.g., Fellin et al., 2007), a tectonic unit formed by a Paleozoic basement overlain by a Triassic – Tertiary metasedimentary sequence, metamorphosed up to the greenschist facies and affected by two main deformation events (D1 and D2 of Carmignani & Kligfield, 1990).

The mineralized cavities have an elongated, sometimes s-shaped cross-section, only a few mm to few cm in width and up to 30 cm high; the length can reach several meters. These cavities lies on definite horizons related to the S1 schistosity surface of the D1 tectonic phase, which was refolded

during the D2 event (Orlandi et al., 1996). In this kind of occurrence, many lead-antimony-arsenic

sulfosalts have been identified, including guettardite (Bracci et al., 1980), robinsonite (Franzini et al., 1992), izoklakeite (Orlandi et al., 2010), zinkenite, boulangerite, sartorite, semseyite, and jordanite (Orlandi et al., 1996). Seravezza is also the type locality for two additional lead sulfosalts, moëloite (Orlandi et al., 2002) and disulfodadsonite (Orlandi et al., 2013a).

Bernarlottiite was identified in few specimens collected in the Ceragiola area. It occurs as mm-sized thin acicular crystals, elongated [010], lead-grey in color (Fig. 1), with a black streak and a metallic luster. It is brittle, without any evident cleavage. Owing to the very small size of the available crystals, micro-hardness was not measured. In plane-polarized incident light, bernarlottiite is white in colour, with abundant red internal reflections. Pleochroism is weak in air, whereas it is distinct in oil, with shades of grey-blue. Bireflectance is distinct. Between crossed polars, bernarlottiite is distinctly to strongly (in oil) anisotropic, with greyish to bluish rotation tints. Twinning revealed by the X-ray diffraction study was not observed. Reflectance values (WTiC as standard) were measured in air and are given in Table 2 and shown in Figure 2.

In the studied specimen, bernarlottiite is associated with Sb-rich sartorite; this latter phase has not been fully characterized yet. The crystallization of these minerals is related to the circulation

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of hydrothermal fluids in the cavities of the marbles during the Tertiary Alpine tectono-metamorphic event.

2.2. Chemical data

Two grains of bernarlottiite (# 2987 and 3819, respectively) were analyzed with a CAMECA SX50 electron microprobe (BRGM-CNRS-University common laboratory, Orléans, France) operating in WDS mode. The operating conditions were: accelerating voltage 20 kV, beam current 20 nA, beam size 5 μm. Standards (element, emission line) were: pyrite (S Kα), galena (Pb Mα), stibnite (Sb Lα), AsGa (As Lα), and metal Cu (Cu Kα).

Electron microprobe data for bernarlottiite are given in Table 3. On the basis of ΣMe = 28 atoms per formula unit, the chemical formula of bernarlottiite can be written as Cu0.07(12)Pb11.71(18)As11.59(21)Sb4.63(9)S35.78(48) and Cu0.02(2)Pb11.92(6)As10.12(14)Sb5.95(8)S36.76(32) for grains #2987

and 3819, respectively. The S excess shown by the analysis on grain #3819 is not supported by the solution of the crystal structure (see below). The small Cu content can be subtracted according to the substitution rule Cu+ + (As,Sb)3+ = 2 Pb2+. The As/(As+Sb) atomic ratio in bernarlottiite varies

between 0.63 (#3819) and 0.71 (#2987), corresponding to the two idealized compositions Pb12As11.5Sb4.5S36 and Pb12As10Sb6S36.

2.3. Crystallography

The X-ray powder diffraction pattern of bernarlottiite was collected using a 114.6 mm Gandolfi camera with Ni-filtered Cu Kα radiation. The observed pattern is reported in Table 4, where it is compared with the calculated one obtained through the software PowderCell (Kraus & Nolze, 1996) using the structural model described below. Owing to the multiplicity of indices for the majority of the diffraction lines, the unit-cell parameters were not refined.

Due to the very small size of available crystals, intensity data of bernarlottiite were collected at the XRD1 beamline at the Elettra synchrotron radiation facility, Basovizza, Trieste, Italy. Data collection were performed by rotating the crystal around one axis by Δφ = 1° and collecting the reflections by means of a 165 mm MarCCD detector with a working distance of 50 mm. Reflections were integrated and intensities corrected for Lorentz-polarization and background effects using the HKL package of software (Otwinowski & Minor, 1997). The statistical tests on the distribution of |E| values (|E2 – 1| = 0.995) suggested the occurrence of a centre of symmetry. The refined unit-cell

parameters are a = 23.704(8), b = 8.386(2), c = 23.501(8) Å, α = 89.91(1), β = 102.93(1), γ = 89.88(1)°, V = 4553(2) Å3, space group P

1

. The a:b:c ratio is 2.827:1:2.802.

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The structure of bernarlottiite was solved by direct methods using Shelxs-97 (Sheldrick, 2008) and then refined through Shelxl-2014 (Sheldrick, 2008). Scattering curves for neutral atoms were taken from the International Tables for Crystallography (Wilson, 1992). After having located the heavier atoms (Pb and some Sb sites), the structure was completed through successive difference-Fourier maps. The examination of the bond distances, as well as bond-valence balance, calculated using the bond parameters given by Brese & O’Keeffe (1991), revealed the presence of several mixed (Pb/Sb) and (Sb/As) sites and their site occupation factors were refined. Finally, the occurrence of a twin axis [010] was introduced, giving a twin ratio of 72:28. After several cycles of anisotropic refinement, the agreement factor R1 converged to 0.088 for 7317 reflections with Fo >

4σ(Fo). Details of the intensity data collection and crystal structure refinement are given in Table 5.

3. Crystal structure description

3.1. General organization

Atomic coordinates, site occupancies, and equivalent isotropic displacement parameters of bernarlottiite are given in Table 6. The unit-cell content is shown in Figure 3.

The general organization of bernarlottiite, as seen down b, is presented in Figure 4. This mineral is a N(1,2) = 3,4 member of the sartorite homologous series and it is homeotypic with baumhauerite, argentobaumhauerite, boscardinite, and écrinsite (Orlandi et al., 2012; Biagioni & Moëlo, 2016; Topa & Makovicky, 2016; Topa et al., 2016). Its crystal structure can be described as formed by the 1:1 alternation along c of layers, one of the sartorite type (N = 3), the second one of the dufrénoysite type (N = 4). These layers are connected through Pb atoms with tricapped trigonal prismatic coordination. The twinning may correspond to layer stacking disorder, for instance (3, 4, 4), i.e. liveingite-type homologue in the ···3-4-3-4-3-4··· sequence.

Within the layers, all sites correspond to pure Pb, As, Sb, or mixed (Pb/Sb) and (As/Sb) positions. Whereas in baumhauerite and boscardinite each type of layer is formed by one kind of oblique ribbon, two different kinds of oblique ribbons occur in each layer of bernarlottiite (Fig. 4). The first one is centrosymmetric, whereas the second one is acentric. Both N = 3 and N = 4 layers are formed by one centrosymmetric ribbon and two acentric ribbons, that induce the 3a superstructure relatively to primitive, Sb-free baumhauerite.

3.2. Cation coordinations and site occupancies

Forty-two independent metal sites and fifty-four S positions have been located in the crystal structure of bernarlottiite. Table 7 gives average bond distances and bond-valence sums, calculated according to the bond parameters of Brese & O’Keeffe (1991).

