Raman Spectroscopy of the Multi Anion Mineral Arsentsumebite Pb2cu (Aso4)(SO4)(OH) and in Comparison with Tsumebite Pb2cu (PO4)(SO4)(OH)

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Frost, Ray, Palmer, Sara,& Xi, Yunfei (2011) Raman spectroscopy of the multi anion mineral arsentsumebite Pb2Cu(AsO4)(SO4)(OH) and in comparison with tsumebite Pb2Cu(PO4)(SO4)(OH). Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 83(1), pp. 449-452.

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2 and in comparison with tsumebite Pb2Cu(PO4)(SO4)(OH)

4 Ray L. Frost,  Sara J. Palmer, Yunfei Xi

5 6 Chemistry Discipline, Faculty of Science and Technology, Queensland University of 7 Technology, GPO Box 2434, Brisbane Queensland 4001, Australia. 8

9 Abstract

10 The mineral arsentsumebite Pb2Cu(AsO4)(SO4)(OH), a copper arsenate-sulfate hydroxide of 11 the brackebuschite group has been characterised by Raman spectroscopy. The 12 brackebuschite mineral group are a series of monoclinic arsenates, phosphates and vanadates

13 of the general formula A2B(XO4)(OH,H2O), where A may be Ba, Ca, Pb, Sr, while B may be 2+ 2+ 3+ 2+ 3+ 14 Al, Cu ,Fe , Fe , Mn , Mn , Zn and XO4 may be AsO4, PO4, SO4,VO4. Bands are 2- 3- 15 assigned to the stretching and bending modes of SO4 AsO4 and HOAsO3 units. Raman 16 spectroscopy readily distinguishes between the two minerals arsentsumebite and tsumebite. 17 Raman bands attributed to arsenate are not observed in the Raman spectrum of tsumebite. 18 Phosphate bands found in the Raman spectrum of tsumebite are not found in the Raman 19 spectrum of arsentsumebite. Raman spectroscopy readily distinguishes the two minerals 20 tsumebite and arsentsumebite.

21 Keywords: Raman spectroscopy, arsentsumebite, tsumebite, phosphate, arsenate, sulphate

 Author to whom correspondence should be addressed ([email protected]) 1

27 Introduction

28 The mineral arsenotsumebite Pb2Cu(AsO4)(SO4)(OH) [1, 2] is a rare mineral found in the 29 oxidized zone of a dolostone-hosted hydrothermal polymetallic ore deposit in Tsumeb, 30 Namibia and in a hydrothermal polymetallic barite-fluorite deposit at the Clara mine, 31 Germany. The mineral belongs to the brackenbuschite mineral group and is the arsenate

32 analogue of tsumebite Pb2Cu(PO4)(SO4)(OH). The mineral is an arsenate-sulphate-hydroxide 33 of lead and copper. The mineral has been known for an extended period of time [1, 2]. 34 Tsumebite and arsentsumebite form a system of solid solutions [3, 4] and this solid solution 35 formation should be reflected in the vibrational spectra [5].

37 The brackebuschite mineral group are a series of monoclinic arsenates, phosphates and

38 vanadates of the general formula A2B(XO4)(OH,H2O) where A may be Ba, Ca, Pb, Sr and B 2+ 2+ 3+ 2+ 3+ 39 may be Al, Cu ,Fe , Fe , Mn , Mn , Zn and XO4 may be AsO4, PO4, SO4,VO4. Related 40 to these minerals are the fornacite-vauquelinite series [6-9]. Minerals in this group are 41 arsenbrackebuschite, arsentsumebite [10, 11], bearthite [12, 13], brackebuschite [14-16], 42 feinglosite [17], gamagarite [18, 19], goedkenite [20], tsumebite [21]. The mineral is 43 monoclinic [2] with Space Group: P21/m. a = 8.85, b = 5.92, c = 7.84, β = 111.5, Z = 2 [2, 44 21]. The mineral is a rare secondary mineral found in the oxidized zone of some arsenic- 45 bearing Pb–Cu deposits and belongs to the brackebuschite group. The mineral has been 46 found in many places worldwide including at Broken Hill, NSW, Australia.

