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A near-infrared and Raman spectroscopic study of the mineral richelsdorfite Ca2Cu5Sb[Cl|(OH)6|(AsO4)4]·6H2O
Spectrochimica Acta Part A 78 (2011) 1302–1304
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
A near-infrared and Raman spectroscopic study of the mineral richelsdorfite Ca2 Cu5 Sb[Cl|(OH)6 |(AsO4 )4 ]·6H2 O Ray L. Frost ∗ , Sara J. Palmer, Silmarilly Bahfenne Chemistry Discipline, Faculty of Science and Technology, Queensland University of Technology, GPO Box 2434, Brisbane, Queensland 4001, Australia
a r t i c l e
i n f o
Article history: Received 16 August 2010 Received in revised form 2 December 2010 Accepted 14 December 2010 Keywords: Raman spectroscopy Arsenate Richelsdorfite Sampleite Lavandulan
a b s t r a c t Raman spectroscopy has enabled insights into the molecular structure of the richelsdorfite Ca2 Cu5 Sb[Cl|(OH)6 |(AsO4 )4 ]·6H2 O. This mineral is based upon the incorporation of arsenate or phosphate with chloride anion into the structure and as a consequence the spectra reflect the bands attributable to these anions, namely arsenate or phosphate and chloride. The richelsdorfite Raman spectrum reflects the spectrum of the arsenate anion and consists of 1 at 849, 2 at 344 cm−1 , 3 at 835 and 4 at 546 and 498 cm−1 . A band at 268 cm−1 is attributed to CuO stretching vibration. Low wavenumber bands at 185 and 144 cm−1 may be assigned to CuCl TO/LO optic vibrations. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The mineral richelsdorfite Ca2 Cu5 Sb[Cl|(OH)6 |(AsO4 )4 ]·6H2 O [1] is monoclinic space group C2/m and is basically a hydrated hydroxy arsenate with calcium, copper and antimony as the cations [2]. The mineral contains a halogen, namely chloride. Phosphates and arsenate minerals of copper are very common. The mineral is a bright blue [3] in colour and is related in structure to lavandulan and sampleite [4,5]. Among these minerals are those of the sampleite group [6–10] including sampleite (NaCaCu5 (PO4 )4 Cl·5H2 O), lavendulan (NaCaCu5 (AsO4 )4 Cl·5H2 O), zdenekite (NaPbCu5 (AsO4 )4 Cl·5H2 O), and the polymorph of lavandulan known as lemanskiite [11–13]. The minerals are often found in arid climates, and in caves derived from copper sulphides in cave walls and the phosphate from bat guano. Some are known as post mining species [12,14]. Also, some been found as alteration products in ancient slag heaps [15,16]. Further it is certain that some of these minerals are of archaeological significance [17] and it is apparent that they were used for colorant effects in cosmetics of the ancient Egyptians [18]. Certainly the appearance of the minerals provides various shades of blue and blue-green. No spectroscopic studies of the mineral richelsdorfite have reported [19–21], even though Raman spectroscopy has proven especially useful for the study of minerals. The aim of this paper is to report the Raman spectra of richelsdorfite and to relate the spectra to the chemistry and molecular structure of the mineral.
∗ Corresponding author. Tel.: +61 7 3138 2407; fax: +61 7 3138 1804. E-mail address: [email protected] (R.L. Frost). 1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2010.12.084
This paper follows the systematic research on Raman and infrared spectroscopy of secondary minerals containing oxy-anions formed in the oxidation zone of minerals. 2. Experimental 2.1. Minerals The mineral richelsdorfite Ca2 Cu5 Sb[Cl|(OH)6 |(AsO4 )4 ]·6H2 O was obtained from the Mineralogical Research Company. The mineral originated from the Wilhelm Mine (Wechselschacht), Bauhaus, Richelsdorf District, North Hesse, Hesse, Germany. The composition of the mineral has been published (Anthony et al. [22], p. 497). The chemical composition is given as CaO 8.97, CuO 28.71, Sb2 O5 11.88, As2 O5 31.18%. The chemical composition as stated for richelsdorfite does not show any P content, even though the Raman spectrum of richelsdorfite does indicate the presence of minor amounts of P. 2.2. Near-infrared (NIR) spectroscopy NIR spectra were collected on a Nicolet Nexus FT-IR spectrometer with a Nicolet Near-IR Fibreport accessory (Madison, Wisconsin). A white light source was used, with a quartz beam splitter and TEC NIR InGaAs detector. Spectra were obtained from 13,000 to 4000 cm−1 (0.77–2.50 m) (770–2500 nm) by the coaddition of 64 scans at a spectral resolution of 8 cm−1 . A mirror velocity of 1.266 m s−1 was used. The spectra were transformed using the Kubelka–Munk algorithm to provide spectra for comparison with published absorption spectra.
