1 2 3 The speciation of in (Tl,Sb,As)-rich 4 1 5 6 2 7 8 9 3 Luke L. Georgea*, Cristian Biagionia†, Giovanni Orazio Leporeb, Maria Lacalamitaa, 10 11 4 Giovanna Agrosìc, Gian Carlo Capitanid, Elena Bonaccorsia, Francesco d’Acapitob 12 13 5 14 15 6 a Dipartimento di Scienze della Terra, Università di Pisa, Via S. Maria 53, I-56126 Pisa, Italy 16 17 7 b CNR-IOM-OGG, 71 Avenue des Martyrs, F-38043 Grenoble, France 18 19 8 c Dipartimento di Scienze della Terra e Geoambientali, Università degli Studi di Bari “Aldo Moro”, 20 21 9 via Orabona 4, I-70125 Bari, Italy 22 23 10 d Dipartimento di Scienze dell’Ambiente e di Scienze della Terra, Università degli Studi di Milano- 24 25 11 Bicocca, Piazza della Scienza 4, I-20126 Milano, Italy 26 27 12 28 29 Abstract 30 13 31 32 14 Three samples of (Tl,Sb,As)-rich pyrite from the ore deposits of southern Apuan Alps (Tuscany, Italy) 33 34 15 were studied through a multi-technique approach in order to constrain the speciation and incorporation 35 36 16 mechanism of thallium in pyrite. High concentrations of Tl, Sb, and As were detected in all the studied 37 38 17 samples through laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS). Average 39 40 18 Tl contents were 1,299 ppm, 1,967 ppm, and 2,623 ppm in samples from Sennari, Canale della Radice, 41 42 19 and Fornovolasco, respectively. The LA-ICP-MS time-resolved down-hole ablation profiles were 43 44 20 smooth indicating that Tl, Sb, and As are dissolved in the pyrite matrix, or occur in homogeneously 45 46 21 distributed nanoparticles (NPs). X-ray absorption spectroscopy (XAS) data revealed that Tl, Sb, and As 47 + 3+ 3+ 1- 48 22 occurs as Tl , Sb , As , and As . In all the studied samples, bond distances and coordination numbers 49 3+ 3+ 50 23 for Sb and As are constant, whereas Tl displays a range of coordination numbers (~3 to ~6), 51 52 53 54 * Luke George present address: School of Chemical Engineering, The University of Adelaide, Adelaide S.A. 55 5005, Australia 56 † Corresponding author: Cristian Biagioni [email protected] 57 58 59 60 61 62 24 revealing the occurrence of different local environments. Moreover, no hints of features corresponding 63 64 25 to coordination shells higher than the first were observed, suggesting an extremely disordered 65 66 26 environment around Tl. Transmission electron microscopy (TEM) investigations revealed the presence 67 68 27 of dislocations and As-rich “fluid” inclusions, whereas no Tl-rich NPs were observed. The study 69 70 28 suggests that Tl+ could occur in structural defects in pyrite, lacking any long-range order. The ‘loose’ 71 72 29 nature of Tl in pyrite favours its ready loss during the metamorphic recrystallization of pyrite and its 73 74 30 redistribution within ore bodies and country rocks, with important implications from an economic and 75 76 31 environmental point of view. 77 78 32 79 80 33 Key-words: pyrite; thallium; speciation; LA-ICP-MS; XAS; TEM 81 82 34 83 84 35 1 Introduction 85 86 36 87 88 37 Thallium has received much scientific attention over the last few decades, given that even low 89 90 38 concentrations are highly toxic for humans, more toxic than As, Hg, Cd or Pb (e.g., Nriagu, 1998; Peter 91 92 39 and Viraraghavan, 2005). Thallium tends to bioaccumulate in living organisms once it enters the 93 94 40 biosphere (Karbowska, 2016), making it a dangerous contaminant in the environment. As such, Tl has 95 been classified as one of the 13 priority metal pollutants (Keith and Telliard, 1979). 96 41 97 42 Although the crust is richer in Tl (average continental crust concentration of 0.52 ppm - 98 99 43 Wedepohl, 1995) than, for example, Ag or Au, this element is rarely concentrated in significant 100 101 44 abundance as other metals may be. Having both lithophile and chalcophile geochemical behaviour, Tl 102 103 45 is generally dispersed within minerals. Given the similarity in ionic radii of Tl+ and K+ (1.70 Å and 1.64 104 105 46 Å in twelve-fold coordination, respectively - Shannon, 1976), Tl commonly substitutes for K in 106 107 + + 47 feldspars and phyllosilicates. Moreover, the ionic radius of Tl is very close to that of NH4 (e.g., 108 109 48 Zelenski et al., 2009) and consequently it could occur in rocks enriched in organic matter, especially 110 111 49 under reducing conditions. 112 113 50 Thallium may become significantly concentrated in sulphides, as proved by the large amounts 114 115 51 of Tl occurring in some sulphide mineral deposits. For instance, Xiangquan, China, the world’s solitary 116 117 118 119 120 121 52 Tl-only mine, hosts around 250 tons of Tl (Zhou et al., 2005), whereas Meggen, Germany, contains 960 122 123 53 tons of Tl, the largest accumulation on Earth (Laznicka, 2010). Thallium is mainly produced as a by- 124 125 54 product of pyrite-rich ore smelting (e.g., Peter and Viraraghavan, 2005) and, indeed, up to 3.5 wt. % Tl 126 127 55 is hosted in some pyrite from Xiangquan, clearly demonstrating the ability of pyrite to host significant 128 129 56 concentrations of this heavy metal. 130 131 57 Recently, the Tl-rich nature of several small pyrite ± ± iron oxide ores hosted in the 132 133 58 metamorphic units of the southern Apuan Alps (northern Tuscany, Italy) was recognised (e.g., Biagioni 134 135 59 et al., 2013; D’Orazio et al., 2017). This is primarily expressed as Tl-rich pyrite (some pyrite samples 136 137 60 concentrate over 4,200 ppm Tl - George et al., 2018a), and as assemblages comprising rare Tl-sulfosalts 138 139 61 (e.g., Orlandi et al., 2012, 2013; Biagioni et al., 2014) mobilised in part as Tl-rich melts after release 140 141 62 from pyrite during greenschist facies metamorphic recrystallization (Biagioni et al., 2013; George et 142 143 63 al., 2018a). Release of Tl from pyrite due to oxidation has also caused severe Tl contamination in both 144 145 64 stream waters and a public water supply system in the vicinity of the Apuan Alps pyrite ores (e.g., 146 147 65 Biagioni et al., 2017; D’Orazio et al., 2017). 148 149 66 Despite pyrite being such an important Tl host worldwide, and its potential to release Tl into 150 151 67 the environment under a range of physico-chemical conditions, the speciation of Tl in pyrite has never 152 153 68 been unequivocally determined. Monovalent Tl should be the dominant oxidation state for Tl in most 154 natural environments (Vink, 1993; Xiong, 2007). Nevertheless, trivalent Tl, the other naturally 155 69 156 70 occurring oxidation state of Tl, may be stabilized by the anionic complexes Cl− or OH− (Batley and 157 158 71 Florence, 1975), or oxidized from Tl+ by Tl-oxidizing bacteria or UV irradiation (Karlsson et al., 2006). 