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Lopez, Andres, Frost, Ray, Scholz, Ricardo, Zigovecki Gobac, Zeljka, & Xi, Yunfei (2013) Vibrational spectroscopy of the plumbotsumite Pb5(OH)10Si4O8 - An assessment of the molecular structure. Journal of Molecular Structure, 1054 -1055, pp. 228-233.

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Notice: Please note that this document may not be the Version of Record (i.e. published version) of the work. Author manuscript versions (as Sub- mitted for peer review or as Accepted for publication after peer review) can be identified by an absence of publisher branding and/or typeset appear- ance. If there is any doubt, please refer to the published source. https://doi.org/10.1016/j.molstruc.2013.09.055 1 Vibrational spectroscopy of the silicate mineral plumbotsumite Pb5(OH)10Si4O8 - an 2 assessment of the molecular structure 3 4 Andrés López a, Ray L. Frost a, Ricardo Scholz b, Željka Žigovečki Gobac c and Yunfei 5 Xi a 6

7 a School of Chemistry, Physics and Mechanical Engineering, Science and Engineering 8 Faculty, Queensland University of Technology, GPO Box 2434, Brisbane Queensland 9 4001, Australia.

10 b Geology Department, School of Mines, Federal University of Ouro Preto, Campus Morro do 11 Cruzeiro, Ouro Preto, MG, 35,400-00, Brazil 12 13 c Institute of Mineralogy and Petrography, Department of Geology, Faculty of Science, 14 University of Zagreb, Horvatovac 95, 10000 Zagreb, Croatia 15 16 17 Abstract: 18 We have used scanning electron microscopy with energy dispersive X-ray analysis to 19 determine the precise formula of plumbotsumite, a rare lead silicate mineral of formula

20 Pb5(OH)10Si4O8. This study forms the first systematic study of plumbotsumite from the 21 Bigadic deposits, Turkey. Vibrational spectroscopy was used to assess the molecular 22 structure of plumbotsumite as the structure is not known. The mineral is characterised by 23 sharp Raman bands at 1047, 1055 and 1060 cm-1 assigned to SiO stretching vibrational 24 modes and sharp Raman bands at 673, 683 and 697 cm-1 assigned to OSiO bending modes.

25 The observation of multiple bands offers support for a layered structure with variable SiO3 26 structural units. Little information may be obtained from the infrared spectra because of 27 broad spectral profiles. Intense Raman bands at 3510, 3546 and 3620 cm-1 are ascribed to 28 OH stretching modes. Evidence for the presence of water in the plumbotsumite structure was 29 inferred from the infrared spectra. 30 31 Key words: Plumbotsumite, molecular structure, Raman spectroscopy, silicate, infrared 32  Author to whom correspondence should be addressed ([email protected]) 1

33 Introduction 34

35 Plumbotsumite is a rare lead silicate mineral of formula Pb5(OH)10Si4O8 [1]. The name is for 36 the chemical composition (lead = PLUMBum) and the type locality (TSUMeb) [1]. 37 Plumbotsumite is found as secondary mineral developed in the oxidation zone above complex 38 sulfide ores, such as Cu-Pb-Zn mineralization in Tsumeb mine, Namibia [1, 2]. 39 Despite the type locality in Tsumeb mine, other occurrences were reported in Mammoth-St. 40 Anthony mine, Tiger, Pinal County, Arizona, USA [3], Blue Bell claims [4] and Otto 41 Mountains [5] near Baker, San Bernardino County, California, USA. Plumbotsumite was also 42 obtained as by-product during hydrothermal syntheses of Pb-zoisite and Pb-lawsonite [6] 43 Plumbotsumite shows importance in the mineral collectors market. 44 45 Plumbotsumite is an orthorhombic mineral [7, 8][7, 8][7, 8][7, 8] with a = 15.875(4), b =

46 9.261(3), c = 29.364(9) Å, space group C2221 and Z = 10 [1]. Structure determination on the 47 plumbotsumite is still unpublished [4]. A proposed new formula of plumbotsumite occurred

48 Pb13[(CO3)6|Si10O27]·3H2O [5], but without published determination. The 49 mineral structure consists of undulating sheets of silicate tetrahedra between which are