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Table 8 gives the chemical distribution of metal sites. There are sixteen pure Pb positions, four mixed (Pb/Sb) sites (two with Pb > Sb and two with Sb > Pb), and twenty-two Me3+ sites (Me3+

= As, Sb). Pure lead sites display coordination numbers ranging from six to nine, corresponding to trigonal prismatic to tricapped trigonal prismatic coordination. Pb1 to Pb12, at the layer junction, are “standing” tricapped trigonal prisms, with average bond lengths ranging from 3.161 (Pb8) and 3.232 Å (Pb11), and bond-valence sums between 1.74 (Pb7) and 1.88 valence unit (v.u.) (Pb11). Pb14 to Pb17, within the dufrénoysite type layer, display distorted octahedral (Pb14) and monocapped trigonal prismatic coordination (Pb15 to Pb17).

The four mixed (Pb/Sb) positions are equally partitioned between sartorite and dufrénoysite type layers. Actually, Pb-dominant sites (i.e., Pb18 and Pb19 sites) are located within the N = 3 layer and display six- (Pb19) and seven-fold (Pb18) coordination. The Sb-dominant sites (Sb13 and Sb33) are hosted within the N = 4 layer. Sb13 has a distorted octahedral coordination, with two Me– S distances shorter than 2.70 Å, three distances ranging between 2.70 and 3.10 Å, and a sixth definitely longer distance at 3.34 Å. The bond-valence sum at the Sb13 site is 2.65 v.u., to be compared with an expected value of 2.77 v.u. Sb33 shows a similar six-fold coordination environment. However, in addition to the two bond distances shorter 2.70 Å, there are only two distances in the range 2.70 – 3.10 Å, whereas the two remaining Me–S bonds are longer (i.e., 3.37 and 3.52 Å). The bond-valence sum (2.59 v.u.) agrees with the expected one (2.68 v.u.).

The twenty-two Me3+ sites can be further subdivided into eight pure As positions, four pure

Sb sites, and ten mixed (As/Sb) positions (eight with As > Sb and two with Sb > As). Pure As sites display the typical three-fold pyramidal coordination, with average <As–S> distance (taking into account distances shorter than 2.70 Å) ranging between 2.257 (As25) and 2.290 (As24) Å. The coordination sphere is usually completed by two additional longer bonds. The pure Sb sites usually display also the typical trigonal pyramidal coordination, with average bond distances (taking into account Sb–S distances shorter than 2.70 Å) varying between 2.515 (Sb31) and 2.549 Å (Sb38). The only exception is represented by the Sb21 site, having only two distances shorter than 2.70 Å, with two additional bonds between 2.75 and 2.85 Å. The coordination environment of the pure Sb positions is completed by three (or two, for Sb31) S atoms, giving rise to Sb-centered distorted octahedra. Finally, the ten mixed (As/Sb) sites show the trigonal pyramidal coordination completed by two or three longer Me–S bonds. The average <Me–S> distances are related to the (As:Sb)at.

ratio, ranging from 2.285 for the As22 site (s.o.f. As0.88Sb0.12) to 2.489 Å for the Sb36 site (s.o.f.

Sb0.71As0.29). According to Table 7, the bond-valence sums of all (As/Sb) positions range between

2.90 (Sb21) and 3.33 v.u. (As20).

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3.3. Polymerization of (Sb/As) sites

Within the sartorite (N = 3) and dufrénoysite (N = 4) type layers, if only the shortest (= strongest) Me3+–S distances are considered (i.e. distances shorter than 2.70 Å, following the

approach of Moëlo et al., 2012), the organization of Me3+ sites into finite Me3+

mSn chain fragments (hereafter “polymers”) can be described. By using such a cut-off distance, As and Sb atoms usually display the classic triangular pyramidal coordination. In the crystal structure of bernarlottiite, the exceptions are represented by the central portions of the two kinds of ribbons within the N = 4 type layer, and in the central portion of the centrosymmetric ribbon within the N = 3 type layer, with Sb or mixed (Sb,Pb) positions having only one or two short distances.

In the N = 3 layers, the two different kinds of oblique ribbons are indicated as Sc and Sa

(subscript c and a indicating the centrosymmetric and non-centrosymmetric natures of the oblique ribbons, respectively). Figure 5 (left) represents the selection of one Sc oblique ribbon. There are

two dualities in cation positions:

1. Sb38 is a mean position between S38 and S53 (sub-position “down” – Sb38–S38 = 2.729 Å; “up” – Sb38–S53 = 2.688 Å);

2. in two neighbouring (Pb,Sb)19 sites, with quite equal occupancies, filling one position by Sb (for instance Sb “up”) will induce the filling of the other position by Pb (Pb “down”), to optimize the local valence equilibrium.

From one side of the ribbon to the other, the choice of combination of Sb38 “up” with Sb19 “up” will imply Pb19 “down” followed by Sb38 “down”, i.e. the polymer sequence AsS3 →

(Sb,As)3S6 → (Sb,As)3S7. The combination of Sb38 “up” with Sb19 “down” will give the polymer

sequence AsS3 → (Sb,As)2S4 → (Sb,As)4S9.

In one Sa oblique ribbon (Fig. 5, right), the main filling of (Pb,Sb)18 by Pb gives two

polymers, (Sb,As)3S7 and (Sb,As)4S9. With Sb18 present, there is only one polymer (Sb,As)7S16.

As in other structures, Sb prefers to be partitioned within the central part of the ribbon, whereas As is preferentially hosted in the marginal group. Indeed, the higher As/(As+Sb)at. ratio

shown by bernarlottiite with respect to boscardinite and (Tl,As)-rich boscardinite favours the As-to-Sb substitution at the isolated trigonal pyramids (As-to-Sb-dominant in boscardinite and As-dominant in (Tl,As)-rich boscardinite – Orlandi et al., 2012; Biagioni & Moëlo, 2016).

The oblique ribbons occurring in the dufrénoysite type layer (N = 4) are shown in Figure 6. The centrosymmetric Dc ribbon (Fig. 6, left) shows lateral [As2(As,Sb)]Σ3S7 chain fragments; the

central portion of the ribbon is occupied by two mixed (Sb,Pb) sites, i.e. Sb13 and Sb33. Consequently, the size of the polymer is determined by the presence or absence of Sb on these positions. These sites have only two Me–S distances shorter than 2.70 Å, probably as a result of

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their average nature. Both sites have two longer Me–S in the range between 2.70 and 3.00 Å. Sb13 has two S atoms at 2.78 (S14) and 2.80 Å (S23), respectively, while Sb33 is at 2.76 and 2.90 Å from S9 and S23, respectively. In our opinion, Sb33–S9 bond could be favoured, because a slight displacement of Sb from the refined position allows a correct three-fold coordination; on the contrary, owing to the similarity in bond distances between Sb13–S14 and Sb13–S23, both solutions could be equally possible. If the first solution occurs, assuming a full Sb occupancy at both Sb13 and Sb33, the polymer [As4(As,Sb)2Sb2]Σ8S17 can be defined. The other configuration

gives rise to the polymer [As4(As,Sb)2Sb4]Σ10S20. The Da ribbon (Fig. 6, right) is composed by two

polymers, mutually perpendicular each other, with metals located on the opposing sides of the ribbon. The two chain fragments are [As2(As,Sb)(Sb,As)]Σ4S9 and [As(As,Sb)2Sb]Σ4S9. In addition,

the Sb21 site can belong to both chains. Indeed, Sb21 is an average position, with only two Me–S distances shorter than 2.70 Å. It could be alternatively bonded to S19 or S16. In the first case, a linear [As(As,Sb)2Sb2]Σ5S11 polymer occurs; in the other case, a branched chain [As2(As,Sb)

(Sb,As)Sb]Σ5S11 can be identified.