159 160 72 As such, some authors have suggested a trivalent, or even native occurrence, of Tl in pyrite (e.g., Huston 161 162 73 et al., 1995; Zhou et al., 2005). Moreover, it has not been clearly shown whether Tl enters the crystal 163 164 74 structure of pyrite or tends to occur as micro- to nano-particles of Tl-bearing phases. Deditius and Reich 165 166 75 (2016) reported a wedge-shaped trend on a Tl vs. As plot, similar to the trend observed by Reich et al. 167 168 76 (2005) for Au vs. As. This was interpreted to define a field where concentrations of Tl are likely present 169 170 77 in solid solution, and thus that the solid solubility of Tl in pyrite is strongly dependent on As 171 172 78 concentration. Similarly, George et al. (2018a) showed an approximately one-to-one correlation 173 174 79 between (Tl + Cu + Ag) and Sb in pyrite from the southern Apuan Alps ore deposits, possibly indicating 175 176 177 178 179 180 80 that Tl (as well as Cu and Ag) could be hosted in pyrite following the substitution mechanism 2Fe2+ ↔ 181 182 81 Tl+ + Sb3+. However, as will be discussed below, the difference in ionic radius between Fe on the one 183 184 82 hand, and Tl and Sb on the other, is likely too large and the proposed substitution mechanism seems 185 186 83 difficult to envision. 187 188 84 Thus, Deditius and Reich (2016) suggested that further work was necessary to elucidate the 189 190 85 speciation of Tl in pyrite. The aforementioned authors recommended that such efforts should combine 191 192 86 determination of Tl composition and depth-profiling by laser ablation inductively-coupled plasma mass 193 194 87 spectrometry (LA-ICP-MS), transmission electron microscopy (TEM) studies to detect the presence of 195 196 88 any potential nano-particles, and synchrotron-based X-ray absorption near edge structure (XANES) and 197 198 89 extended X-ray absorption fine structure (EXAFS) studies to determine the oxidation state and the local 199 200 90 bonding environment of Tl. 201 202 91 Here, we attempt to combine these different methods (LA-ICP-MS, XANES, EXAFS, and 203 204 92 TEM) in order to determine the speciation of Tl in the (Tl, Sb, As)-rich pyrite from the southern Apuan 205 206 93 Alps. We report XANES and EXAFS data for Tl, as well as for Sb and As. Moreover, LA-ICP-MS and 207 208 94 TEM investigations were made with the aim to determine if Tl is hosted in the pyrite , 209 210 95 or in micro- to nanoparticles (NPs) of other Tl-bearing phases. Understanding the speciation of Tl in 211 212 96 pyrite is important since it should help clarify the mechanisms of release and dispersion of Tl in natural 213 systems, particularly during pyrite oxidation, with significant environmental implications. 214 97 215 98 216 217 99 2 Sample material 218 219 100 220 221 101 Three Tl-rich pyrite samples, previously investigated by George et al. (2018a), were used in 222 223 102 this study. One was collected from Sennari (SEN1), a small occurrence of pyrite cropping out on the 224 225 103 road from Sant’Anna di Stazzema to Case Sennari, in the southern Apuan Alps, Tuscany, Italy. Another 226 227 104 sample came from the Canale della Radice mine (sample CDR4) and the last from the Fornovolasco 228 229 105 mine (sample FOR19). These two mines exploited small orebodies belonging to a series of pyrite ± 230 231 106 baryte ± iron oxide ore deposits located within a ~10 km NE-SW belt in the southern Apuan Alps 232 233 107 (D’Orazio et al., 2017 and references therein). The Apuan Alps represent a tectonic window exposing 234 235 236 237 238 239 108 the most complete succession of northern Apennine units in a complex tectonic nappe formed due to 240 241 109 numerous over-thrusts during the Tertiary Apennine Orogeny (Fellin et al., 2007 and references 242 243 110 therein). 244 245 111 The orebodies at Canale della Radice and Fornovolasco were mostly monomineralic lens- 246 247 112 shaped bodies with pyrite or magnetite and minor baryte. The geological setting of these deposits is 248 249 113 described in D’Orazio et al. (2017) and references therein. The orebodies were metamorphosed to the 250 251 114 greenschist facies during the Apennine Orogeny, partly remobilising the ore and recrystallizing pyrite 252 253 115 (George et al., 2018a and references therein). While the Sennari sample does not come from a mine, it 254 255 116 is hosted in the same metamorphic units and shares similar mineralogy, textures and pyrite 256 257 117 geochemistry to samples from Apuan Alps deposits (George et al., 2018a). 258 259 118 SEN1 contains networks and bands of very fine to fine subhedral to anhedral pyrite in baryte, 260 261 119 benstonite and lesser ankerite (Fig. 1A); accessory cymrite was observed. Pyrite sometimes forms 262 263 120 granoblastic aggregates (Fig. 1B). CDR4 hosts coarse, sometimes granoblastic, aggregates and 264 265 121 disseminations of very fine to fine grained euhedral to anhedral pyrite (Fig. 1C, D), sometimes aligned 266 267 122 in rough bands. Some medium to coarse-grained relict euhedral pyrite and very fine grained framboidal 268 269 123 pyrite is also present. Matrix minerals include siderite, dolomite, “chlorite”, “biotite”, cymrite, albite 270 271 124 and quartz. FOR19 contains very coarse grained colloform banded pyrite (Fig. 1E, F) and lesser fine 272 grained anhedral pyrite in siderite, dolomite and quartz. In some places, finer grained pyrite is being 273 125 274 126 replaced by pyrrhotite and arsenopyrite. 275 276 127 277 278 128 3 Methodology 279 280 129 281 282 130 3.1 Laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) 283 284 131 285 286 132 LA-ICP-MS data were collected using an ASI M-50-LR 193 nm Excimer laser attached to an 287 288 133 Agilent 7700cx Quadrupole inductively coupled-mass spectrometer (Adelaide Microscopy, University 289 290 134 of Adelaide) following the methods set out in George et al. (2018a). LA-ICP-MS spot analyses were 291 292 135 used for accurate quantitative concentration measurements and monitoring of potential micro- to nano- 293 294 295 296 297 298 136 inclusions in pyrite using time-resolved down-hole ablation profiles. LA-ICP-MS element mapping was 299 300 137 used to monitor trace element variation and zonation. Laser repetition rate was maintained at 5 Hz for 301 302 138 spot analysis and 10 Hz for mapping. The total acquisition time for each individual spot analysis was 303 304 139 70 s: 30 s of background measurement followed by 40 s of sample ablation. For mapping, a 10 s 305 306 140 background measurement was acquired before the ablation of each trench. A 20 s delay time was 307 308 141 allowed after each spot or trench analysis to ensure the ablation cell was sufficiently washed-out and 309 310 142 the gases had stabilized. 311 312 143 During spot analysis and mapping, the following suite of isotopes were measured: 27Al, 29Si, 313 314 144 43Ca, 49Ti, 53Cr, 55Mn, 57Fe, 59Co, 60Ni, 65Cu, 66Zn, 69Ga, 71Ga, 75As, 77Se, 95Mo, 107Ag, 111Cd, 115In, 118Sn, 315 316 145 121Sb, 125Te, 137Ba, 182W, 197Au, 202Hg, 205Tl, 206Pb, 207Pb, 208Pb and 209Bi. During spot analysis, the dwell 317 318 146 time for most elements was set at 0.02 s, while Se, Au and Tl were set at 0.04 s, Co, Ni, Cu and Zn were 319 320 147 set at 0.01 s and Al, Si, Ca, Mn, Fe and Ba were set at 0.005 s. Aluminium, Si, Ca and Ba counts were 321 322 148 monitored to ensure gangue phases were not analysed; these elements were not quantified. During 323 324 149 mapping, the dwell time for most elements was 0.01 s, while Ti, Cr, Mn, Co, Ni, Cu, Zn, As, Ba, Pb 325 326 150 and Bi were set at 0.005 s and Al, Si, Ca and Fe were set at 0.002 s. 327 328 151 Two 51-µm analyses were made on the STDGL3 standard (Belousov et al., 2015) every 20 329 330 152 unknown spot analyses, as well as before and after a map acquisition. All data calculations were carried 331 out using Iolite (Paton et al., 2011), an open source software package for ICP-MS data processing 332 153 333 154 developed by the Melbourne Isotope Group as an add-in for the data analysis program Igor by 334 335 155 WaveMetrics. Iron was used as the internal standard assuming stoichiometric pyrite. 