50 located Pb atoms and channels containing H2O (and Pb2+ lone-pair electrons). The silicate 51 sheets can be described as consisting of zigzag pyroxene-like (SiO3)n chains joined laterally 52 into sheets with the unshared tetrahedral apices in successive chains pointed alternately up 53 and down, a configuration also found in pentagonite [4]. 54 55 Lead silicates in Mammoth-St. Anthony mine, Tiger, Pinal County, Arizona, USA are found 56 in unusual oxidation zone assemblages of rare minerals [3]. In “normal” oxidation zone in 57 Mammoth-St. Anthony mine primary sulphides were subjected to weathering, and, according 58 to this process, secondary copper sulphides, oxides, carbonates, and silicates were developed 59 [3]. Locally, above this “normal” oxidation zone, due to retention or reintroduction of some 60 components from hydrothermal solutions, derivation of some components from supergene 61 alteration, or possibly a supply of some components from groundwater, suites of complex and 62 rare minerals, including plumbotsumite, were formed [3]. 63 64 Raman spectroscopy has proven most useful for the study of mineral structures [9, 10]. The 65 objective of this research is to report the Raman and infrared spectra of plumbotsumite and to 66 relate the spectra to the molecular structure of the minerals. The number of plumbotsumite 2

67 occurrences is limited and this is the first report of a systematic spectroscopic study of 68 plumbotsumite. 69 70 Experimental 71 72 Samples and preparation 73 Off white plumbotsumite single crystals were obtained from the collection of the Geology 74 Department of the Federal University of Ouro Preto, Minas Gerais, Brazil, with sample code 75 SAB-090. The mineral originated from Pinal Co., Mammoth District, St. Anthony deposit, 76 Arizona, USA [3]. The studied sample was gently crushed and the associated minerals were 77 removed under a stereomicroscope Leica MZ4. 78 79 Scanning electron microscopy (SEM) 80 Experiments and analyses involving electron microscopy were performed in the Center of 81 Microscopy of the Universidade Federal de Minas Gerais, Belo Horizonte, Minas Gerais, 82 Brazil (http://www.microscopia.ufmg.br). 83 84 A fragment of a plumbotsumite crystal was placed in a carbon tape. The sample was analyses 85 without coating to eliminate the presence of unwanted chemical elements. Secondary 86 Electron and Backscattering Electron images were obtained using a JEOL JSM-6360LV 87 equipment. Qualitative and semi-quantitative chemical analyses in the EDS mode were 88 performed with a ThermoNORAN spectrometer model Quest and was applied to support the 89 mineral characterization. The EDS analysis was performed in a low vacuum condition. 90 91 Raman spectroscopy 92 Crystals of plumbotsumite were placed on a polished metal surface on the stage of an 93 Olympus BHSM microscope, which is equipped with 10x, 20x, and 50x objectives. The 94 microscope is part of a Renishaw 1000 Raman microscope system, which also includes a 95 monochromator, a filter system and a CCD detector (1024 pixels). The Raman spectra were 96 excited by a Spectra-Physics model 127 He-Ne laser producing highly polarised light at 633 97 nm and collected at a nominal resolution of 2 cm-1 and a precision of ± 1 cm-1 in the range 98 between 200 and 4000 cm-1. Repeated acquisitions on the crystals using the highest 99 magnification (50x) were accumulated to improve the signal to noise ratio of the spectra.

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100 Spectra were calibrated using the 520.5 cm-1 line of a silicon wafer. The Raman spectrum of 101 at least 10 crystals was collected to ensure the consistency of the spectra. 102 103 Infrared spectroscopy 104 Infrared spectra were obtained using a Nicolet Nexus 870 FTIR spectrometer with a smart 105 endurance single bounce diamond ATR cell. Spectra over the 4000525 cm-1 range were 106 obtained by the co-addition of 128 scans with a resolution of 4 cm-1 and a mirror velocity of 107 0.6329 cm/s. Spectra were co-added to improve the signal to noise ratio. 108 109 Spectral manipulation such as baseline correction/adjustment and smoothing were performed 110 using the Spectracalc software package GRAMS (Galactic Industries Corporation, NH, 111 USA). Band component analysis was undertaken using the Jandel ‘Peakfit’ software package 112 that enabled the type of fitting function to be selected and allows specific parameters to be 113 fixed or varied accordingly. Band fitting was done using a Lorentzian-Gaussian cross-product 114 function with the minimum number of component bands used for the fitting process. The 115 Gaussian-Lorentzian ratio was maintained at values greater than 0.7 and fitting was 116 undertaken until reproducible results were obtained with squared correlations of r2 greater 117 than 0.995. 118