4. Discussion

4.1. Crystal-chemistry of bernarlottiite

The crystal-chemical formula of bernarlottiite, as obtained through the refinement of its crystal structure, is Pb11.93As10Sb6.07S36 (Z = 3), with the relative error on the valence equilibrium Ev(%), as defined in the footnote of Table 3, of +0.1. With respect to the chemical data, the formula derived from the crystal structure refinement agrees with sample #3819.

Taking into account electron-microprobe data, the homologue order of bernarlottiite can be calculated according to Makovicky & Topa (2015), resulting in N = 3.47 and 3.49 for samples #2987 and 3819, respectively, in very good agreement with the expected value N = 3.5. Indeed, as stated above, the crystal structure of bernarlottiite displays the 1:1 alternation, along c, of two kinds of layers, sartorite type (N = 3) and dufrénoysite type (N = 4). Both layer types are formed by two kinds of ribbons. In the sartorite type layer, the centric Sc and acentric Sa ribbons have chemical

composition [Pb4(Pb1.02Sb0.98)Σ2(As3.60Sb2.40)Σ6.00S16]-1.02 and [Pb4(Pb0.81Sb0.19)Σ1.00(As4.64Sb2.36)Σ7.00S16]-0.81,

giving an overall composition of the N = 3 layer corresponding to [Pb12(Pb2.64Sb1.36)Σ4(As12.88Sb7.12)Σ20.00S48]-2.64. The dufrénoysite type layer is formed by two ribbons Dc

and Da having chemical composition [Pb6.00(Sb2.86Pb1.14)Σ4(As5.68Sb0.32)Σ6.00S20]+0.86 and

[Pb7.00(As5.72Sb3.28)Σ9.00S20]+1. The overall composition of the N = 4 layer is

[Pb20(Sb2.86Pb1.14)Σ4(As17.12Sb6.88)Σ24.00S60]+2.86. Consequently, the chemical formula of bernarlottiite is

Pb32(Sb4.22Pb3.78)Σ8(As30.00Sb14.00)Σ44.00S108. On the basis of the general formula Me8NS8N+8 (e.g., 10 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 19

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Makovicky & Topa, 2015), with N = 3.5, it could be written as Pb10.67(Sb1.41Pb1.26)Σ2.67(As10.00Sb4.67)Σ14.67S36 (Z = 3).

The simplified chemical formula of the sartorite-type layer is close to Pb5Me3+7S16, against

Pb4Me3+8S16 for ideal sartorite; that of the dufrénoysite-type layer is Pb7Me3+9S20, against Pb8Me3+8S20

for ideal dufrénoysite (Table 8). In other words, to minimize the distorsion between the two layers, the large cation Pb2+ partly substitutes As3+ in the ideally As-pure sartorite layer, while, on the

contrary, the reverse substitution operates in the dufrénoysite-type layer, also permitting to respect the valence equilibrium. It is a clear example of interlayer crossed-substitution rule, i.e. As3+

sart +

Pb2+

dufr → Pb2+sart + As3+dufr.

Considering the partitioning of As and Sb between the two kinds of layers, the As/(As+Sb)at.

ratio is 0.603 and 0.637 in the sartorite and dufrénoysite type layers, respectively. In the two kinds of ribbons forming the N = 3 layer, the As/(As+Sb)at. ratio indicates the preferential partitioning of

Sb in the Sc ribbon (atomic ratio = 0.516) with respect to the Sa ribbons (atomic ratio = 0.645). In

the dufrénoysite layer, on the contrary, there is no significant difference between the acentric and centric ribbons, having As/(As+Sb)at. ratios of 0.636 and 0.641, respectively.

This inter-ribbon partitioning in the sartorite layer has an important consequence: the “symmetrization” of the Sb-rich ribbon, while all ribbons are dissymmetric in Sb-free baumhauerite (as well as in dufrénoysite – Ribar et al., 1969). Such a partial ribbon “symmetrization” by Sb enrichment appears as the essential crystal chemical factor for the formation of a new baumhauerite homeotype by stabilization of a 3a superstructure.

4.2. Chemical variability of “baumhauerites” and the occurrence of superstructures

Bernarlottiite is a new addition to the N = 3.5 homeotypes within the sartorite homologous series. Baumhauerite was originally described as a pure Pb-As sulfosalt from the Lengenbach quarry, Binn Valley, Switzerland (Solly & Jackson, 1902); Laroussi et al. (1989), examining minerals belonging to the sartorite series from this Swiss locality, found less than 0.5 wt% Sb in baumhauerite, corresponding to an As/(As+Sb)at. ratio close to 0.99. Similarly, baumhauerite from

Moosegg, Salzburg, Austria, was Sb-free (Topa & Makovicky, 2016).

The crystal-chemical studies of N = 3.5 members of the sartorite group highlighted three different kinds of substitutions:

a) the heterovalent substitution Ag+ + (As/Sb)3+ = 2 Pb2+;

b) the heterovalent substitution Tl+ + (As/Sb)3+ = 2 Pb2+;

c) the homovalent substitution As3+ = Sb3+. 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343

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The occurrence of Ag in baumhauerite was accurately described by Laroussi et al. (1989) who reported two types of baumhauerite in syntactic lamellar intergrowth, i.e. Ag-free and Ag-rich (~ 1.5 wt%). Pring et al. (1990) pointed out the occurrence of a 2a superstructure (actually a 2c superstructure, assuming the axial setting given in Table 1), naming this new homeotype baumhauerite-2a. The appearance of the superstructure reflections was related to some kind of cation ordering; this hypothesis was confirmed by Topa & Makovicky (2016) who solved the crystal structure of baumhauerite-2a and renamed it as argentobaumhauerite. Indeed, the doubling of the c parameter is related to the occurrence of two different kinds of N = 4 slabs, displaying different degrees of substitution of As and Pb by Ag. In addition, Pring & Graeser (1994) observed the occurrence of a pseudo-orthorhombic 3c phase (3a in the axial setting of those authors).

Thallium was reported in “baumhauerites” from Lengenbach by Laroussi et al. (1989) in only small amount, up to 0.65 wt% in argentobaumhauerite. The recent findings of boscardinite (Orlandi et al., 2012) and écrinsite (Topa et al., 2016) allowed the description of the first homeotypes in which Tl is an essential component. In addition, both mineral species have significant Sb substituting for As, with As/(As+Sb)at. ratios ranging from 0.55 in écrinsite (Topa et al., 2016) down to 0.21-0.31 in boscardinite (Orlandi et al., 2012; Biagioni & Moëlo, 2016). Boscardinite is actually the first N = 3.5 homeotype in which Sb dominates over As.