336 337 156 338 339 157 3.2 X-ray absorption spectroscopy (XAS) 340 341 158 342 343 159 X-ray absorption spectroscopy (XAS) measurements on pyrite at the Tl L3-edge (12,658 eV), 344 345 160 As K-edge (11,867 eV) and Sb K-edge (30,491 eV) were made at the LISA beamline (BM-08; d’Acapito 346 347 161 et al., 2019) at the European Synchrotron Radiation Facility (ESRF, Grenoble, France) during three 348 349 162 different experimental sessions. Pyrite was selected in order to avoid the gangue minerals and an aliquot 350 351 163 of the powdered samples was checked through X-ray powder diffraction using a Bruker D2 Phaser 352 353 354 355 356 357 164 diffractometer operating at 30 kV and 10 mA (Ni-filtered Cu Kα radiation) and equipped with a one- 358 359 165 dimensional Linxeye detector. Pyrite was the only mineral above detection limit (> 1 vol%). However, 360 361 166 the occurrence of minor amounts of accessory sulphides and gangue minerals cannot be avoided. 362 363 167 Since measurements were taken in a wide energy range, Pd- or Pt-coated mirrors were used for 364 365 168 harmonics rejection (Ecutoff ≈ 18 KeV and 40 KeV, respectively). Model compounds and sample material 366 367 169 were powdered, mixed with cellulose and pressed in pellets. The amount of material used was such as 368 369 170 to keep the maximum total absorption (μ) around 1.5. All samples were measured in fluorescence mode 370 371 171 by means of a 12-element solid state (high purity Germanium) detector while all reference compounds, 372

373 172 with the exception of protochabournéite at the Tl L3-edge (see text below), were measured in 374 375 173 transmission mode. All the fluorescence detected data were deadtime corrected; in order to check for 376 377 174 the presence of over-absorption effects, self-absorption corrections were run on collected spectra at all 378 379 175 absorption edges in ATHENA (Ravel and Newville, 2005), using the FLUO (available on Daniel Haskel 380 381 176 webpage at APS, https://www3.aps.anl.gov/haskel/) and Booth (Booth and Bridges; 2005) algorithms 382 383 177 for XANES and EXAFS regions, respectively. Results of self-absorption corrections (see Appendix A) 384 385 178 rule out the possibility of over-absorption effects since experimental and absorption-corrected data are 386 387 179 essentially superimposed at all measured edges. In order to reduce the damping of the signal due to the 388 389 180 thermal contribution and to prevent possible beam-induced redox reactions, all samples were measured 390 at 80 K using a liquid N cold finger cryostat (Puri et al., 2019). 391 181 2 392 182 Spectra at the Tl L -edge were acquired in the energy range 12,458-13,206 eV. The energy 393 3 394 183 sampling interval in the higher resolution near edge region (12,638–12,688 eV) was 0.5 eV. A fixed 395 396 184 exit sagittally focusing monochromator (d’Acapito et al., 2014) with a pair of Si [311] crystals were 397 398 185 used; beam size was ~2000 (H) μm × 150 (V) μm. Measured reference compounds were Tl2O3, 399 400 186 protochabournéite (~Tl2Pb(Sb9–8As1–2)Σ10S17 - Orlandi et al., 2013) and Tl2SO4. Synthetic Tl2O3 and 401 402 187 Tl2SO4 were collected in transmission mode. Suitable filters were used for fluorescence measurements 403 404 188 in order to attenuate for Fe and As fluorescence emission. Pyrite samples bearing significant As, as well 405

406 189 as protochabournéite, showed strong As fluorescence emission (Kα1 = 10,543 eV) which almost 407

408 190 superimposes on the Tl emission line (Lα1 = 10,269 eV) considering that the energy resolution of the 409 410 191 detector is about 300 eV. Therefore, spectra were collected with a Ga filter (K-edge = 10,367 eV) 411 412 413 414 415 416 192 associated with a 40 μm Al filter to damp the Ga fluorescence. The Ga filter was specifically prepared 417 418 193 for this experiment by deposing Ga O on various layers of teflon filters to obtain a thickness equivalent 419 2 3 420 194 to 80 μm of Ga2O3, absorbing most part of the fluorescence emission above the Ga K-edge. The teflon 421 422 195 layers are very fragile and, probably due to the presence of holes and fractures in the filter, the final 423 424 196 effect was closer to a filter with an equivalent thickness of 30 μm of Ga2O3. The actual effect of the 425 426 197 filter on transmissivity at the different energies is summarized in Appendix B; the count rate on the 427 428 198 detector in the Tl emission region was ~500 cts/s (with total counts on the detector always lower than 429 430 199 10/20k cts/s). The small Se amounts (Table 1) present in the samples were found to be below detection 431

432 200 limit; besides, the Se emission line (Kα1 = 11,224 eV) is well separated from that of Tl and its emission 433 434 201 would be mainly absorbed by the Ga filter. A Se (Se K-edge = 12,657.8 eV) model compound was 435 436 202 placed in a second experimental chamber, allowing simultaneous spectrum acquisition with each 437 438 203 measurement on the samples and thus accurate energy calibration. A minimum of ten spectra were 439 440 204 collected for each sample and no hints of beam damage effects were shown in each series of 441 442 205 measurements. 443 444 206 For As investigation, samples were measured after the first part of the BM08 refurbishment 445 446 207 (d’Acapito et al., 2019) using a pair of flat Si [111] monochromator crystals. Beam size was ~2000 (H) 447 448 208 μm × 150 (V) μm Spectra were acquired in the energy range 11,667-12,850 eV. Seven As compounds 449 were also measured as standards in transmission mode: skutterudite (CoAs ), protochabournéite (As3+ 450 209 3 451 210 sulphide), arsenopyrite (FeAsS), adamite (Zn AsO OH), (As S ), As O and As O 452 2 4 2 3 2 3 2 5 453 211 (hydrated). 454 455 212 For Sb investigation, samples were measured with the BM08 setup (d’Acapito et al., 2019), 456 457 213 using a pair of flat Si [311] monochromator crystals and Pt coated focusing mirrors (Ecutoff ≈ 40 KeV). 458 459 214 Beam size was ~200 (H) μm × 200 (V) μm. Measured reference compounds were Sb2S3, Sb2O3 and 460 461 215 Sb2O5. A reference Sb foil was also measured at the same time in order to accurately calibrate the 462 463 216 energy. For each sample, six spectra were collected and no hints of beam damage effects were shown 464 465 217 in each series of measurements. 466 467 218 The software ATHENA (Ravel and Newville, 2005) was used to average multiple spectra. 