119 Results and Discussion

120 Chemical characterization 121 The SEM/BSI image of the plumbotsumite crystal studied in this work is shown in Figure 1. 122 The crystal shows a perfect . Qualitative chemical analysis of plumbotsumite shows 123 a Pb silicate and no other chemical elements are observed (Figure 2). The crystal doesn´t 124 show chemical zonation and can be considered a single mineral phase and a type material. 125 126 127 Vibrational spectroscopy 128 The Raman spectrum of plumbotsumite over the 100 to 4000 cm-1 spectral range is displayed 129 in Figure 3a. This spectrum shows the position and relative intensities of the Raman bands. 130 It is obvious that there are large parts of the spectrum where no intensity is observed and 131 therefore the spectrum is subdivided into sections based upon the type of vibration being 132 examined. The infrared spectrum of plumbotsumite over the 500 to 4000 cm-1 spectral range 4

133 is reported in Figure 3b. This figure shows the position and relative intensities of the infrared 134 bands. Again, there are large parts of the spectrum where little or no intensity is observed. 135 Hence, the spectrum is subdivided into subsections based upon the type of vibration being 136 studied. 137 138 The Raman spectrum over the 800 to 1300 cm-1 spectral range is illustrated in Figure 4a. 139 Intense Raman bands are found at 1047, 1055 and 1060 cm-1. Raman bands of lower 140 intensity are found at 839, 844 and 1084 cm-1. The three Raman bands at 1047, 1055 and 141 1060 cm-1 are assigned to the SiO stretching bands. The exact structure of plumbotsumite is 142 unknown, however it is likely to be a layered type structure. The infrared spectrum of 143 plumbotsumite over the 500 to 1300 cm-1 spectral range is reported in Figure 4b. Compared 144 with the Raman spectrum, the infrared spectrum shows a broad spectral profile which may be 145 resolved into component bands. Infrared bands are determined at 984, 1023, 1050 and 1090 146 cm-1. These bands are described as SiO stretching vibrations. According to Kampf et al. [4], 147 plumbotsumite has a structure resembling pentagonite and its structurally related mineral 4+ 148 Ca(V O)Si4O10·4H2O. The Raman spectrum of cavansite is dominated by an 149 intense band at 981 cm-1 and pentagonite by a band at 971 cm-1 attributed to the stretching

150 vibrations of (SiO3)n units. Cavansite is characterised by two intense bands at 574 and 672 151 cm-1 whereas pentagonite by a single band at 651 cm-1, assigned to OSiO bending vibrational 152 modes.

153

154 If this is the case, then a comparison may be made with the type silicate minerals. -1 155 Dowty showed that the -SiO3 units had a unique band position of 980 cm [11] (see Figures 2

156 and 4 of this reference). Dowty also showed that Si2O5 units had a Raman peak at around -1 157 1100 cm . Apophyllite-(KF) consists of continuous sheets of Si2O6 parallel to the 001 plane. -1 158 The band at 1059 cm is assigned to the SiO stretching vibration of these Si2O6 units. Adams 159 et al. [12] reported the single crystal Raman spectrum of apophyllite. Adams and co-workers 160 reported the factor group analysis of apophyllite. Based upon Adams [12] assignment this

161 band is the A1g mode. It is predicted that there should be three A1g modes. However, only one 162 is observed, perhaps because of accidental coincidence. Narayanan [13] collected the 163 spectrum of an apophyllite mineral but did not assign any bands. Raman bands of 164 significantly lower intensity are observed at 970, 1007, 1043, 1086 and 1114 cm-1. The -1 165 Raman bands at 1043, 1086 and 1114 cm are assigned to the A2u modes. Vierne and Brunel 5

166 [14] published the single crystal infrared spectrum of apophyllite and found the two A2 167 modes, at 1048 and 1129 cm-1. The significance of this observation is that it shows that both 168 the Si-O bridge and terminal bonds yield stretching wavenumbers at comparable positions.