48 To the best of our knowledge, few infrared and no Raman spectroscopic studies have 49 been undertaken on these minerals. Farmer reported the results of an infrared spectrum of

50 brackebuschite (Pb2(Mn,Fe)(VO4)(OH) [22]. However, the formula presented for the mineral -1 51 is different from that now accepted. Infrared bands were found at 890, 860 cm (ν1), 740 and -1 -1 52 730 cm (ν3), 495 and 475 cm (ν4). The values correspond with recently published 53 spectroscopic data for vanadates [23, 24]. The infrared spectrum of tsumebite was also -1 54 published by Farmer [22]. Infrared bands for tsumebite were found at 932 cm (ν1), 475, 425 -1 -1 -1 55 and 415 cm (ν2), 1090, 1048, 1015 and 970 cm (ν3), 630, 615, 596, 575, 548 and 515 cm

56 (ν4). The values correspond with that of the phosphate anion in a distorted environment [23,

57 25-27]. No results of the infrared spectra of the hydroxyl and water OH stretching region 58 were published.

60 Raman spectroscopy has proven very useful for the study of minerals, [28-34] especially for 61 the study of diagenetically related minerals as often occurs with minerals containing arsenate 62 and phosphate groups, including tsumebite and arsenotsumebite. This paper is a part of 63 systematic studies of vibrational spectra of minerals of secondary origin in the oxide 64 supergene zone. In this work we attribute bands at various wavenumbers to vibrational modes 65 of tsumebite using Raman spectroscopy complimented with infrared spectroscopy and relate 66 the spectra to the structure of the mineral.

67 Experimental

68 Minerals

69 The mineral arsenotsumebite was supplied by The Mineralogical Research Company. Details 70 of the mineral have been published (page 35) [35].

71 Raman spectroscopy

72 Crystals of brushite were placed on a polished metal surface on the stage of an Olympus 73 BHSM microscope, which is equipped with 10x, 20x, and 50x objectives. The microscope is 74 part of a Renishaw 1000 Raman microscope system, which also includes a monochromator, a 75 filter system and a CCD detector (1024 pixels). The Raman spectra were excited by a 76 Spectra-Physics model 127 He-Ne laser producing highly polarised light at 633 nm and 77 collected at a nominal resolution of 2 cm-1 and a precision of ± 1 cm-1 in the range between 78 100 and 4000 cm-1. Repeated acquisition on the crystals using the highest magnification (50x) 79 was accumulated to improve the signal to noise ratio in the spectra. Spectra were calibrated 80 using the 520.5 cm-1 line of a silicon wafer.

81 Infrared spectroscopy

82 Infrared spectra were obtained using a Nicolet Nexus 870 FTIR spectrometer with a smart 83 endurance single bounce diamond ATR cell. Spectra over the 4000525 cm-1 range were 84 obtained by the co-addition of 64 scans with a resolution of 4 cm-1 and a mirror velocity of 85 0.6329 cm/s. Spectra were co-added to improve the signal to noise ratio.

86 Band component analysis was undertaken using the Jandel ‘Peakfit’ (Erkrath, 87 Germany) software package which enabled the type of fitting function to be selected and 88 allowed specific parameters to be fixed or varied accordingly. Band fitting was done using a 89 Lorentz-Gauss cross-product function with the minimum number of component bands used 90 for the fitting process. The Lorentz-Gauss ratio was maintained at values greater than 0.7 and 91 fitting was undertaken until reproducible results were obtained with squared correlations ( r2) 92 greater than 0.995. Band fitting of the spectra is quite reliable providing there is some band 93 separation or changes in the spectral profile.