R.L. Frost et al. / Spectrochimica Acta Part A 78 (2011) 1302–1304
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2.3. Raman spectroscopy Crystals of richelsdorfite were placed on a polished metal surface on the stage of an Olympus BHSM microscope, which is equipped with 10×, 20×, and 50× objectives. The microscope is part of a Renishaw 1000 Raman microscope system, which also includes a monochromator, a filter system and a CCD detector (1024 pixels). The Raman spectra were excited by a Spectra-Physics model 127 He–Ne laser producing highly polarised light at 633 nm and collected at a nominal resolution of 2 cm−1 and a precision of ±1 cm−1 in the range between 100 and 4000 cm−1 . Repeated acquisition on the crystals using the highest magnification (50×) was accumulated to improve the signal to noise ratio in the spectra. Spectra were calibrated using the 520.5 cm−1 line of a silicon wafer. Band component analysis was undertaken using the Jandel ‘Peakfit’ (Erkrath, Germany) software package which enabled the type of fitting function to be selected and allowed specific parameters to be fixed or varied accordingly. Band fitting was done using a Lorentz–Gauss cross-product function with the minimum number of component bands used for the fitting process. The Lorentz–Gauss ratio was maintained at values greater than 0.7 and fitting was undertaken until reproducible results were obtained with squared correlations (r2 ) greater than 0.995. Band fitting of the spectra is quite reliable providing there is some band separation or changes in the spectral profile. 3. Results and discussion 3.1. Colour of the mineral – NIR spectrum The mineral is a very bright turquoise-blue colour. The Near infrared spectrum of richelsdorfite exhibits broad bands at around 8050 and 10,250 cm−1 which were attributed to Cu(II) atoms in the structure of richelsdorfite. The mineral richelsdorfite is characterised by the divalent copper ion bands in their electronic spectra. Cu(II) (d9 ) is considered with one unpaired electron and gives one free ion term 2 D. The 2 D term splits into ground state 2 Eg and excited 2 Tg state in an octahedral crystal field. But 2 Eg is unstable and splits under the influence of Jahn–Teller effect and the 2 T excited state also splits. That is why Cu2+ complex experig ences distortion from regular octahedral coordination usually to elongated octahedron. As a result of distortion of octahedral symmetry, copper complexes usually show three bands corresponding to 2 B1g → 2 A1g , 2 B1g → 2 B2g and 2 B1g → 2 Eg transitions. Two bands appear in near-infrared and third one is observable in visible spectrum. The Jahn–Teller effect is most often encountered in octahedral complexes of the transition metal ions and is very commonly occurs in six-coordinate copper(II) complexes. The Jahn–Teller effect can be demonstrated experimentally by investigating the UV–VIS–NIR absorbance spectra of inorganic compounds, where Cu2+ often causes splitting of bands. Many studies reported on Cu(II) minerals [23–31]. For example: two bands observed in connellite [Cu19 (SO4 )Cl4 (OH)32 ·3H2 O] at 8335 and 12,050 cm−1 are assigned to 2 B1g → 2 A1g and 2 B1g → 2 B2g transitions. Near infrared spectra of smithsonite minerals exhibits bands at 8050 and 10,310 cm−1 were attributed to Cu(II) [32,33].
Fig. 1. Raman spectrum of richelsdorfite over the 100–3700 cm−1 range. The figure shows the relative intensity of the bands.