169

170 The Raman spectra of plumbotsumite in the 300 to 800 cm-1 and in the 100 to 300 cm-1 are 171 shown in Figures 5a and 5b. The first spectrum is dominated by an intense Raman band at 172 683 cm-1 with two shoulders at 673 and 697 cm-1. Dowty calculated the band position of 173 these bending modes for different siloxane units [11] and demonstrated the band position of -1 174 the bending modes for SiO3 units at around 650 cm . This calculated value is in harmony 175 with the higher wavenumber band observed at 683 cm-1 observed for plumbotsumite. A 176 lartge number of low intensity bands are observed in Figure 5a. These bands are found at 177 346, 396, 432, 458 and 481 cm-1. Other bands are observed at 581, 609, 636, 729 and 772

178 cm-1.

179

180 Strong Raman bands are discovered in the 100 to 300 cm-1 spectral range. Intense Raman 181 bands are found at 143, 154 and 179 cm-1. Other medium intensity bands are found at 103 182 and 107 cm-1 and bands of lower intensity are found at 227, 246, 280 and 248 cm-1. Strong 183 Raman bands were also reported by Adams et al. [12] in the single crystal Raman spectrum 184 of apophyllite in this spectral region. Adams et al. showed the orientation dependence of the 185 spectra. Bands in these positions are due to framework vibrations and probably also involve 186 water. The intense band at 143 cm-1 of plumbotsumite may involve hydrogen bonding of 187 water. However, until the Raman spectrum of deuterated plumbotsumite is measured, then 188 no firm conclusions can be made.

189

190 The Raman and infrared spectra of plumbotsumite in the 2600 to 3800 cm-1 spectral range is 191 shown in Figure 6a and 6b. The Raman spectrum shows three bands at 3510, 3546 and 3620 192 cm-1. These Raman bands are assigned to the stretching vibrations of the OH units in the 193 plumbotsumite structure. The observations of multiple bands lead to the conclusion that the 194 OH units in the structure of plumbotsumite are non-equivalent. No Raman bands that could 195 be attributed to water stretching vibrations are observed in the Raman spectrum. In

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196 comparison, the infrared spectrum displays a broad spectral profile with a series of 197 overlapping bands that may be curve resolved into component bands at 2978, 3248, 3435, 198 3570 and 3632 cm-1. The latter two bands are the infrared equivalent of the Raman bands at 199 3546 sand 3620 cm-1. The other infrared bands in this spectral region are assigned to water 200 stretching vibrations. The Raman spectrum of apophyllite and cavansite are reported in 201 Figure 7. The Raman spectrum of cavansite in the hydroxyl stretching region shows bands at 202 3504, 3546, 3577, 3604 and 3654 cm-1 whereas pentagonite is a single band at 3532 cm-1.

203

204 Overall two features are observed in the Raman spectrum of apophyllite, namely bands due to 205 water stretching vibrations and hydroxyl stretching bands. It is noted that the 206 hydroxyapophyllite Raman spectrum has two OH stretching bands. The Raman spectrum of 207 the apophyllite shows a complex set of bands which may be resolved into component bands 208 at 2813, 2893, 3007, 3085 and 3365 cm-1. These bands are attributed to water stretching 209 vibrations. Neutron diffraction studies have shown that water is hydrogen bonded to the 210 silicate framework structure [15]. In the model of Prince [15] approximately one-eighth of 211 the water molecules are replaced by OH- and the remaining protons bonded to fluoride to

212 form HF molecules. Both OH- and H20 are hydrogen bonded to the silicate framework. Bartl 213 and Pfeifer [16] presented a model of apophyllite in which some hydroxyl units are replaced 214 by fluoride ions. This model seems more appropriate as the sizes of F- and OH- ions are very 215 close. There are many examples in nature where in minerals the OH- units are either 216 completely or partially replaced by F- ions.