468 469 219 Standard procedures (Lee et al., 1981) were followed to extract the structural EXAFS signal (k•χ(k)): 470 471 472 473 474 475 220 pre-edge background removal, spline modelling of bare atomic background, edge step normalization 476 477 221 using a far above the edge region, and energy calibration. Model atomic clusters centred on the absorber 478 479 222 atom were obtained by ATOMS (Ravel, 2001); theoretical amplitude and phase functions were 480 481 223 generated using the FEFF8 code (Ankudinov et al., 1998). EXAFS spectra were fitted through the 482 483 224 ARTEMIS software (Ravel and Newville, 2005) in the Fourier-Transform (FT) space. 484 485 225 486 487 226 3.3 Transmission electron microscopy (TEM) 488 489 227 490 491 228 Transmission electron microscopy investigations were performed on the samples FOR19 and 492 493 229 CDR4 (the samples having the highest average Tl content) at the Dipartimento di Scienze della Terra e 494 495 230 Geoambientali of the University of Bari, and at the Dipartimento di Scienze Fisiche, della Terra e 496 497 231 dell’Ambiente of the University of Siena, with two analogous JEOL-JEM 2010, having the following 498 499 232 characteristics: operating voltage 200 kV, LaB6 source, nominal point resolution of ~2.0 Å and spherical 500 501 233 aberration of 0.5 mm. In order to distinguish between artefacts induced by sample preparation and the 502 503 234 intrinsic micro/nano structures of the samples, two different methods, ion milling and grinding, were 504 505 235 used. For ion milled mounts, the samples were prepared as 30 m thick double polished petrographic 506 507 236 thin sections using a hot melt adhesive to fix the mineral to the glass slide. Copper-rings, 3 mm in 508 509 237 diameter, were then glued on selected areas of the sections using Araldite®. Once hardened, the rings 510 511 238 and the underneath mineral were detached with the aid of a scalpel and a hot plate. The mineral was 512 finally ion milled down to electron transparency with Ar ions in a Gatan Precise Ion Polishing System 513 239 514 240 (PIPS) at the Dipartimento di Scienze della Terra of the University of Milan. Before TEM experiments, 515 516 241 ion milled mounts were carbon coated to avoid electrostatic charging during observations. Some 517 518 242 fragments from the same FOR19 sample were also manually ground in a carborundum mortar. The 519 520 243 obtained powder was ultrasonically dispersed in acetone, thereafter a few drops of the suspension were 521 522 244 deposited onto a 3 mm wide holey carbon Cu-grid. Both single and double-tilt specimen holders (±20°) 523 524 245 were used for powdered samples and polished samples. 525 526 246 Digital images were recorded with a Gatan US 1000 CCD (Bari) and an Orius CCD camera 527 528 247 (Siena). Energy-dispersive qualitative X-ray analyses (EDS) were obtained with Oxford Link (Siena) 529 530 531 532 533 534 248 and Oxford-INCA (Bari) EDS spectrometers, both equipped with a Si(Li) detector and ultra-thin 535 536 249 window. Quantitative analyses were obtained using the standard-less method within the Van Cappellen 537 538 250 and Doukhan (1994) approximation. 539 540 251 High Resolution Transmission Electron Microscopy (HRTEM) images were rotationally 541 542 252 filtered (Kilaas, 1998) with the HRTEM filter (Mitchell, 2007), as implemented in the Gatan Digital 543 544 253 Micrograph version 3.9, in order to remove noise contrast from amorphous materials. 545 546 254 547 548 255 4. Results 549 550 256 551 552 257 4.1 Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) 553 554 258 555 556 259 Pyrite in all three samples contained high concentrations of Tl, Sb and As (Figure 2 and Table 557 558 260 1). On average, pyrite in SEN1 contained 1,299 ppm Tl, 1,299 ppm Sb and 364 ppm As, CDR4 pyrite 559 560 261 hosted 1,967 ppm Tl, 1,871 ppm Sb and 1,857 ppm As, while FOR19 hosted 2,623 ppm Tl, 1,896 ppm 561 562 262 Sb and 1,602 ppm As. Based on LA-ICP-MS mapping of a handful of samples from Canale della Radice 563 564 263 and Fornovolasco, minor zoning of trace elements was only noted in some coarse inclusion-free late 565 566 264 metamorphic pyrite, and the colloform bands in FOR19 (Figure 2B). 567 For elements hosted in solid solution in a mineral, LA-ICP-MS time-resolved down-hole 568 265 569 266 ablation profiles will appear smooth, whereas sub-surface inclusions usually show as peaks on such 570 571 267 profiles if they are sufficiently large and not homogeneously distributed (e.g., Cook et al., 2009; George 572 573 268 et al., 2015, 2018b). Using Iolite, the LA-ICP-MS down-hole ablation profile for each element in each 574 575 269 analysis was carefully checked for peaks that may indicate an inclusion was accidentally ablated. 576 577 270 Overall, spectra were smooth. While some spectra revealed occasional tiny peaks that may be due to 578 579 271 nano-inclusions of distinct phases, the selected time interval for integration always excluded tiny peaks 580 581 272 (Figure 2). Thus, the LA-ICP-MS data may be assumed to reflect trace elements in solid solution or 582 583 273 related to homogeneously distributed NPs. Especially for Tl, Sb and As, most LA-ICP-MS down-hole 584 585 274 ablation profiles appeared smooth, even those corresponding to exceptionally rich concentrations of Tl, 586 587 588 589 590 591 592 593 275 i.e., colloform banded pyrite in FOR19 (Figure 2B and C). Similarly, LA-ICP-MS element maps 594 595 276 revealed smooth distributions of Tl, Sb and As, even when some zonation is present, i.e., Figure 2B. 596 597 277 Figure 3A shows a positive correlation between Tl and Sb (as mol. %). Such a correlation may 598 599 278 be explained by a coupled substitution between these elements into the pyrite structure, i.e., 2Fe2+ ↔ 600 601 279 Tl+ + Sb3+. George et al. (2018a) noted that Tl does not correlate well with As in pyrite from the southern 602 603 280 Apuan Alps ore deposits, as shown here in Figure 3B. Pyrite in FOR19 and CDR4 typically contains 604 605 281 far more As than Tl (comparing mol. %), likely reflecting the possible incorporation of anionic As as 606 607 282 well as As3+, in agreement with XANES data. Indeed, As-1 is the most common oxidation state of As in 608 609 283 pyrite in deposits that are not oxidised, e.g., high-sulfidation epithermal and porphyry deposits (Simon 610 611 284 et al., 1999a, 1999b; Savage et al., 2000; Reich et al., 2005, 2013; Deditius et al., 2008; Qian et al., 612 613 285 2013). 614 615 286 616 617 287 4.2 X-ray absorption spectroscopy (XAS) 618 619 288 620 621 289 4.2.1 Tl L3-edge 622 623 290 624 625 291 XANES spectra of measured samples and reference compounds indicate that Tl in pyrite is 626 present as Tl+ (Figure 4). Pyrite samples and protochabournéite show a single absorption peak, typical 627 292 628 293 of monovalent Tl compounds (e.g., Scheckel et al., 2004; Agarwal and Vishnoi, 2005; Dutrizac et al., 629 630 294 2005; Peacock and Moon, 2012; Biagioni et al., 2017) at 12,663 eV (first derivative maximum) while 631 632 633 295 Tl2O3 shows a weak peak, followed by a shoulder on rising absorption, culminating in a marked 634 635 296 peak at at 12,669 eV (first derivative maximum).. 