at 463 cm−1 [34]. Of all the tetrahedral oxyanion spectra, the wavenumbers of the arsenate vibrations are lower than those of any of the other naturally occurring mineral oxyanions. The effect on the arsenate ion in a crystal will be to remove the degeneracies and allow splitting of the bands according to factor group analysis. The Raman spectrum of richelsdorfite over the full wavenumber range is shown in Fig. 1. The figure displays the relative intensity of all the bands in the Raman spectrum. Details of the Raman spectrum in the 100–1700 cm−1 range are displayed in Fig. 2. The intense Raman band at 849 cm−1 is attributed to the (AsO4 )3− symmetric stretching vibration. Two shoulders are observed at 835 and 910 cm−1 . The first band is attributed to the (AsO4 )3− antisymmetric stretching vibrations and the latter band is assigned to the stretching vibration of HAsO4 units. Low intensity bands are observed at 988 and 1082 cm−1 and are attributed to (PO4 )3− stretching vibrations. These low intensity bands provide evidence for the replacement of some arsenate units by phosphate anions. A strong Raman band for richelsdorfite is found at 546 cm−1 and is assigned to the 4 (AsO4 )3− bending vibration. A low intensity band appearing as a shoulder at 498 cm−1 is also attributed to this vibrational mode. The observation of more than one band supports the reduction in symmetry of the arsenate anion in the richelsdorfite structure. Both the lemanskiite and the lavendulan samples also show an intense Raman band centred around 545 cm−1 . This band is also assigned to OH bending vibrations. A similar band is observed at 546 cm−1 for zdenekite (see formulae above). Low intensity Raman bands are observed at 1376 and 1564 cm−1 . These bands may be assigned to overtones of the (AsO4 )3− fundamentals.
3.2. Raman spectroscopy Raman spectra of the tetrahedral phosphate and arsenate anions in aqueous systems are well known [34,35]. The symmetric stretching vibration of the arsenate anion (1 ) is observed at 837 cm−1 and may be coincident with the position of the antisymmetric stretching mode (3 ). The symmetric bending mode (2 ) is observed at 349 cm−1 and the out-of-plane bending modes (4 ) is observed
Fig. 2. Raman spectrum of richelsdorfite over the 100–1700 cm−1 range.
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R.L. Frost et al. / Spectrochimica Acta Part A 78 (2011) 1302–1304
A Raman band of lower intensity is observed at 344 cm−1 and is assigned to the 2 AsO4 bending mode. Raman bands are observed for richelsdorfite at 268 and 185 cm−1 . These bands are attributed to CuCl longitudinal and transverse optical vibrations. The observation of this mode at two different positions supports the fact that the two sampleite minerals have slightly different molecular structures. Two other bands are observed at 282 and 224 cm−1 for the sampleite polymorph from Victoria and at 289 and 221 cm−1 for sampleite. Bands are also observed at around 190 and 172 cm−1 attributed to CuCl stretching vibrations. It is possible that longitudinal and transverse optic vibrations are observed in these minerals. It is probable that the band around 224 cm−1 is the longitudinal optic vibration and the band at 190 cm−1 is the transverse optic vibration [36,37]. Bands in this OH stretching region are of very low intensity and the spectrum suffers from a low signal to noise. Raman bands may be observed at 3036, 3171, 3254, 3338 and 3482 cm−1 . These bands are assigned to the OH stretching vibrations of the water and OH units. It is difficult to assign bands in this spectral region with absolute precision. The higher wavenumber band at 3482 cm−1 is the likely band position of the symmetric stretching mode of the OH units. The other bands are assigned to water stretching vibrations. 4. Conclusions The mineral richelsdorfite has been studied by Raman spectroscopy and the Raman spectra related to the molecular structure of the mineral. Raman bands were attributed to arsenate stretching and bending modes. A comparison of the Raman spectrum of richelsdorfite is made with the Raman spectra of the sampleite minerals containing the arsenate anion. Raman spectroscopy [4] has been previously used to characterise the sampleite mineral group namely sampleite (NaCaCu5 (PO4 )4 Cl·5H2 O), lavendulan (NaCaCu5 (AsO4 )4 Cl·5H2 O), zdenekite (NaPbCu5 (AsO4 )4 Cl·5H2 O), and the polymorph of lavendulan known as lemanskiite. The minerals are characterised by the spectroscopy of the two anions present in the structure either phosphate in the case of sampleite or arsenate in the case of zdenekite, lavendulan and lemanskiite. The (PO4 )3− stretching vibrations are found at high wavenumbers compared with many phosphate minerals. This reflects the strength of the bonding of the phosphate anion to the copper atom. Acknowledgements The financial and infra-structure support of the Queensland University of Technology Chemistry Discipline of the Faculty of Sci-
ence and Technology, is gratefully acknowledged. The Australian Research Council (ARC) is thanked for funding the instrumentation.