217

218 The Raman and infrared spectrum in the 1300 to 1800 cm-1 spectral region are reported in 219 Figures 8a and 8b. The Raman spectrum shows low intensity bands at 1685, 1709, 1716, 220 1732 and 1744 cm-1 which are attributed to OH deformation modes. No water bending modes 221 were observed in the Raman spectrum. Intense Raman bands are observed at 1379, 1424 and 222 1479 cm-1. These bands are attributed to the SiO antisymmetric stretching vibrations. These 223 bands are observed as broad bands in the infrared spectrum with resolved bands at 1312, 224 1389, 1435 and 1462 cm-1. Infrared bands are observed at 1626 and 1646 cm-1 and are 225 assigned to the water bending modes. The two infrared bands at 1728 and 1741 cm-1 are 226 attributed to hydroxyl deformation modes.

7

227 228 229 Conclusions 230 231 We have undertaken a study of the silicate mineral plumbotsumite, of formula

232 Pb5(OH)10Si4O8 using a combination of SEM with EDX and a combination of Raman and 233 infrared spectroscopy. EDX analysis shows the mineral to be pure with no extraneous 234 elements. The structure consists of consists of undulating sheets of silicate tetrahedra 2+ 235 between which are located Pb atoms and channels containing H2O (and Pb lone-pair 236 electrons) [1]. The silicate sheets can be described as consisting of zigzag pyroxene-like

237 (SiO3)n chains joined laterally into sheets with the unshared tetrahedral apices in successive 238 chains pointed alternately up and down [1], a configuration also found in pentagonite. 239 240 The structure of plumbotsumite was assessed using a combination of Raman and infrared 241 spectroscopy. The mineral is characterised by sharp Raman bands at 1047, 1055 and 1060 242 cm-1 assigned to SiO stretching vibrational modes and sharp Raman bands at 673, 683 and 243 697 cm-1 assigned to OSiO bending modes. The observation of multiple bands offers support

244 for a layered structure with variable SiO3 structural units. Intense Raman bands at 3510, 245 3546 and 3620 cm-1 are ascribed to OH stretching modes. Evidence for the presence of water 246 in the plumbotsumite structure was inferred from the infrared spectra. 247 248 Acknowledgments

249 The financial and infra-structure support of the Queensland University of Technology, 250 Chemistry discipline is gratefully acknowledged. The Australian Research Council (ARC) is 251 thanked for funding the instrumentation. The authors would like to acknowledge the Center 252 of Microscopy at the Universidade Federal de Minas Gerais 253 (http://www.microscopia.ufmg.br) for providing the equipment and technical support for 254 experiments involving electron microscopy. R. Scholz thanks to CNPq – Conselho Nacional 255 de Desenvolvimento Científico e Tecnológico (grant No. 306287/2012-9). Ž. Žigovečki 256 Gobac thanks to Ministry of Science, Education and Sports of the Republic of Croatia, under 257 Grant No. 119-0000000-1158. 258

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259 References 260

261 [1] P. Keller, P.J. Dunn, Chemie der Erde, 41 (1982) 1-6.

262 [2] M. Fleischer, I.J. Cabri, G.Y. Chao, J.A. Mandarino, A. Pabst, Amer. Miner. 67 (1982)

263 1075-1076.

264 [3] J.W. Anthony, S.A. Williams, R.A. Bideaux, R.W. Grant, Mineralogy of Arizona, 3rd

265 Edition ed., University of Arizona Press, Tuscon, 1995.

266 [4] A.R. Kampf, G.R. Rossman, R.M. Housley, , Amer. Miner. 94 (2009) 1198-1204.

267 [5] J. Marty, A.R. Kampf, R.M. Housley, S.J. Mills, S. Weiß, Lapis, 35 (2010) 42-51.

268 [6] G. Dorsam, A. Liebscher, B. Wunder, G. Franz, M. Gottschalk, Neues Jahr.Miner.

269 Ab.188 (2011) 99-110.

270 [7] D.M.C. Huminicki, F.C. Hawthorne, Can. Miner.38 (2000) 1425-1432.

271 [8] M. Mrose, D.E. Appleman, Zeit. fuer Krist.117 (1962) 16-36.