636 637 297 Results of EXAFS quantitative analysis are shown in Table 2. Thallium L3-edge EXAFS and 638 639 298 Fourier transforms of measured samples are shown in Figure 5; the corresponding multiparameter fits 640 641 299 are also shown. Fit results (Table 2) indicate that Tl in the analysed pyrite samples has a first 642 643 300 coordination shell constituted by S atoms; attempts to fit the first shell with oxygen atoms or with a 644 645 301 combination of S and O were unsuccessful. First shell distances show substantial variations among the 646 647 648 649 650 651 652 302 three studied samples. Specifically, Tl–S distances are much lower in FOR19 pyrite compared to CDR4 653 654 303 and SEN1 pyrite. Given the high error on refined distances and the frequent extreme inhomogeneity of 655 656 304 the Tl+ coordination environment (e.g., Sabrowsky et al., 1979; Gostojić et al., 1982; Orlandi et al., 657 658 305 2013), it is difficult to determine a precise coordination number (CN) for the studied samples. 659 660 306 Nevertheless, a crystal chemical comparison of Tl–S distances in the studied samples with data on Tl 661 662 307 minerals allows these results to be strengthened. Indeed, coordination numbers appear in agreement 663 664 308 with those expected by the bond valence model (BVM; Brown and Altermatt, 1985), according to which 665 666 309 the distance fitted in the quantitative analyses would be compatible with a coordination of ~3.2(2), 667

668 310 5.3(7) and 6.0(4) for FOR19, SEN1 and CDR4, respectively. The choice of the R0 value (2.545 Å) 669 670 311 proposed by Brown and Altermatt (1985) seems the most suitable for Tl–S bonds; Biagioni et al. (2014) 671 672 312 have indeed noticed that the R0 reported by Brese and O’Keeffe (1991), i.e. 2.63 Å, usually results in 673 674 313 an overbonding of Tl and suggested an R0 of 2.55 Å, almost identical to the one proposed by Brown 675 676 314 and Altermatt (1985). Figure 6 shows the relationship between CN and Tl–S average bond distances 677 678 315 taken from the Tl sites of several Tl minerals. The position of the studied samples fit well with data 679 680 316 from the literature, confirming a substantially different environment for Tl between FOR19, which 681 682 317 shows features similar to carlinite (Tl2S - Giester et al., 2002), where Tl is at the vertex of a pyramid 683 684 318 coordinating 3 S atoms, and CDR4 and SEN1, which appear more similar to phases such as raguinite 685 (TlFeS - Welz et al., 1989), ellisite (Tl AsS - Gostojić, 1980) and fangite (Tl AsS - Wilson et al., 686 319 2 3 3 3 4 687 320 1993) (Tl2 site), where Tl is hosted in 5/6-coordinated sites. Coordination numbers obtained by the 688 689 321 EXAFS multiparameter fits and BVM are in excellent agreement with only a very small discrepancy 690 691 322 shown by sample FOR19, thus supporting the choice of the bond valence parameters and the fact that 692 693 323 no self-absorption effects played a significant role in the data collection. 694 695 324 No hints of features corresponding to coordination shells higher than the first could be 696 697 325 distinguished in the EXAFS signal. This may be due to the quality of available data that could be related 698 699 326 to an extremely disordered environment around Tl atoms or to the very small size of Tl-rich clusters 700 701 327 within pyrite. 702 703 328 704 705 329 4.2.2 As K-edge 706 707 708 709 710 711 330 712 713 331 Measurements at the As K-edge were focused on the XANES region. Despite the fact that 714 715 332 measurements were taken in vacuum at 80 K, there was no way to avoid the partial oxidation of As to 716 717 333 As5+. The effect is clear by looking at the linear combination fit (LCF) results (Table 3) performed with 718 719 334 ATHENA (Ravel and Newville, 2005), and it is evidenced by the presence of a small bump on the 720 721 335 measured spectra at ~11874 eV, in correspondence of the main edge crest of adamite [Zn2(AsO4)(OH)] 722 723 336 (Figure 7). All the studied samples’ spectra can indeed be approximated by a combination of 724 725 337 arsenopyrite, protochabournéite, and adamite (see Table 3 and Figure 7). The oxidation was markedly 726 727 338 evident for FOR19 where oxidation gradually increased after each measurement, while in the case of 728 729 339 SEN1 and CDR4 the process does not seem to increase with the exposure time. This effect is illustrated 730 731 340 in Appendix C, where the gradual oxidation of As in sample FOR19 in three consecutive measurements 732 733 341 is shown. Data indicated that As oxidises mainly at the expense of arsenopyrite, while the contribution 734 735 342 of As3+ sulphide does not seem to change with the exposure to the beam. According to the correlation 736 737 343 observed in Appendix A, the oxidized contribution should reach the 0 value with an arsenopyrite content 738 739 344 of ~0.8 (relative fraction of total As), which would be in good agreement with the approximately 740 3+ 741 345 constant value of As sulphide of ~0.2. As a consequence of this effect, only the first measured 742 743 346 spectrum has been used for the analysis of sample FOR19, while the sum of four consecutive spectra 744 was employed for the other samples. 745 347 746 348 It is reasonable to conclude that all the “arsenate” contributions in pyrite highlighted by the 747 748 349 LCF is likely due to the oxidation of As during the preparation of the pellets and exposure to the X-ray 749 750 350 beam. Indeed, following the mineralogical characterization performed by George et al. (2018) on these 751 752 351 samples, the occurrence of natural arsenates seems to be unlikely. The contribution of the As5+ 753 754 352 component is not always negligible, nonetheless it is possible to infer that As is present at least as both 755 756 353 As-1 and As3+, the latter bonded to S. 757 758 354 759 760 355 4.2.3 Sb K-edge 761 762 356 763 764 765 766 767 768 769 770 357 XANES spectra at the Sb K-edge for measured samples and standards are shown in Figure 8. 771 772 358 The edge position clearly indicates that Sb is present in the trivalent state with all the studied samples 773 774 359 having the maximum of the first derivative at 30,492.1 eV, in perfect agreement with that shown by 775 776 360 Sb2S3. Sb K-edge EXAFS and Fourier transform of measured samples and standard compounds are 777 778 361 shown in Figure 9, as are the corresponding multiparameter fits; fit results are shown in Table 4. Both 779 780 362 the period of oscillations and the position of the main peak in the Fourier transform space indicate 781

782 363 considerable similarity between the studied samples and pure Sb2S3. The EXAFS quantitative analyses 783 784 364 (see Table 4) shows that Sb has a first coordination shell constituted by S atoms; first shell distances 785

786 365 show no substantial variations among the three studied samples and are very similar to those of Sb2S3. 787