References [1] P. Suesse, G. Schnorrer-Koehler, Neues Jahrb. Miner. (1983) 145–150. [2] P. Suesse, B. Tillmann, Zeit. Krist. 179 (1987) 323–334. [3] H. Sarp, B. Dominik, P.-J. Chiappero, Schweiz. Miner. Petrog. Mitt. 74 (1994) 273–277. [4] R.L. Frost, M.L. Weier, P.A. Williams, P. Leverett, J.T. Kloprogge, J. Raman Spectrosc. 38 (2007) 574–583. [5] G. Giester, U. Kolitsch, P. Leverett, P. Turner, P.A. Williams, Eur. J. Mineral. 19 (2007) 75–93. [6] P. Ondrus, F. Veselovsky, R. Skala, J. Sejkora, R. Pazout, J. Fryda, A. Gabasova, J. Vajdak, Can. Mineral. 44 (2006) 523–531. [7] W.F. Foshag, Am. Mineral. 9 (1924) 29–30. [8] C.S. Hurlbut Jr., Am. Mineral. 27 (1942) 586–589. [9] H. Strunz, Fortschr. Mineral. 37 (1959) 87–90. [10] R. Arana Castillo, M. Rodriguez Gallego, J.A. Alcover, Cuad. Geol. Univ. Granada 3 (1973) 55–67. [11] P.-J. Chiappero, H. Sarp, Eur. J. Mineral. 7 (1995) 553–557. [12] R.S.W. Braithwaite, J.R. Knight, Mineral. Mag. 54 (1990) 129–131. [13] P.J. Bridge, M.W. Pryce, R.M. Clarke, M.B. Costello, Mineral. Mag. 42 (1978) 369–371. [14] C. Guillemin, Bull. Soc. Fr. Mineral. Crist. 79 (1956) 7–95. [15] G. Pallix, Mineral. Rec. 9 (1978) 69–73. [16] C. Rewitzer, R. Hochleitner, Riv. Mineral. Ital. (1989) 83–100. [17] V.V. Zaykov, A.P. Bushmakin, A.M. Yuminov, E.V. Zaykova, G.B. Zdanovich, A.D. Tairov, R.J. Herrington, Geol. Soc. Spec. Publ. 165 (1999) 165–176. [18] J.M. Peixoto Cabral, Quimica (Lisboa) 66 (1997) 17–24. [19] M.T. Paques-Ledent, P. Tarte, Spectrochim. Acta 30A (1974) 673–689. [20] S.V. Gevork’yan, A.S. Povarennykh, Konst. Svoistva Miner. 9 (1975) 73–81. [21] R.S.W. Braithwaite, Mineral. Mag. 47 (1983) 51–57. [22] J.W. Anthony, R.A. Bideaux, K.W. Bladh, M.C. Nichols, Handbook of Mineralogy, Mineral Data Publishing, Tuscon, Arizona, 1997. [23] R.L. Frost, B.J. Reddy, E.C. O’Keeffe, J. Near Infrared Spectrosc. 18 (2010) 177–189. [24] B.J. Reddy, E.C. Keeffe, R.L. Frost, Transit. Met. Chem. 35 (2010) 279–287. [25] R.R.S. Reddy, S.L. Reddy, P.S. Rao, R.L. Frost, Spectrochim. Acta 75 (2010) 28–31. [26] S.L. Reddy, K.N.M. Reddy, G.S. Reddy, T. Endo, R.L. Frost, Mol. Phys. 108 (2010) 1467–1470. [27] B.J. Reddy, R.L. Frost, M.J. Dickfos, Spectrochim. Acta 71A (2009) 1762–1768. [28] B.J. Reddy, L. Frost Ray, J. Dickfos Marilla, Spectrochim. Acta A 71 (2008) 1762–1768. [29] B.J. Reddy, R.L. Frost, A. Locke, Transit. Met. Chem. 33 (2008) 331–339. [30] S.L. Reddy, K.N.M. Reddy, G.S. Reddy, B.J. Reddy, R.L. Frost, T. Endo, Spectrochim. Acta 69 (2008) 503–508. [31] B.J. Reddy, R.L. Frost, J. Near Infrared Spectrosc. 15 (2007) 115–121. [32] R.L. Frost, B.J. Reddy, M.C. Hales, D.L. Wain, J. Near Infrared Spectrosc. 16 (2008) 75–82. [33] R.L. Frost, B.J. Reddy, D.L. Wain, M.C. Hales, J. Near Infrared Spectrosc. 14 (2006) 317–324. [34] K. Nakamoto, Infrared Spectra of Inorganic and Coordination Compounds, 1963. [35] V.C. Farmer, Mineralogical Society Monograph 4: The Infrared Spectra of Minerals, 1974. [36] R.L. Frost, W. Martens, J.T. Kloprogge, P.A. Williams, J. Raman Spectrosc. 33 (2002) 801–806. [37] R.L. Frost, P.A. Williams, W. Martens, J.T. Kloprogge, J. Raman Spectrosc. 33 (2002) 752–757.