94 Results and discussion 95 One means of distinguishing between tsumebite and arsentsumebite is to use Raman 96 spectroscopy. Raman spectroscopy will identify the phosphate and arsenate bands as well as 97 the sulphate bands. The Raman spectra of arsentsumebite and tsumebite are displayed in 98 Figures 1a and 1b. There is obvious difference between the Raman spectra. The band at -1 2- 99 around 972 cm is common in both spectra and is assigned to the SO4 ν1 symmetric -1 100 stretching mode. The Raman band at 935 cm assigned to the ν1 symmetric stretching mode 101 of hydrogen phosphate units. It is proposed in the structure of tsumebite that the protons 102 from the OH units interact with oxygen of the phosphate units, resulting in the formation of -1 103 HOPO3 units. The band at 935 cm is therefore assigned to the symmetric stretching -1 104 vibration of these HOPO3 units. A low intensity band at 906 cm is observed in the spectrum 105 of arsentsumebite which may some substitution of phosphate units in the arsentsumebite -1 3- 106 structure. The strong sharp Raman band at 814 cm is assigned to the AsO4 ν1 symmetric 107 stretching mode. A broad band is observed at 827 cm-1 in the Raman spectrum of tsumebite. 3- 108 This band is also assigned to the AsO4 ν1 symmetric stretching mode, thus proving that 109 substitution of phosphate by arsenate occurs in the structure of tsumebite. The higher 110 wavenumber bands at 1070, 1121, 1161 (arsentsumebite) and 1061 and 1097 cm-1 (tsumebite) 2- 111 are attributed to SO4 ν3 antisymmetric stretching mode.

112

113 The Raman spectrum of arsentsumebite in the 275 to 675 cm-1 region is shown in Figure 2. 114 This spectral region is where the sulphate and arsenate bending modes occur. The two -1 2- 115 prominent bands at 604 and 620 cm are assigned to the 4 (SO4) bending modes. In the 116 Raman spectrum of tsumebite two bands are found at 598 and 602 cm-1. The Raman bands -1 2- 117 observed at 412, 442 and 464 cm are attributable to the doubly degenerate 2 (SO4)

118 bending modes. Two very intense Raman bands are observed at 308 and 340 cm-1 and are 3- + 119 assigned to the AsO4 4 bending modes. Raman bands for tsumebite are observed at 540 120 and 554 cm-1. These bands are not observed in the Raman spectrum of tsumebite. The bands 3- + 121 are attributed to the PO4 4 bending modes.

122

123 The far low wavenumber region of arsentsumebite is reported in Figure 3. These bands may 124 be simply described as lattice vibrations. Intense Raman bands are found at 102, 145 and171 125 cm-1. The latter band is attributed to PbO stretching vibrations. Two other more low intensity 126 bands are observed at 241 and 248 cm-1. It is considered that these bands are associated with 127 hydrogen bonding in the arsentsumebite structure. A Raman band at 390 cm-1 (Figure 2) is 128 assigned to CuO stretching vibrations. Raman bands are found in very similar positions in 129 the Raman spectrum of tsumebite.

130 The Raman spectrum of arsentsumebite in the 2700 to 3600 cm-1 region is displayed in 131 Figure 4. The poor signal to noise ratio makes curve fitting and mode assignment more 132 speculative. Nevertheless, Raman bands are found at 2857, 2876, 2925, 3324 and 3458 cm-1. 133 The latter two bands are assigned to the OH stretching vibrations. The first three bands are

134 thought to be associated with HO-AsO3 vibrations.

135 Conclusions

136 The mineral arsentsumebite Pb2Cu(AsO4)(SO4)(OH), a copper phosphate-sulfate hydroxide 137 of the brackebuschite group has been characterised by Raman and infrared spectroscopy. 2- 3- 138 Bands are assigned to the stretching and bending modes of SO4 , PO4 and HOPO3 units.

139 The mineral is the arsenate analogue of tsumebite. Raman spectroscopy shows spectra 140 differences between the two minerals. Raman spectroscopy identifies differences in the 141 Raman spectra of the two minerals.

142 Acknowledgments

143 The financial and infra-structure support of the Queensland University of Technology, 144 Chemistry discipline is gratefully acknowledged. The Australian Research Council (ARC) is 145 thanked for funding the instrumentation.

146

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199 200

201

202

203 List of Figures

204 Figure 1a Raman spectrum of arsentsumebite in the 700 to 1600 cm-1 region.

205 Figure 1b Raman spectrum of tsumebite in the 700 to 1600 cm-1 region.

206 Figure 2 Raman spectrum of arsentsumebite in the 275 to 675 cm-1 region.

207 Figure 3 Raman spectrum of arsentsumebite in the 75 to 275 cm-1 region.

208 Figure 4 Raman spectrum of arsentsumebite in the 2700 to 3600 cm-1 region.

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Figure 1a Figure 1b 217

218

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222 Figure 2

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226 Figure 3

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231 Figure 4

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