788 366 On the contrary, it is possible to notice from Figure 9 how the spectrum of Sb2O3 shows a markedly 789 790 367 different oscillation period. Nonetheless, attempts to fit the first shell with oxygen atoms or with a 791 792 368 combination of S and O were tentatively tried resulting unsuccessful. 793 794 369 795 796 370 4.3 Transmission electron microscopy (TEM) 797 798 371 799 800 372 The TEM-EDS investigations on samples CDR4 and FOR19 revealed different nano-structural 801 802 373 features. The prominent ones are dislocations which appear either long and curved, straight and long, 803 or short and straight (Figure 10A). The different appearance of the dislocations may be due to different 804 374 805 375 operating Burgers vectors and to different orientations of the dislocation lines (Figure 10A, B). High 806 807 376 resolution images of short and straight dislocations, tagged with black arrows in Figure 10A, were taken 808 809 810 377 along [01 1] and showed (011) lattice planes offset by ½ [011] from one side to the other of the 811 dislocation line (Figure 10C, D). 812 378 813 379 Another nano-structural feature of the investigated samples is the occurrence of “fluid” 814 815 816 380 inclusions with polygonal shape (Figure 11). High-resolution images of the inclusions taken along [01 817 818 381 1] show they are confined within the {011}, {111}, and {100} crystallographic planes of pyrite (Figure 819 11). The absence of lattice fringes other than those of pyrite and/or moiré suggests that no crystalline 820 382 821 383 precipitate is present within the inclusions. The attenuation, up to disappearance, of lattice fringes 822 823 824 825 826 827 828 829 384 towards the interior of the inclusions indicates that these inclusions may contain amorphous material. 830 831 385 Qualitative EDS chemical data indicate an As-rich nature of these inclusions (Figure 12A). 832 833 386 TEM-EDS analyses reveal no significant compositional differences between defect-free pyrite 834 835 387 and pyrite showing dislocations. is homogenously distributed, even if areas with high density 836 837 388 of “fluid” inclusions show a significant As-enrichment, up to 2.4(5) wt% (Figure 12A). Thallium 838 839 389 content is generally below the detection limit for TEM-EDS. However, some areas of the ground sample 840 841 390 FOR19 exhibit locally detectable levels of Tl [1.9(8) wt%] and Sb [0.8(2) wt%] (Figure 12B). 842 843 391 Unfortunately, these areas are too thick to obtain images with microstructural contrasts. 844 845 392 846 847 393 5. Discussion 848 849 394 850 851 395 5.1 A step in the understanding of thallium speciation in pyrite 852 853 396 In accord with previous investigations (e.g., D’Orazio et al., 2017; George et al., 2018a), Table 854 855 397 1 shows the complex geochemistry of pyrite samples from the southern Apuan Alps ore deposits, having 856 857 398 variable amounts of trace elements. These trace elements may be incorporated into pyrite through 858 859 399 complex mechanisms and their speciation and concentration are controlled by several factors (e.g., 860 861 400 Reich et al., 2005; Deditius et al., 2014; Deditius and Reich, 2016). In particular, two types of speciation 862 863 401 can be identified, i.e. structurally bound elements vs. nanoparticle-hosted elements. Several crystal- 864 865 402 chemical considerations (i.e., atomic radii, formal charges) constrain the possibility of hosting elements 866 in the crystal structure of pyrite, whereas NPs can have varying degrees of crystallinity (e.g., amorphous 867 403 868 404 Fe-As-S inclusions in arsenian pyrite from Pueblo Viejo, Dominican Republic – Deditius et al., 2009) 869 870 405 and complexity, i.e., from native elements to complex nanophases (e.g., Deditius et al., 2011). 871 872 406 873 874 407 5.1.1 Thallium in pyrite from the southern Apuan Alps ore deposits and the role of As and Sb 875 876 408 877 878 409 The high Tl content of pyrite ores from the ore deposits of the southern Apuan Alps was first 879 880 410 noted by Biagioni et al. (2013) and fully described by D’Orazio et al. (2017). George et al. (2018a) 881 882 411 detailed the geochemical evolution of pyrite during metamorphic recrystallization and the relationships 883 884 885 886 887 888 412 occurring among several hosted elements, suggesting the potential role played by the coupled 889 890 413 heterovalent substitution 2Fe2+ ↔ Tl+ + Sb3+. Indeed, SEM observations of pyrite samples and the 891 892 414 smooth nature of the LA-ICP-MS profiles did not reveal the occurrence of Tl-bearing inclusions. These 893 894 415 results agree with previous studies on Tl-bearing pyrite, that were not able to detect the occurrence of 895 896 416 Tl-bearing NPs (e.g., Kouzmanov et al., 2010), suggesting that Tl preferentially dissolves in the pyrite 897 898 417 matrix. However, as noted by Cook et al. (2009), the occurrence of homogeneously distributed NPs of 899 900 418 Tl-bearing phases (only few tens of nanometers in size) could lead to misinterpretation of the LA-ICP- 901 902 419 MS data. 903 904 420 Previous authors suggested that the occurrence of anionic or cationic As species (As1-, Simon 905 906 421 et al., 1999a; Abraitis et al., 2004; As3+, Deditius et al., 2008), as well as cationic Sb (Sb3+, George et 907 908 422 al., 2018a), favours the incorporation of several trace elements in pyrite as a result of the combination 909 910 423 of electrical (p-type conductivity vs n-type conductivity) and crystal-chemical (expansion of the unit 911 912 424 cell and creation of structural distortion) effects that As and Sb may induce in pyrite. In addition, another 913 914 425 important factor controlling the concentration of trace elements in pyrite is the crystal size: the smaller 915 916 426 the crystal size the larger is the specific surface is, which favours the adsorption of trace elements. 917 918 427 However, nanoparticulate semiconductors experience self-purification, expelling non-stoichiometric 919 920 428 impurities toward grain boundaries (Erwin et al., 2005 and references therein). Consequently, for trace 921 elements incorporation from adsorption, the growth rate of pyrite must exceed the equilibration and 922 429 923 430 removal rate of trace elements. The fine crystal size and the colloform banded texture shown by pyrite 924 925 431 from the ore deposits of southern Apuan Alps suggest it formed through pristine kinetically-controlled 926 927 432 crystallization under lower T conditions, promoting the distortion of the pyrite structure and the 928 929 433 incorporation of abundant trace elements. This incorporation is likely further favoured by the 930 931 434 occurrence of Sb3+, as first proposed by George et al. (2018a). 