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
A near-infrared and Raman spectroscopic study of the mineral richelsdorfite Ca2 Cu5 Sb[Cl|(OH)6 |(AsO4 )4 ]·6H2 O Ray L. Frost ∗ , Sara J. Palmer, Silmarilly Bahfenne Chemistry Discipline, Faculty of Science and Technology, Queensland University of Technology, GPO Box 2434, Brisbane, Queensland 4001, Australia
a r t i c l e
i n f o
Article history: Received 16 August 2010 Received in revised form 2 December 2010 Accepted 14 December 2010 Keywords: Raman spectroscopy Arsenate Richelsdorfite Sampleite Lavandulan
a b s t r a c t Raman spectroscopy has enabled insights into the molecular structure of the richelsdorfite Ca2 Cu5 Sb[Cl|(OH)6 |(AsO4 )4 ]·6H2 O. This mineral is based upon the incorporation of arsenate or phosphate with chloride anion into the structure and as a consequence the spectra reflect the bands attributable to these anions, namely arsenate or phosphate and chloride. The richelsdorfite Raman spectrum reflects the spectrum of the arsenate anion and consists of 1 at 849, 2 at 344 cm−1 , 3 at 835 and 4 at 546 and 498 cm−1 . A band at 268 cm−1 is attributed to CuO stretching vibration. Low wavenumber bands at 185 and 144 cm−1 may be assigned to CuCl TO/LO optic vibrations. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The mineral richelsdorfite Ca2 Cu5 Sb[Cl|(OH)6 |(AsO4 )4 ]·6H2 O [1] is monoclinic space group C2/m and is basically a hydrated hydroxy arsenate with calcium, copper and antimony as the cations [2]. The mineral contains a halogen, namely chloride. Phosphates and arsenate minerals of copper are very common. The mineral is a bright blue [3] in colour and is related in structure to lavandulan and sampleite [4,5]. Among these minerals are those of the sampleite group [6–10] including sampleite (NaCaCu5 (PO4 )4 Cl·5H2 O), lavendulan (NaCaCu5 (AsO4 )4 Cl·5H2 O), zdenekite (NaPbCu5 (AsO4 )4 Cl·5H2 O), and the polymorph of lavandulan known as lemanskiite [11–13]. The minerals are often found in arid climates, and in caves derived from copper sulphides in cave walls and the phosphate from bat guano. Some are known as post mining species [12,14]. Also, some been found as alteration products in ancient slag heaps [15,16]. Further it is certain that some of these minerals are of archaeological significance [17] and it is apparent that they were used for colorant effects in cosmetics of the ancient Egyptians [18]. Certainly the appearance of the minerals provides various shades of blue and blue-green. No spectroscopic studies of the mineral richelsdorfite have reported [19–21], even though Raman spectroscopy has proven especially useful for the study of minerals. The aim of this paper is to report the Raman spectra of richelsdorfite and to relate the spectra to the chemistry and molecular structure of the mineral.
∗ Corresponding author. Tel.: +61 7 3138 2407; fax: +61 7 3138 1804. E-mail address: [email protected] (R.L. Frost). 1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2010.12.084
This paper follows the systematic research on Raman and infrared spectroscopy of secondary minerals containing oxy-anions formed in the oxidation zone of minerals. 2. Experimental 2.1. Minerals The mineral richelsdorfite Ca2 Cu5 Sb[Cl|(OH)6 |(AsO4 )4 ]·6H2 O was obtained from the Mineralogical Research Company. The mineral originated from the Wilhelm Mine (Wechselschacht), Bauhaus, Richelsdorf District, North Hesse, Hesse, Germany. The composition of the mineral has been published (Anthony et al. [22], p. 497). The chemical composition is given as CaO 8.97, CuO 28.71, Sb2 O5 11.88, As2 O5 31.18%. The chemical composition as stated for richelsdorfite does not show any P content, even though the Raman spectrum of richelsdorfite does indicate the presence of minor amounts of P. 2.2. Near-infrared (NIR) spectroscopy NIR spectra were collected on a Nicolet Nexus FT-IR spectrometer with a Nicolet Near-IR Fibreport accessory (Madison, Wisconsin). A white light source was used, with a quartz beam splitter and TEC NIR InGaAs detector. Spectra were obtained from 13,000 to 4000 cm−1 (0.77–2.50 m) (770–2500 nm) by the coaddition of 64 scans at a spectral resolution of 8 cm−1 . A mirror velocity of 1.266 m s−1 was used. The spectra were transformed using the Kubelka–Munk algorithm to provide spectra for comparison with published absorption spectra.