The very first evidences of high Sb contents in “baumhauerites” were given by Jambor (1967) who found an As:Sb atomic ratio close to 1 in samples from Madoc, Ontario, Canada. Further chemical analyses indicated a slight dominance of Sb over As, with Sb-to-As ratios of 1.03 and 1.12 in the two studied samples (Jambor et al., 1982). The occurrence of bearing and Sb-rich baumhauerite was reported by other authors, e.g., Robinson & Harris (1987) who found an As/ (As+Sb)at. ratio of 0.86 in a “baumhauerite” from Quiruvilca, Perù. Dunn (1995) reported the

occurrence of an antimonian baumhauerite from Sterling Hill, New Jersey, USA; its As/(As+Sb)at.

ratio is 0.715, close to the values found for bernarlottiite, i.e. 0.63-0.71. It is worth noting that Dunn (1995) observed the same 3a supercell found in bernarlottiite; consequently it is very likely that antimonian baumhauerite from Sterling Hill is actually bernarlottiite.

4.3. Building operators in baumhauerite homeotypes and other homologues

Three main crystal-chemical factors operate for the building of the structure of bernarlottiite and its differentiation from baumhauerite.

1. Interlayer As/Pb crossed-substitution favouring steric adjustment between the sartorite-and the dufrénoysite-type layers. The Pb-enriched sartorite layer has a composition close to Pb5As7S16, like in “baumhauerite-2a” (Engel & Nowacki, 1969). This interlayer steric 12 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 23

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adjustment also operates in liveingite (= rathite II), according to Engel & Nowacki (1970): the sartorite layer has effectively the formula Pb5As7S16, although in the double

dufrénoysite layer the As/Pb ratio is a little bit too high (Pb6.75As9.25S20, ideally

Pb7.5As8.5S20 for the charge balance). Such a crystal-chemical mechanism may be the

main factor minimizing the combination energy of the N = 3 and 4 layers relatively to a mixture of primitive homologues sartorite and dufrénoysite.

2. Polymerization of Me3+–S short bonds reflects the steric adjustment of Me3+-rich ribbons

to columns or slabs of standing trigonal prismatic Pb atoms (prism axis parallel to the elongation). An increase of the As content relatively to Sb as well Pb induces a general shortening of the polymers (Doussier et al., 2008).

3. Sb-versus-As inter-ribbon partitioning, with symmetrization of the Sb-rich ribbon, which controls the 3a superstructure of bernarlottiite. In Sb- and Tl-free species, all ribbons are dissymmetric. On the contrary, with a high Sb/As atomic ratio, the symmetrization is complete, e.g., in boscardinite, a (Tl,Sb)-rich homeotype of baumhauerite (Orlandi et al., 2012), as well as in twinnite (Makovicky & Topa, 2012) and guettardite (Makovicky et al., 2012), two Sb-rich homeotypes of sartorite.

In synthetic Sb-pure sartorite members BaSb2S4 (Cordier et al., 1984) and BaSb2Se4 (Cordier

& Schaefer, 1979), ribbons are necessary centrosymmetric (no Sb/As or Pb/(As,Sb) mixing). It is also the case in Sb dufrénoysite derivatives Ba3Sb4.667S10 and Ba2.62Pb1.39Sb4S10 (Choi & Kanatzidis,

2000), although with a small dissymmetric partitioning of Pb and Ba in this last compound. In philrothite (Bindi et al., 2015), owing to the replacement of 2 Pb by (Tl + Sb), there is no Tl within the ribbon, that induces centrosymmetry.

5. Conclusion

Bernarlottiite is the first N(1,2) = 3,4 member of the sartorite homologous series having a 3a superstructure. Thus, the pair baumhauerite – bernarlottiite is a new example in the group of sulfosalts related the PbS or SnS archetypes where a small change in the chemistry (i.e., the Sb/As ratio here) gives rise to homeotypic pairs, controlling a crystallographic discontinuity between close species. Indeed, the different partitioning of As and Sb in the crystal structures of sulfosalts can give rise to superstructure reflections, e.g., as in the chabournéite – protochabournéite pair (Orlandi et al., 2013b), the Sb-to-As substitution being related to an increase in the size of the polymeric (Sb/As)mSn units, in agreement with Doussier et al. (2008).

Bernarlottiite structure enhance the role of three building operators: 1) the interlayer As-versus-Pb crossed substitution, stabilizing the combined N = (3,4) baumhauerite homologue, 2) the 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411

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inter-ribbon Sb partitioning in the sartorite layer, with “symmetrization” of the Sb-rich ribbon, that induces the 3a superstructure, and 3) the common (As,Sb) polymerization through short (As,Sb) – S bonds.

Bernarlottiite exemplifies the importance of an accurate crystal-chemical study of complex sulfosalts. Indeed, the As-to-Sb substitution could give rise to isotypic series (e.g., the bournonite – seligmannite and geocronite – jordanite pairs) or it could promote the appearance of new structural configurations (e.g., occurrence of superstructure reflections, changes in space group symmetry) and, consequently, it could favour the crystallization of new mineral species. The occurrence of superstructures seems to be a phenomenon characteristic of several members of the sartorite homologous series. In addition to bernarlottiite, several new minerals have been recently defined (e.g., enneasartorite, hendekasartorite, heptasartorite) whose occurrence is related to chemical changes. Finally, it is worth noting that polloneite (Topa et al., 2015b), a N = 4 homologue related to dufrénoysite, presented a 3 × 7.9 Å superstructure apparently similar to that observed in bernarlottiite. Consequently, the sartorite homologous series is probably one of the most prolific research field today in sulfosalt systematics.

Acknowledgments: Stefano Conforti is thanked for providing us with the studied specimens.

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Table captions

Table 1 – Members of the sartorite homologous series. Table 2 – Reflectance data (%) for bernarlottiite in air.

Table 3 – Electron microprobe analyses of bernarlottiite: chemical composition as wt% and

chemical formula (in atoms per formula unit, apfu) on the basis of ΣMe = 28 apfu.

Table 4 –X-ray powder diffraction data for bernarlottiite. Intensities and dhkl were calculated using

the software PowderCell 2.3 (Kraus and Nolze, 1996) on the basis of the structural model given in Table 4. The seven strongest reflections are given in bold. Only reflections with Icalc ≥ 10 were

reported, if not observed. Observed intensities were visually estimated (s = strong; m = medium; mw = medium-weak; w = weak; vw = very weak).

Table 5 – Crystal data and summary of parameters describing data collection and refinement for

bernarlottiite.

Table 6 – Atomic coordinates, site occupation factors, and equivalente isotropic displacement

parameters (Å2) for bernarlottiite.

Table 7 – Average bond-distances (in Å) and bond-valence sums (in valence units) for metal sites

in bernarlottiite.

Table 8 – Metal site distribution within the crystal structure of bernarlottiite. Sc and Sa represent the

centric and acentric ribbons in the N = 3 layer, respectively; Dc and Da represent the centric and

acentric ribbons in the N = 4 layers.

Figure captions

Figure 1 – Bernarlottiite, black acicular crystals. Ceragiola area, Seravezza, Apuan Alps, Tuscany,

Italy. Collection Museo di Storia Naturale, Università di Pisa. Catalogue number 19687.