932 933 435 934 935 436 5.1.2 Speciation of thallium in pyrite 936 937 437 938 939 438 Whereas some authors proposed the occurrence of Tl3+ in collomorphic pyrite (Huston et al., 940 941 439 1995), EXAFS and XANES data proves that Tl is present as Tl+. This valence state is the most common 942 943 944 945 946 947 440 in natural Tl compounds; indeed, among the 69 known Tl minerals, only three contain Tl3+, i.e. 948 949 441 avicennite, Tl O , chrysothallite, K Cu Tl3+Cl (OH) ·H O, and kalithallite, K Tl3+Cl ·2H O. The 950 2 3 6 6 17 4 2 3 6 2 951 442 occurrence of Tl+ in pyrite from southern Apuan Alps ores is coupled with the presence of Sb3+ and 952 953 443 As3+, as indicated by XAS data. Arsenic also occurs as As-1, implying a dual crystal-chemical role for 954 955 444 this element in the studied samples. The role played by trivalent Sb and As in favouring the 956 957 445 incorporation of Tl+ in pyrite agrees with previous data reported by Deditius and Reich (2016). 958 959 446 EXAFS data indicate that the thallium coordination numbers range from ~3 to ~6 in the studied 960 961 447 samples, thus differing from the usually higher coordination numbers observed in sulfosalts (e.g., 962 963 448 Makovicky, 2018). Thus the occurrence of sulfosalt NPs may be excluded. 964 965 449 The Tl–S distances are in the range ~3-3.2 Å, significantly larger than Fe–S distances (~2.26 966 967 450 Å, Rieder et al., 2007). Arsenic–S and Sb–S distances are ideally 2.26 and 2.45 Å. In agreement with 968 969 451 XAS data, these two elements display the typical trigonal pyramidal coordination, observed in As3+ and 970 971 452 Sb3+-compounds. The range of different Tl coordination numbers among the three studied samples 972 973 453 indicate the occurrence of different local environments around this element. This observation is not 974 2+ + 3+ 975 454 compatible with a simple coupled substitution 2Fe ↔ Tl + Sb . Indeed, assuming the incorporation 976 977 455 of Tl through this substitution, the same local environment around Tl should be observed in all the 978 979 456 studied samples. Moreover, the complete lack of signal about coordination shells higher than the first 980 in the EXAFS signal may be hypothetically related the occurrence of an extremely disordered 981 457 982 458 environment around Tl atoms, possibly with the absence of long-range order, i.e., an amorphous 983 984 459 environment. This is not entirely surprising, since the replacement of Fe by Tl would cause too short 985 986 460 metal–S distances, requiring an important reorganization of the local structure of pyrite. TEM imaging 987 988 461 revealed a number of defects affecting the pyrite structure, such as dislocations and “fluid” inclusions, 989 990 462 but no Tl enrichment could be detected on these defects, since Tl was always below the detection limit 991 992 463 as in defect-free pyrite. However, the possibility that Tl, ‘loosely’ tied to the pyrite structure, could 993 994 464 diffuse away during sample preparation and/or observation should not be discarded, as it is well known 995 996 465 that during ion milling there is non-negligible local heating of the sample (e.g., Barna et al., 1999), and 997 998 466 that the highly focused electron beam required for EDS analysis may cause severe elemental diffusion 999 1000 467 (e.g., Capitani et al., 2016). Significant Tl levels were only detected in few thick grains in samples 1001 1002 1003 1004 1005 1006 468 prepared by grinding. In this case, the greater thickness of the sample, and therefore the lower electron 1007 1008 469 dose, may have prevented elemental diffusion. The drawback of this situation is that the thickness of 1009 1010 470 the sample also prevents any microstructural information to be obtained. 1011 1012 471 The ‘loose’ nature of the Tl incorporation in pyrite is further confirmed by its expulsion under 1013 1014 472 pyrite low-grade metamorphic recrystallization. As shown by George et al. (2018a), after 1015 1016 473 recrystallization, pyrite from the ore deposits of southern Apuan Alps is depleted in As, Sb, Tl, Hg, Cu, 1017 1018 474 Zn, Ag, and Mn, whereas Ni, Co, and As are sometimes even enriched. These results fully agree with 1019 1020 475 those reported by Large et al. (2007, 2011), who suggested that trace elements occurring as 1021 1022 476 nanoinclusions or that are ‘loosely’ held in the pyrite structure are readily expelled during 1023 1024 477 metamorphism, whereas those elements replacing Fe or S in the crystal structure remain and even 1025 1026 478 become enriched during metamorphic recrystallization. 1027 1028 479 At the present stage of investigation, Tl-hosting NPs have not been observed in pyrite. On the 1029 1030 480 contrary, XAS data suggest its occurrence as a monovalent cation, with variable local environments, 1031 1032 481 compatible with its occurrence in structural defects lacking long-range order. In conclusion, we 1033 1034 482 confidently suggest that Tl could be present in the defective structure of pyrite. 1035 1036 483 1037 1038 484 5.1.3 Arsenic-rich “fluid” inclusions in pyrite 1039 1040 485 1041 486 An interesting feature shown by HRTEM is the occurrence of As-rich “fluid” inclusions, similar 1042 1043 487 to those observed by Deditius et al. (2009) who revealed As-enrichment in amorphous Fe-As-S 1044 1045 488 nanoscale “liquid” inclusions in arsenian pyrite from Pueblo Viejo, Dominican Republic. There are two 1046 1047 489 alternative ways to interpret the occurrence of these inclusions. Following Deditius et al. (2009), a 1048 1049 490 possible interpretation is that these inclusions are primary features, trapped during the crystallization of 1050 1051 491 host pyrite, in agreement with the observation of the precipitation of an amorphous Fe-As-S phase via 1052 1053 492 bacterial activity by some authors (e.g., Morin and Calas, 2006). On the contrary, taking into account 1054 1055 493 the results reported by George et al. (2018a), these As-rich “liquid” inclusions could be sulphide melt 1056 1057 494 nano-droplets exsolved during the final stages of the Apuan Alps metamorphism. According to Deditius 1058 1059 495 and Reich (2016), these kinds of nanoparticles may be able to sequester migrating/diffusing elements, 1060 1061 1062 1063 1064 1065 496 like, for instance, Tl. It is worth noting that Guillemin et al. (1970) reported the occurrence of an 1066 1067 497 amorphous Tl(As,Sb) S phase in the Jas Roux deposit, Hautes-Alpes, France. 1068 10 16 1069 498 1070 1071 499 5.2 Environmental implications 1072 1073 500 1074 1075 501 The confirmation that Tl is present in the monovalent state in pyrite has significant 1076 1077 502 environmental implications. Xiong (2009) calculated that Tl+ is the dominant Tl species over 1078 1079 503 geochemically reasonable pH ranges for soils. Similarly, Vink (1993) showed that Tl+ is the most stable 1080 1081 504 state of Tl under ‘normal’ Eh-pH conditions. Monovalent Tl is very mobile in the environment, being 1082 1083 505 easily transported in aqueous form due to its high solubility (e.g., Vink, 1993; Peter and Viraraghavan, 1084 1085 506 2005). In a study of the toxicity and availability of both monovalent and trivalent Tl, Ralph and Twiss 1086 1087 507 (2002) showed that, though Tl3+ is likely orders of magnitude more toxic than Tl+ to algae, 1088 1089 508 concentrations of available Tl3+, even in polluted environments, are far below dangerous toxicity levels. 