R.L. Frost et al. / Spectrochimica Acta Part A 78 (2011) 1302–1304
1303
2.3. Raman spectroscopy Crystals of richelsdorfite were placed on a polished metal surface on the stage of an Olympus BHSM microscope, which is equipped with 10×, 20×, and 50× objectives. The microscope is part of a Renishaw 1000 Raman microscope system, which also includes a monochromator, a filter system and a CCD detector (1024 pixels). The Raman spectra were excited by a Spectra-Physics model 127 He–Ne laser producing highly polarised light at 633 nm and collected at a nominal resolution of 2 cm−1 and a precision of ±1 cm−1 in the range between 100 and 4000 cm−1 . Repeated acquisition on the crystals using the highest magnification (50×) was accumulated to improve the signal to noise ratio in the spectra. Spectra were calibrated using the 520.5 cm−1 line of a silicon wafer. Band component analysis was undertaken using the Jandel ‘Peakfit’ (Erkrath, Germany) software package which enabled the type of fitting function to be selected and allowed specific parameters to be fixed or varied accordingly. Band fitting was done using a Lorentz–Gauss cross-product function with the minimum number of component bands used for the fitting process. The Lorentz–Gauss ratio was maintained at values greater than 0.7 and fitting was undertaken until reproducible results were obtained with squared correlations (r2 ) greater than 0.995. Band fitting of the spectra is quite reliable providing there is some band separation or changes in the spectral profile. 3. Results and discussion 3.1. Colour of the mineral – NIR spectrum The mineral is a very bright turquoise-blue colour. The Near infrared spectrum of richelsdorfite exhibits broad bands at around 8050 and 10,250 cm−1 which were attributed to Cu(II) atoms in the structure of richelsdorfite. The mineral richelsdorfite is characterised by the divalent copper ion bands in their electronic spectra. Cu(II) (d9 ) is considered with one unpaired electron and gives one free ion term 2 D. The 2 D term splits into ground state 2 Eg and excited 2 Tg state in an octahedral crystal field. But 2 Eg is unstable and splits under the influence of Jahn–Teller effect and the 2 T excited state also splits. That is why Cu2+ complex experig ences distortion from regular octahedral coordination usually to elongated octahedron. As a result of distortion of octahedral symmetry, copper complexes usually show three bands corresponding to 2 B1g → 2 A1g , 2 B1g → 2 B2g and 2 B1g → 2 Eg transitions. Two bands appear in near-infrared and third one is observable in visible spectrum. The Jahn–Teller effect is most often encountered in octahedral complexes of the transition metal ions and is very commonly occurs in six-coordinate copper(II) complexes. The Jahn–Teller effect can be demonstrated experimentally by investigating the UV–VIS–NIR absorbance spectra of inorganic compounds, where Cu2+ often causes splitting of bands. Many studies reported on Cu(II) minerals [23–31]. For example: two bands observed in connellite [Cu19 (SO4 )Cl4 (OH)32 ·3H2 O] at 8335 and 12,050 cm−1 are assigned to 2 B1g → 2 A1g and 2 B1g → 2 B2g transitions. Near infrared spectra of smithsonite minerals exhibits bands at 8050 and 10,310 cm−1 were attributed to Cu(II) [32,33].