Figure 2 – Reflectance spectra of bernarlottiite. For sake of comparison, the reflectance spectra of

its Tl-Sb homeotype boscardinite (Orlandi et al., 2012) are shown.

Figure 3 – Unit-cell content of bernarlottiite, as seen down b.

Figure 4 – General organization of bernarlottiite, as seen down b. Dotted and dashed lines delimit

the N =3 and N = 4 ribbons, respectively. Red and blue lines indicate the centrosymmetric and acentric ribbons, respectively.

Figure 5 – Polymeric organization of (Sb/As) atoms with S atoms (short bonds, thick dark green

lines) in the sartorite type layer. The two ribbons Sc (left) and Sa (right) are shown. Thick tie-lines 20 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 39

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correspond to longer bonds, related to mean Sb positions, or mixed (Pb,Sb) sites. In Sc ribbon (left),

the lower part indicates the two possible polymer combinations (separated by red tie-line), according to the “up” or “down” positions of Sb atoms.

Figure 6 – Polymeric organization of (Sb/As) atoms with S atoms (short bonds, thick dark green

lines) in the dufrénoysite type layer. The two ribbons Dc (left) and Da (right) are shown. 612 613 614 615 616 617

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Table 1 – Members of the sartorite homologous series. Unit-cell parameters are given as AE (elongation axis), AL (layer stacking axis), and AR

(ribbon stacking axis).

Mineral Chemical formula AE (Å) AR (Å) AL (Å) Angles ≠ 90° V (Å3) S.G. Ref.

N = 3,3 Sartorite PbAs2S4 4.19 7.89 19.62 648.6 P21/n [1] Guettardite Pb(Sb0.56As0.44)2S4 8.53 7.97 20.10 101.8 1337.3 P21/c [2] Twinnite Pb(Sb0.63As0.37)2S4 8.63 8.00 19.52 91.1 (AE/AR) 1347.4 P21/n [3] Heptasartorite Tl7Pb22As55S108 29.27 7.88 20.13 102.1 (AE/AL) 4537.9 P21/c [4] Enneasartorite Tl6Pb32As70S140 37.61 7.88 20.07 101.9 (AE/AL) 5818.6 P21/c [5, 6] Hendekasartorite Tl2Pb48As82S172 31.81 7.89 28.56 99.0 (AE/AL) 7076.4 P21/c [7] Pierrotite TlSb5S8 8.82 7.99 38.75 2728.9 Pna21 [8] Parapierrotite Tl(Sb,As)5S8 9.06 8.10 19.42 92.0 (AE/AR) 1423.0 Pn [9] Synth. BaSb2S4 8.99 8.20 20.60 101.4 (AE/AL) 1488.7 P21/c [10] Synth. BaSb2Se4 9.24 8.55 20.76 91.2 (AE/AR) 1639.4 P21/n [11] N = 3, 4 AE/AL AR/AL AE/AR Baumhauerite Pb12As16S36 8.34 7.88 22.81 90.1 97.3 90.1 1488.8 P1 [12] Argentobaumhauerite AgPb10As17S36 8.47 7.91 44.41 84.6 86.5 89.8 2954.2 P-1 [12] Bernarlottiite Pb12(As10Sb6)Σ16S36 8.39 23.70 23.50 89.9 102.9 89.9 4553 P-1 [13]

Écrinsite AgTl3Pb4(As11Sb9)Σ20S36 8.53 8.08 22.61 90.2 97.2 90.8 1546.7 P1 [14]

(Tl,As)-rich boscardinite AgTl3Pb4(Sb14As6)Σ20S36 8.65 8.10 22.56 90.7 97.2 90.8 1569.6 P-1 [15] Boscardinite AgTl2Pb6(Sb15As4)Σ19S36 8.76 8.09 22.50 90.9 97.2 90.8 1582.0 P-1 [16] N = 4, 3, 4 Liveingite Pb20As24S56 8.37 7.91 70.49 90.1 (AE/AR) 4669.8 P21/c [17] N = 4, 4 Dufrénoysite Pb8As8S20 8.37 7.90 25.74 90.4 (AE/AL) 1702.0 P21 [18] Veenite Pb8(Sb,As)8S20 8.42 8.96 26.2 117.4 (AE/AR) 1747.6 P21 [19] Polloneite Ag0.17Pb7.67(As4.33Sb3.83)Σ8.16S20 8.41 23.82 25.90 90.0 (AE/AL) 5189.8 P21 [20] Rathite (AgAs)Pb6As8S20 8.50 7.97 25.12 100.7 (AE/AL) 1672.2 P21/c [21]

Carducciite (AgSb)Pb6(As,Sb)8S20 8.49 8.02 25.40 100.4 (AE/AL) 1701.6 P21/c [22]

Barikaite* Ag0.5(AgAs)Pb5(As,Sb)8.5S20 8.53 8.08 24.95 100.7 (AE/AL) 1688.8 P21/c [23]

Philrothite** Tl4As12S20 8.62 8.01 24.83 90.1 (AE/AR) 1715.9 P21/n [24]

Synth. Li0.73Eu3As4.43S10 8.46 7.82 24.62 99.8 (AR/AL) 1605.0 P21/c [25]

Synth. Na0.66Eu2.86As4.54S10 8.43 7.80 25.14 100.2 (AR/AL) 1626.9 P21/c [25]

Synth. Ba3Sb4.667S10 8.96 8.22 26.76 100.3 (AE/AL) 1939.0 P21/c [26]

Synth. Ba2.6Pb1.4Sb4S10 8.84 8.20 26.76 99.5 (AE/AL) 1914.3 P21 [26]

Synth. “shift” derivative Ba2Sb2Se5 4.64 8.40 27.57 1075.3 Pbam [27]

22 618

619

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[1] Nowacki et al. (1961); [2] Makovicky et al. (2012); [3] Makovicky & Topa (2012); [4] Topa et al. (2015d); [5] Topa et al. (2015a); [6] Berlepsch et al. (2003); [7] Topa et al. (2015c); [8] Engel et al. (1983); [9] Engel (1980); [10] Cordier et al. (1984); [11] Cordier & Schaefer (1979); [12] Topa & Makovicky (2016a); [13] this work; [14] Topa et al. (2016); [15] Biagioni & Moëlo (2016); [16] Orlandi et al. (2012); [17] Engel & Nowacki (1970); [18] Marumo & Nowacki (1967); [19] Topa & Makovicky (2016b); [20] Topa et al. (2015b); [21] Berlepsch et al. (2002); [22] Biagioni et al. (2014); [23] Makovicky & Topa (2013); [24] Bindi et al. (2014); [25] Bera et al. (2007); [26] Choi & Kanatzidis (2000); [27] Wang et al. (2015).

* Unit-cell transformed in the conventional space group P21/c though the matrix R = [1 0 0 | 0 -1 0 | -1 0 -1].

** Unit-cell transformed in order to compare with dufrénoysite through the matrix R = [-1 0 0 | 0 -1 0 | 1 0 1].

620 621 622 623 624 625 626 627

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Table 2 – Reflectance data (%) for bernarlottiite in air.