272 [9] J. Cejka, J. Sejkora, S. Bahfenne, S.J. Palmer, J. Plasil, R.L. Frost, J. Raman Spectrosc. 42

273 (2011) 214-218.

274 [10] R.L. Frost, S. Bahfenne, J. Cejka, J. Sejkora, J. Plasil, S.J. Palmer, E.C. Keeffe, I.

275 Nemec, J. Raman Spectrosc. 42 (2011) 56-61.

276 [11] E. Dowty, Phys. Chem.Miner.14 (1987) 80-93.

277 [12] D.M. Adams, R.S. Armstrong, S.P. Best, Inorg. Chem. 20 (1981) 1771-1776.

278 [13] P.S. Narayanan, Current Sc. 20 (1951) 94-95.

279 [14] R. Vierne, R. Brunel, Bull. Soc. Franc. Miner. Crist. 92 (1969) 409-419.

280 [15] E. Prince, Amer. Miner.56 (1971) 1241-1249.

281 [16] H. Bartl, G. Pfeifer, Fort. Miner. 53 (1975) 3.

282

283

284 9

285

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286 List of Figures 287 288 Figure 1 BSI image of plumbotsumite 289 290 Figure 2 EDS analysis of plumbotsumite 291 292 Figure 3 (a) Raman spectrum of plumbotsumite in the 100 to 4000 cm-1 region (b) infrared 293 spectrum of plumbotsumite in the 500 to 4000 cm-1 region. 294 295 Figure 4 (a) Raman spectrum of plumbotsumite in the 800 to 1400 cm-1 region (b) infrared 296 spectrum of plumbotsumite in the 500 to 1300 cm-1 region. 297 298 Figure 5 (a) Raman spectrum of plumbotsumite in the 300 to 800 cm-1 region (b) Raman 299 spectrum of plumbotsumite in the 100 to 300 cm-1 region 300 301 Figure 6 (a) Raman spectrum of plumbotsumite in the 2400 to 3800 cm-1 region (b) infrared 302 spectrum of plumbotsumite in the 2500 to 3700 cm-1 region. 303 304 Figure 7 (a) Raman spectrum of cavansite in the 2400 to 3800 cm-1 region (b) Raman 305 spectrum of apophyllite in the 2500 to 3700 cm-1 region. 306 307 Figure 8 (a) Raman spectrum of plumbotsumite in the 1400 to 1800 cm-1 region (b) infrared 308 spectrum of plumbotsumite in the 1300 to 1800 cm-1 region. 309 310

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311 312 Figure 1 313 314

12

315 316 Figure 2

13

317 318

Figure 3a Raman spectrum of plumbotsumite (upper spectrum) over the 100 to 4000 cm-1 spectral range and Figure 3b infrared spectrum of plumbotsumite (lower spectrum) over the 500 to 4000 cm-1 spectral range 319 320 321

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322

Figure 4a Raman spectrum of plumbotsumite (upper spectrum) in the 800 to 1400 cm-1 spectral range and Figure 4b infrared spectrum of plumbotsumite (lower spectrum) in the 500 to 1300 cm-1 spectral range 323

15

324

Figure 5a Raman spectrum of plumbotsumite (upper spectrum) in the 300 to 800 cm-1 spectral range and Figure 5b Raman spectrum of plumbotsumite (lower spectrum) in the 100 to 300 cm-1 spectral range 325

16

326

Figure 6a Raman spectrum of plumbotsumite (upper spectrum) in the 2600 to 4000 cm-1 spectral range and Figure 6b infrared spectrum of plumbotsumite (lower spectrum) in the 2600 to 4000 cm-1 spectral range 327 328 329

17

330

Figure 7a Raman spectrum of apophyllite (upper spectrum) in the 2600 to 4000 cm-1 spectral range and Figure 7b Raman spectrum of cavansite (upper spectrum) in the 2600 to 4000 cm-1 spectral range 331

18

332

Figure 8a Raman spectrum of plumbotsumite (upper spectrum) in the 1400 to 2000 cm-1 spectral range and Figure 8b infrared spectrum of plumbotsumite (lower spectrum) in the 1300 to 1800 cm-1 spectral range 333 334

19