Fig. 1. Raman spectrum of richelsdorfite over the 100–3700 cm−1 range. The figure shows the relative intensity of the bands.
at 463 cm−1 [34]. Of all the tetrahedral oxyanion spectra, the wavenumbers of the arsenate vibrations are lower than those of any of the other naturally occurring mineral oxyanions. The effect on the arsenate ion in a crystal will be to remove the degeneracies and allow splitting of the bands according to factor group analysis. The Raman spectrum of richelsdorfite over the full wavenumber range is shown in Fig. 1. The figure displays the relative intensity of all the bands in the Raman spectrum. Details of the Raman spectrum in the 100–1700 cm−1 range are displayed in Fig. 2. The intense Raman band at 849 cm−1 is attributed to the (AsO4 )3− symmetric stretching vibration. Two shoulders are observed at 835 and 910 cm−1 . The first band is attributed to the (AsO4 )3− antisymmetric stretching vibrations and the latter band is assigned to the stretching vibration of HAsO4 units. Low intensity bands are observed at 988 and 1082 cm−1 and are attributed to (PO4 )3− stretching vibrations. These low intensity bands provide evidence for the replacement of some arsenate units by phosphate anions. A strong Raman band for richelsdorfite is found at 546 cm−1 and is assigned to the 4 (AsO4 )3− bending vibration. A low intensity band appearing as a shoulder at 498 cm−1 is also attributed to this vibrational mode. The observation of more than one band supports the reduction in symmetry of the arsenate anion in the richelsdorfite structure. Both the lemanskiite and the lavendulan samples also show an intense Raman band centred around 545 cm−1 . This band is also assigned to OH bending vibrations. A similar band is observed at 546 cm−1 for zdenekite (see formulae above). Low intensity Raman bands are observed at 1376 and 1564 cm−1 . These bands may be assigned to overtones of the (AsO4 )3− fundamentals.
3.2. Raman spectroscopy Raman spectra of the tetrahedral phosphate and arsenate anions in aqueous systems are well known [34,35]. The symmetric stretching vibration of the arsenate anion (1 ) is observed at 837 cm−1 and may be coincident with the position of the antisymmetric stretching mode (3 ). The symmetric bending mode (2 ) is observed at 349 cm−1 and the out-of-plane bending modes (4 ) is observed
Fig. 2. Raman spectrum of richelsdorfite over the 100–1700 cm−1 range.
1304
R.L. Frost et al. / Spectrochimica Acta Part A 78 (2011) 1302–1304
A Raman band of lower intensity is observed at 344 cm−1 and is assigned to the 2 AsO4 bending mode. Raman bands are observed for richelsdorfite at 268 and 185 cm−1 . These bands are attributed to CuCl longitudinal and transverse optical vibrations. The observation of this mode at two different positions supports the fact that the two sampleite minerals have slightly different molecular structures. Two other bands are observed at 282 and 224 cm−1 for the sampleite polymorph from Victoria and at 289 and 221 cm−1 for sampleite. Bands are also observed at around 190 and 172 cm−1 attributed to CuCl stretching vibrations. It is possible that longitudinal and transverse optic vibrations are observed in these minerals. It is probable that the band around 224 cm−1 is the longitudinal optic vibration and the band at 190 cm−1 is the transverse optic vibration [36,37]. Bands in this OH stretching region are of very low intensity and the spectrum suffers from a low signal to noise. Raman bands may be observed at 3036, 3171, 3254, 3338 and 3482 cm−1 . These bands are assigned to the OH stretching vibrations of the water and OH units. It is difficult to assign bands in this spectral region with absolute precision. The higher wavenumber band at 3482 cm−1 is the likely band position of the symmetric stretching mode of the OH units. The other bands are assigned to water stretching vibrations. 4. Conclusions The mineral richelsdorfite has been studied by Raman spectroscopy and the Raman spectra related to the molecular structure of the mineral. Raman bands were attributed to arsenate stretching and bending modes. A comparison of the Raman spectrum of richelsdorfite is made with the Raman spectra of the sampleite minerals containing the arsenate anion. Raman spectroscopy [4] has been previously used to characterise the sampleite mineral group namely sampleite (NaCaCu5 (PO4 )4 Cl·5H2 O), lavendulan (NaCaCu5 (AsO4 )4 Cl·5H2 O), zdenekite (NaPbCu5 (AsO4 )4 Cl·5H2 O), and the polymorph of lavendulan known as lemanskiite. The minerals are characterised by the spectroscopy of the two anions present in the structure either phosphate in the case of sampleite or arsenate in the case of zdenekite, lavendulan and lemanskiite. The (PO4 )3− stretching vibrations are found at high wavenumbers compared with many phosphate minerals. This reflects the strength of the bonding of the phosphate anion to the copper atom. Acknowledgements The financial and infra-structure support of the Queensland University of Technology Chemistry Discipline of the Faculty of Sci-
ence and Technology, is gratefully acknowledged. The Australian Research Council (ARC) is thanked for funding the instrumentation.
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