λ (nm) Rmin Rmax λ (nm) Rmin Rmax

400 30.3 - 560 30.1 37.1 420 29.4 39.6 580 29.9 36.9 440 29.9 38.3 589 29.7 36.8 460 29.8 38.1 600 29.8 36.7 470 30.0 37.5 620 29.2 35.9 480 29.9 37.6 640 29.5 36.4 500 29.9 37.2 650 29.3 36.2 520 30.2 37.4 660 28.8 35.5 540 30.4 37.5 680 28.4 36.9 546 30.3 37.3 700 28.4 36.9

Table 3 – Electron microprobe analyses of bernarlottiite: chemical composition as wt% and

chemical formula (in atoms per formula unit, apfu) on the basis of ΣMe = 28 apfu.

2987 (n = 3) 3819 (n = 5)

Element wt% range e.s.d. wt% range e.s.d.

Cu 0.09 0.00 – 0.27 0.16 0.02 0.00 – 0.07 0.03 Pb 48.89 47.48 – 49.92 1.26 47.43 47.24 – 47.88 0.26 As 17.48 17.25 – 17.68 0.22 14.56 14.31 – 14.92 0.24 Sb 11.36 11.29 – 11.47 0.10 13.92 13.69 – 14.12 0.18 S 23.11 22.85 – 23.47 0.32 22.64 22.37 – 22.81 0.17 Total 100.93 99.48 – 102.22 1.38 98.58 98.19 – 99.25 0.46 Cu 0.07 0.00 – 0.20 0.12 0.02 0.00 – 0.05 0.02 Pb 11.71 11.52 – 11.87 0.18 11.92 11.84 – 11.98 0.06 As 11.59 11.34 – 11.75 0.21 10.12 9.95 – 10.30 0.14 Sb 4.63 4.58 – 4.74 0.09 5.95 5.85 – 6.02 0.08 S 35.78 35.22 – 36.07 0.48 36.76 36.25 – 37.10 0.32 Ev* 0.8 -0.5 – 2.6 1.6 -2.0 -0.6 - -2.9 0.9 Pbcorr.** 11.85 11.52 – 12.28 0.39 11.95 11.84 – 12.08 0.09 (As+Sb)corr.** 16.15 15.72 – 16.48 0.39 16.05 15.92 – 16.16 0.09 As/(As+Sb)at. 0.71 0.71 – 0.72 0.00 0.63 0.62 – 0.64 0.01

*Relative error on the valence equilibrium (%), calculated as [Σ(val+) – Σ(val-)]×100/Σ(val-). **Pbcorr. and (As+Sb)corr. on the basis of the substitution Cu+ + (As,Sb)3+ = 2Pb2+.

24 628 629 630 631 632 633 47

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Table 4 –X-ray powder diffraction data for bernarlottiite. Intensities and dhkl were calculated using

the software PowderCell 2.3 (Kraus and Nolze, 1996) on the basis of the structural model given in Table 4. The seven strongest reflections are given in bold. Only reflections with Icalc ≥ 10 were

reported, if not observed. Observed intensities were visually estimated (s = strong; m = medium; mw = medium-weak; w = weak; vw = very weak).

Iobs dobs Icalc dcalc h k l Iobs dobs Icalc dcalc h k l

w 7.7 13 7.70 0 0 3 ms 2.823 46 2.825 -7 0 6 w 7.2 25 7.18 -2 0 3 16 2.816 -3 2 6 w 6.2 18 6.15 -3 0 3 12 2.812 -3 -2 6 w 5.8 25 5.82 2 0 3 23 2.806 -6 -2 3 w 5.17 13 5.18 -4 0 3 14 2.803 -6 2 3 w 4.91 7 4.901 3 0 3 vs 2.748 63 2.763 1 2 6 vw 4.583 9 4.581 5 0 0 63 2.756 1 -2 6 w 4.399 14 4.393 -5 0 3 56 2.728 -4 2 6 ms 4.170 57 4.170 4 0 3 58 2.725 -4 -2 6 48 4.126 1 2 0 m 2.667 15 2.668 5 0 6 48 4.123 1 -2 0 18 2.657 2 2 6 11 4.024 5 1 0 18 2.649 2 -2 6 w 3.927 25 3.925 -2 0 6 w 2.621 11 2.617 -1 0 9 s 3.851 87 3.850 0 0 6 w 2.568 6 2.567 0 0 9 s 3.794 65 3.796 -3 0 6 9 2.567 -4 0 9 62 3.773 -6 0 3 w 2.510 13 2.507 8 0 3 m 3.661 49 3.665 1 0 6 vw 2.448 2 2.451 6 0 6 12 3.622 -2 2 3 s 2.363 54 2.367 8 2 0 ms 3.592 60 3.599 5 0 3 55 2.362 8 -2 0 13 3.589 -4 0 6 m 2.287 17 2.284 3 0 9 18 3.581 1 2 3 11 2.283 -7 0 9 23 3.572 1 -2 3 w 2.257 21 2.257 7 0 6 634 635 636 637 638 639

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ms 3.442 69 3.466 -3 2 3 vs 2.221 23 2.222 -1 2 9 72 3.465 -3 -2 3 22 2.222 -3 2 9 32 3.406 2 2 3 25 2.218 -3 -2 9 30 3.397 2 -2 3 23 2.218 -1 -2 9 12 3.386 4 2 0 12 2.208 -9 -2 3 11 3.380 4 -2 0 17 2.206 -9 2 3 w 3.347 18 3.338 -5 0 6 vw 2.170 8 2.168 -8 0 9 13 3.288 -7 0 3 vw 2.131 4 2.134 -11 0 3 s 3.278 100 3.272 7 0 0 vw 2.120 4 2.118 6 2 6 39 3.260 -4 -2 3 m 2.087 87 2.096 0 4 0 41 3.260 -4 2 3 ms 2.056 22 2.052 5 0 9 ms 3.167 59 3.191 3 2 3 14 2.051 -9 0 9 58 3.182 3 -2 3 vw 2.009 4 2.008 10 -2 0 33 3.165 3 0 6 vw 1.965 4 1.962 -4 0 12 26 3.151 6 0 3 5 1.955 -1 0 12 s 3.075 75 3.077 -6 0 6 s 1.935 14 1.936 -5 0 12 ms 3.043 55 3.034 -5 -2 3 13 1.928 4 2 9 57 3.032 -5 2 3 11 1.926 -8 2 9 m 2.954 71 2.961 4 2 3 10 1.925 0 0 12 84 2.952 4 -2 3 13 1.924 4 -2 9 ms 2.903 36 2.908 4 0 6 ms 1.869 17 1.869 8 2 6 28 2.904 -8 0 3 18 1.864 8 -2 6 30 2.876 -1 2 6 m 1.833 13 1.829 -11 -2 6 21 2.870 -1 -2 6 11 1.828 -11 2 6 26 51

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Table 5 – Crystal data and summary of parameters describing data collection and refinement for

bernarlottiite.