1090 3+ 1091 509 This is because Tl is largely locked within Tl(OH)3 under near-neutral pH conditions. On the contrary, 1092 + + 1093 510 the bioavailable fraction of Tl is approximately equal to the total amount of dissolved Tl in water. 1094 + 3+ 1095 511 Thus, Tl is far more of an environmental concern than Tl . 1096 + 1097 512 Given the stability of Tl in the environment, as well as its high mobility in this state, the release 1098 of Tl+ from pyrite will lead to its uninhibited movement into the environment. Under normal Eh-pH 1099 513 1100 514 conditions, free Tl+ may readily be transported in ground or surface waters and dispersed. Importantly, 1101 1102 515 Tl in the monovalent state is readily bioavailable; no chemical transformation is thus necessary, after 1103 1104 516 release from pyrite, before Tl can be transported and made available for uptake into biota. The thallium 1105 1106 517 contamination related to the ore deposits of southern Apuan Alps is an interesting case study (e.g., 1107 1108 518 Biagioni et al., 2017). Given that, historically, Tl has not been as economically valuable as many other 1109 1110 519 trace metals present in sulphide ores, it has rarely been recovered during the processing of such ores 1111 1112 520 (Zitko et al., 1975). As such, Tl-bearing pyrite would be frequently discarded with ore tailings. 1113 1114 521 Therefore, the oxidation of pyrite contained in mine dumps and tailings in Tl-rich mining areas 1115 1116 522 worldwide potentially represents a severe environmental hazard. 1117 1118 523 1119 1120 1121 1122 1123 1124 524 Acknowledgements 1125 1126 525 1127 1128 526 This research received support from Ministero dell'Istruzione, dell'Università e della Ricerca through 1129 1130 527 the project SIR 2014 “THALMIGEN – Thallium: Mineralogy, Geochemistry, and Environmental 1131 1132 528 Hazards”, granted to CB (Grant no. RBSI14A1CV). The critical reading and the suggestions of Yves 1133 1134 529 Moëlo were greatly appreciated. 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Samples are from Sennari (A, B), Canale della Radice (C, 12 6 D), and Fornovolasco (E, F). 13 14 7 15 16 8 Figure 2 17 18 9 19 10 Representative LA-ICP-MS time-resolved down-hole ablation profiles from CDR4 (A), FOR19 (B) and (C), and 20 21 11 SEN1 (D). Y axis = counts per second, X axis = time (s). Relatively flat spectra reflect solid solution or 22 23 12 homogeneously distributed NPs. Reflected light images of the respective pyrite grains are also shown. Red dots 24 25 13 indicate where the LA-ICP-MS spot was made (dot not to scale). LA-ICP-MS element map of Tl in colloform 26 pyrite from FOR19 also shown in (B). 27 14 28 15 29 30 16 Figure 3 31 32 17 33 34 18 Plots showing inter-element correlations in pyrite. (A) Tl vs. Sb and (B) Tl vs. As. Data plotted as mol. %. Straight 35 19 line is ideal 1:1 correlation. 36 37 20 38 39 21 Figure 4 40 41 22 42 23 Normalized Tl L3-edge XANES for measured samples and standards. The vertical dashed lines indicate the 43 44 24 position of the main inflection point, which, by convention, is considered to be the absorption edge energy. It can 45 + 46 25 be seen that all Tl standard compounds have an absorption edge at ~12663 eV, while Tl2O3 at ~12669 eV. 47 48 26 49 Figure 5 50 27 51 28 52 2 53 29 (A) Tl L3-edge k -weighted EXAFS region of measured samples. (B) Fourier transforms (uncorrected for phase 54 55 30 shifts) of the EXAFS of measured samples. Solid lines are data, red lines are fits. Fits were performed in R space. 56 57 58 59 60 61 62 31 63 64 32 Figure 6 65 66 33 67 68 34 Coordination numbers (CNs) vs. Tl–S average bond distances for several Tl sulphide sites. Data from: Pasava et 69 35 al., 1989 (bernardite); Giester et al., 2002 (carlinite); Ohmasa and Nowacki, 1971 (vrbaite); Brown and Dickson, 70 71 36 1976 (christite); Balić-Žunić and Engel, 1983 (edenharterite); Orlandi et al., 2013 (protochabournéite); Rey et al., 72 73 37 1983 (weissbergite); Engel, 1980 (parapierrotite); Berlepsch, 1996 (jentschite); Engel et al., 1982 (simonite); 74 75 38 Balić-Žunić et al., 2008 (picotpaulite); Berger, 1989 (thalcusite); Fleet, 1973 (lorándite); Bindi et al., 2014 76 (philrothite); Graeser et al., 2001 (sicherite); Wilson et al., 1983 (fangite); Gostojić, 1980 (ellisite); Engel et al., 77 39 78 40 1983 (pierrotite). 79 80 41 81 82 42 Figure 7 83 84 43 85 44 Calibrated and normalized As K-edge XANES spectra of studied samples (black line) and measured reference 86 87 45 compounds (black dotted line). Linear combination fit (LCF) of studied samples are shown in red. Vertical dashed 88 89 46 lines in correspondence of the main inflection point of arsenopyrite (As-), protochabournéite (As3+) and adamite 90 5+ 91 47 (As ). 92 48 93 94 49 Figure 8 95 96 50 97 98 51 Normalized Sb K-edge XANES for measured samples and standards. Vertical dashed lines in correspondence of 99 the main inflection point of Sb O , Sb S and Sb O . 100 52 2 3 2 3 2 5 101 53 102 103 54 Figure 9 104 105 55 106 2 107 56 (A) Sb K-edge k -weighted EXAFS region of measured samples. (B) Fourier transforms (uncorrected for phase 108 57 shifts) of the EXAFS of measured samples. Solid lines are data, red lines are fits. Fits were performed in R space. 109 110 58 Figure 10 111 112 59 113 114 115 116 117 118 119 120 121 60 Ion-milled sample CDR4. (A) BF image showing different dislocations whose contrast is due to different 122 123 61 operating Burger vectors and to different orientations of dislocation lines. White arrows mark long and curved 124 125 62 dislocations; black arrows show short and straight dislocations. (B) BF image showing a lamella (white arrow) 126 127 63 and several straight dislocations. (C) HR image taken along [011] (SAED pattern shown in the inset) on two short 128 and straight dislocations. (D) Enlargement of the square region in C (see text for details) showing HR image taken 129 64 130 65 along [011] with (011) lattice planes offset by ½ [011] from one side to the other of the dislocation line. 131 132 66 133 134 67 Figure 11 135 136 68 137 69 (A) BF images showing inclusions with polygonal shape (some them indicated by arrows) of the ion-milled 138 139 70 sample CDR4. (B) HRTEM images taken along [011] (related SAED pattern in the inset) of the ion-milled sample 140 141 71 CDR4. The polygonal shape of the inclusion in the HR image is determined by pyrite crystallographic planes 142 143 72 (indexed). 144 145 73 146 74 Figure 12 147 148 75 149 150 76 (A) EDS spectrum of fluid inclusions showing As-enrichment. (B) EDS spectrum of thick area from the ground 151 152 77 FOR19 sample, exhibiting Tl and Sb. Nickel, Cu and Cr peaks are due to grid. 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 1 2 3 Table 1. Summary of trace element concentrations in pyrite determined by LA-ICP-MS (data in ppm). 4 5 Deposit Sample Mn Co Ni Cu Zn As Se Mo Ag Cd In Sn Sb Te W Au Hg Tl Pb Bi 6 Mean 14 14 151 10 20 364 1.0 18 0.74 0.7 0.002 0.06 1299