Crystal data

X-ray formula Pb11.93As10.00Sb6.07S36

Crystal size (mm3) 0.03 x 0.005 x 0.005

Cell setting, space group Triclinic, P

1

a (Å) 23.704(8) b (Å) 8.386(2) c (Å) 23.501(8) α (°) 89.91(1) β (°) 102.93(1) γ (°) 89.88(1) V (Å3) 4553(2) Z 3

Data collection and refinement

Radiation, wavelength (Å) synchrotron, λ = 0.7

Temperature (K) 293 2θmax 42.30 Measured reflections 10709 Unique reflections 8048 Reflections with Fo > 4σ(Fo) 7317 Rint 0.0461 0.1189 Range of h, k, l 24 ≤ h ≤ 24, 7 ≤ k ≤ 7, 22 ≤ l ≤ 24 R [Fo > 4σ(Fo)] 0.0885 R (all data) 0.0958 wR (on Fo2) 0.2631 Goof 1.095 Number of least-squares parameters 880 Maximum and

minimum residual peak (e Å-3) -2.03 (at 1.04 Å from Pb19)2.91 (at 2.07 Å from S41) 640 641 642 643 644 645 646 647

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Table 6 – Atomic coordinates, site occupation factors, and equivalente isotropic displacement

parameters (Å2) for bernarlottiite.

Site s.o.f. x/a y/b z/c Ueq

Pb1 Pb1.00 0.1400(1) 0.6147(4) -0.7667(1) 0.0718(8) Pb2 Pb1.00 0.1425(1) 0.1250(4) -0.7614(1) 0.0588(7) Pb3 Pb1.00 -0.5247(1) 0.6307(4) -0.7656(1) 0.0684(8) Pb4 Pb1.00 0.4774(1) 0.1274(4) -0.7692(1) 0.0594(7) Pb5 Pb1.00 0.8112(1) 0.1316(4) -0.7637(1) 0.0598(7) Pb6 Pb1.00 -0.1911(1) 0.6404(4) -0.7648(1) 0.0636(7) Pb7 Pb1.00 0.0037(1) 0.1205(4) -0.6565(1) 0.0661(8) Pb8 Pb1.00 0.0018(1) 0.3736(4) -0.3388(1) 0.0614(7) Pb9 Pb1.00 0.3307(1) 0.3763(4) -0.3435(1) 0.0585(7) Pb10 Pb1.00 0.3334(1) -0.1266(4) -0.3416(1) 0.0592(7) Pb11 Pb1.00 0.6618(1) 0.3550(4) -0.3341(1) 0.0625(8) Pb12 Pb1.00 0.6754(1) -0.1395(4) -0.3405(1) 0.0591(7) Sb13 Sb0.75(2)Pb0.25(2) 0.0041(1) 0.1281(5) 0.0701(2) 0.059(2) Pb14 Pb1.00 0.3364(1) 0.1069(4) 0.0743(1) 0.0560(7) Pb15 Pb1.00 0.6634(1) 0.6236(4) 0.0882(1) 0.0533(6) Pb16 Pb1.00 0.9964(1) 0.6375(4) 0.0870(1) 0.0562(7) Pb17 Pb1.00 0.3258(1) 0.6172(3) 0.0815(1) 0.0532(6) Pb18 Pb0.81(2)Sb0.19(2) 0.4186(1) 0.3860(4) -0.4808(1) 0.055(1) Pb19 Pb0.51(3)Sb0.49(3) 0.0739(2) 0.3723(5) -0.4847(2) 0.075(2) As20 As0.79(4)Sb0.21(4) 0.7336(2) 0.3880(7) -0.5077(2) 0.053(2) Sb21 Sb1.00 0.3288(2) -0.1196(5) -0.0669(2) 0.048(1) As22 As0.88(4)Sb0.12(4) 0.6074(2) 0.3152(8) 0.1736(2) 0.055(2) As23 As1.00 0.9367(2) 0.3804(8) 0.1854(2) 0.045(1) As24 As1.00 0.2716(2) 0.3210(8) 0.1726(2) 0.050(2) As25 As1.00 0.6021(2) -0.1289(8) 0.1845(2) 0.049(1) As26 As1.00 0.9412(2) -0.0630(8) 0.1743(2) 0.050(2) As27 As1.00 0.2667(2) -0.1305(8) 0.1848(2) 0.050(2) As28 As0.79(4)Sb0.21(4) 0.4728(2) 0.3800(8) 0.0692(2) 0.053(2) Sb29 Sb0.68(4)As0.32(4) 0.8042(2) 0.4350(6) 0.0635(2) 0.055(2) As30 As0.84(4)Sb0.16(4) 0.1433(2) 0.3799(8) 0.0648(2) 0.053(2) Sb31 Sb1.00 0.4701(1) -0.1690(6) 0.0532(2) 0.052(1) As32 As0.73(4)Sb0.27(4) 0.8091(2) -0.1274(8) 0.0646(2) 0.054(2) Sb33 Sb0.68(2)Pb0.32(2) 0.1375(1) -0.1602(5) 0.0544(2) 0.066(2) As34 As1.00 0.5194(2) 0.6231(9) -0.3800(2) 0.049(1) As35 As1.00 0.8408(2) 0.6284(9) -0.3892(2) 0.049(2) Sb36 Sb0.71(4)As0.29(4) 0.1778(2) 0.6441(7) -0.3908(2) 0.054(2) As37 As0.77(4)Sb0.23(4) 0.5105(2) 0.0800(8) -0.3912(2) 0.053(2) Sb38 Sb1.00 0.8423(1) 0.1489(6) -0.3917(2) 0.054(2) As39 As1.00 0.1766(2) 0.1183(9) -0.3884(2) 0.048(2) As40 As0.79(4)Sb0.21(4) 0.4010(2) -0.1280(8) -0.5076(2) 0.050(2) Sb41 Sb1.00 0.7271(1) -0.1441(6) -0.4910(2) 0.049(1) As42 As0.80(4)Sb0.20(4) 0.0689(2) -0.1296(8) -0.5064(2) 0.054(2) S1 S1.00 0.6651(4) 0.674(2) 0.2097(5) 0.044(4) S2 S1.00 0.9994(4) 0.573(2) 0.2093(5) 0.034(3) S3 S1.00 0.3314(4) 0.673(2) 0.2116(5) 0.049(4) S4 S1.00 0.6628(4) 0.089(2) 0.2072(5) 0.039(3) S5 S1.00 0.9978(4) 0.157(2) 0.2104(5) 0.044(3) S6 S1.00 0.3285(4) 0.092(2) 0.2055(5) 0.037(3) S7 S1.00 0.5272(4) 0.589(2) 0.1125(5) 0.042(3) S8 S1.00 0.8634(4) 0.662(2) 0.1164(5) 0.042(3) S9 S1.00 0.1956(5) 0.582(2) 0.1136(6) 0.050(4) S10 S1.00 0.5339(5) 0.174(2) 0.1172(6) 0.046(3) S11 S1.00 0.8692(5) 0.076(2) 0.1171(6) 0.050(4) S12 S1.00 0.2008(5) 0.177(2) 0.1151(6) 0.054(4) S13 S1.00 0.7538(4) 0.394(2) 0.1387(6) 0.047(3) S14 S1.00 0.0775(4) 0.370(2) 0.1226(6) 0.044(3) 28 648 649 55

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S15 S1.00 0.4112(5) 0.364(2) 0.1298(6) 0.047(3) S16 S1.00 0.7448(4) -0.120(2) 0.1262(5) 0.045(3) S17 S1.00 0.0828(4) -0.137(2) 0.1349(5) 0.042(3) S18 S1.00 0.4180(4) -0.126(2) 0.1298(5) 0.043(3) S19 S1.00 0.6017(4) 0.355(2) 0.0027(5) 0.046(3) S20 S1.00 0.9379(5) 0.352(2) 0.0070(5) 0.046(3) S21 S1.00 0.2642(5) 0.389(2) -0.0013(5) 0.042(3) S22 S1.00 0.6048(5) -0.091(2) 0.0143(6) 0.043(3) S23 S1.00 0.9335(5) -0.102(2) 0.0030(6) 0.042(3) S24 S1.00 0.2643(4) -0.139(2) 0.0000(5) 0.041(3) S25 S1.00 0.4296(4) 0.638(2) -0.2520(5) 0.037(3) S26 S1.00 0.7626(4) 0.641(2) -0.2533(5) 0.042(3) S27 S1.00 0.1012(4) 0.619(2) -0.2647(5) 0.038(3) S28 S1.00 0.4363(4) 0.129(2) -0.2633(5) 0.038(3) S29 S1.00 0.7715(5) 0.130(2) -0.2624(5) 0.046(3) S30 S1.00 0.0967(4) 0.112(2) -0.2521(5) 0.039(3) S31 S1.00 0.5960(5) 0.621(2) -0.3041(6) 0.046(3) S32 S1.00 0.9202(5) 0.630(2) -0.3169(5) 0.047(3) S33 S1.00 0.2511(5) 0.628(2) -0.3029(5) 0.045(3) S34 S1.00 0.5814(5) 0.112(2) -0.3093(5) 0.043(3) S35 S1.00 0.9184(5) 0.133(2) -0.3036(6) 0.048(3) S36 S1.00 0.2546(5) 0.117(2) -0.3158(5) 0.044(3) S37 S1.00 0.4687(5) 0.427(2) -0.3504(5) 0.042(3) S38 S1.00 0.7978(5) 0.418(2) -0.3531(5) 0.043(3) S39 S1.00 0.1313(5) 0.325(2) -0.3560(5) 0.045(3) S40 S1.00 0.4700(5) -0.157(2) -0.3567(5) 0.040(3) S41 S1.00 0.7991(4) -0.154(2) -0.3572(5) 0.040(3) S42 S1.00 0.1325(4) -0.103(2) -0.3582(6) 0.042(3) S43 S1.00 0.3555(4) -0.381(2) -0.4339(5) 0.038(3) S44 S1.00 0.6842(4) -0.351(2) -0.4379(5) 0.036(3) S45 S1.00 0.0223(5) -0.397(2) -0.4306(5) 0.047(4) S46 S1.00 0.3576(4) 0.136(2) -0.4270(5) 0.035(3) S47 S1.00 0.6906(5) 0.104(2) -0.4389(5) 0.040(3) S48 S1.00 0.0215(4) 0.135(2) -0.4369(5) 0.036(3) S49 S1.00 0.5677(4) 0.334(2) -0.4449(5) 0.037(3) S50 S1.00 0.8960(5) 0.337(2) -0.4451(6) 0.046(3) S51 S1.00 0.2303(5) 0.410(2) -0.4447(5) 0.038(3) S52 S1.00 0.5606(5) -0.082(2) -0.4434(5) 0.040(3) S53 S1.00 0.8967(5) -0.070(2) -0.4438(5) 0.041(3) S54 S1.00 0.2310(5) -0.176(2) -0.4450(5) 0.041(3) 650

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Table 7 – Average bond-distances (in Å) and bond-valence sums (in valence units) for metal sites

in bernarlottiite.

Site <Me–S> BVS Site <Me–S> BVS Site <Me–S> BVS

Pb1 3.191 1.78 Pb15 3.045 2.03 Sb29 2.438 2.93 Pb2 3.211 1.87 Pb16 3.044 2.02 As30 2.291 3.25 Pb3 3.204 1.85 Pb17 3.034 2.03 Sb31 2.515 2.94 Pb4 3.188 1.84 Pb18 3.040 2.08 As32 2.356 3.09 Pb5 3.201 1.87 Pb19 2.918 2.28 Sb33 2.947 2.59 Pb6 3.186 1.80 As20 2.297 3.33 As34 2.266 3.12 Pb7 3.203 1.74 Sb21 2.468 2.90 As35 2.270 3.08 Pb8 3.161 1.84 As22 2.285 3.21 Sb36 2.489 2.97 Pb9 3.186 1.84 As23 2.262 3.20 As37 2.341 3.07 Pb10 3.189 1.75 As24 2.290 3.00 Sb38 2.549 3.14 Pb11 3.232 1.88 As25 2.257 3.19 As39 2.268 3.11 Pb12 3.164 1.84 As26 2.276 3.05 As40 2.308 3.25 Sb13 2.863 2.65 As27 2.265 3.14 Sb41 2.530 2.93 Pb14 2.973 2.08 As28 2.301 3.16 As42 2.293 3.32

Note: for pure As, pure Sb, and mixed (Sb/As) sites, <Me–S> distances are calculated taking into account Me–S distances shorter than 2.70 Å.

30 651 652 653 654 655 59

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Table 8 – Metal site distribution within the crystal structure of bernarlottiite. Sc and Sa represent the

centric and acentric ribbons in the N = 3 layer, respectively; Dc and Da represent the centric and

acentric ribbons in the N = 4 layers.

N = 3 N = 4 Position number Metal site Sc Sa (× 2) Dc Da (× 2) Pb (interlayer) 4 4 4 4 12 Pb (intralayer) 0 0 2 2 4 (Pb/Sb) 2 (0.51/0.49) 0.81/0.19 0 0 2 (Sb/Pb) 0 0 2 (0.68/0.32) 2 (0.75/0.25) 0 2 Sb 2 1 0 2 4 (Sb/As) 0 0.71/0.29 0 0.68/0.32 2 (As/Sb) 2 (0.80/0.20) 0.77/0.23 0.79/0.21 (× 2) 2 (0.84/0.16) 0.73/0.27 0.79/0.21 0.88/0.12 8 As 2 2 4 4 8 Total/Ribbon Centric Pb5.02Sb3.38As3.60S16 Pb7.14Sb3.18As5.68S20 Acentric (× 2) Pb4.81Sb2.55As4.64S16 Pb7.00Sb3.28As5.72S20 Mean Pb4.88(Sb2.83As4.29)Σ7.12S16 Pb7.05(Sb3.25As5.71)Σ8.96S20 Simplified Pb5(Sb3As4)Σ7S16 Pb7(Sb3As6)Σ9S20

Ideal Pb4As8S16 (sartorite) Pb8As8S20 (dufrénoysite)

Total/Layer (Pb14.64Sb8.48As12.88S48)-2.64 (Pb21.14Sb9.74As17.12S60)+2.86

Formula unit

(Z = 3) Pb11.93(Sb6.07As10.00)Σ16.07S36

Note: in the upper part, integers correspond to number of atom positions, whereas non-integer values to site occupancy factors in mixed positions.

656 657 658 659 660 661

(32)

Figure 1 – Bernarlottiite, black acicular crystals. Ceragiola area, Seravezza, Apuan Alps, Tuscany,

Italy. Collection Museo di Storia Naturale, Università di Pisa. Catalogue number 19687.

32 662

663

664

(33)

Figure 2 – Reflectance spectra of bernarlottiite. For sake of comparison, the reflectance spectra of

its Tl-Sb homeotype boscardinite (Orlandi et al., 2012) a shown.

665 666

Riferimenti

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