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An infrared and Raman spectroscopic study of natural zinc phosphates
Spectrochimica Acta Part A 60 (2004) 1439–1445
An infrared and Raman spectroscopic study of natural zinc phosphates Ray L. Frost∗ Inorganic Materials Research Program, School of Physical and Chemical Sciences, Queensland University of Technology, GPO Box 2434, Brisbane Qld 4001, Australia Received 17 July 2003; received in revised form 17 July 2003; accepted 20 August 2003
Abstract Zinc phosphates are important in the study of the phosphatisation of metals. Raman spectroscopy in combination with infrared spectroscopy has been used to characterise the zinc phosphate minerals. The minerals may be characterised by the patterns of the hydroxyl stretching vibrations in both the Raman and infrared spectra. Spencerite is characterised by a sharp Raman band at 3516 cm−1 and tarbuttite by a single band at 3446 cm−1 . The patterns of the Raman spectra of the hydroxyl stretching region of hopeite and parahopeite are different in line with their differing crystal structures. The Raman spectrum of the PO4 stretching region shows better band separated peaks than the infrared spectra which consist of a complex set of overlapping bands. The position of the PO4 symmetric stretching mode can be used to identify the zinc phosphate mineral. It is apparent that Raman spectroscopy lends itself to the fundamental study of the evolution of zinc phosphate films. © 2003 Elsevier B.V. All rights reserved. Keywords: Hopeite; Parahopeite; Spencerite; Scholzite; Tarbuttite; Phosphate; Raman spectroscopy
1. Introduction The study of zinc phosphates is important for a number of reasons. Firstly, because the phosphatisation of metals can result in the formation of zinc and other metal phosphates [1–3]. Secondly, the use of these types of minerals has application in dental cements [4]. Thirdly, the formation of such minerals is important for the fundamental understanding of the oxidised zones of metal deposits [3]. Further many of these minerals may form in landfills and their stability means that there presence is permanent [5]. It is also probable that many of the zinc and calcium phosphate minerals also occur when replacing human and animal bones. Whereas the copper phosphate minerals are many and common [6–8], the phosphates of the divalent cations are not. The zinc phosphate minerals are confined to the dimorphs of hopeite [9,10] and parahopeite (Zn3 (PO4 )2 ·4H2 O) [11,12], tarbuttite (Zn2 PO4 (OH)) [13,14], spencerite (Zn2 PO4 (OH)·1.5H2 O) [15,16], together with the mixed
∗
Tel.: +61-7-3864-2407; fax: +61-7-3864-1804. E-mail address: [email protected] (R.L. Frost).
1386-1425/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2003.08.009
cationic minerals scholzite (CaZn2 (PO4 )2 ·3H2 O) [17,18], and parascholzite (CaZn2 (PO4 )2 ·2H2 O) [19–21]. Infrared spectroscopy has proven most useful for the study of these phosphates particularly for phosphate coatings [4,22–24]. Raman spectroscopy has been used to a limited extent [25,26]. In aqueous systems, Raman spectra of phosphate oxyanions show a symmetric stretching mode (ν1 ) at 938 cm−1 , the antisymmetric stretching mode (ν3 ) at 1017 cm−1 , the symmetric bending mode (ν2 ) at 420 cm−1 and the ν4 mode at 567 cm−1 [7,27,28]. The Raman spectroscopy of some phosphate minerals have been studied [7,8,27]. The vibrational spectra of related minerals such as pseudo-malachite and reichenbachite are different in line with the differences in the crystal structures. The pseudo-malachite vibrational spectrum consists of ν1 at 953 cm−1 , ν2 at 422 and 450 cm−1 , ν3 at 1025 and 1096 cm−1 and ν4 at 482, 530, 555 and 615 cm−1 . Libethenite vibrational modes occur at 960 cm−1 (ν1 ), 445 cm−1 (ν2 ), 1050 cm−1 (ν3 ) and 480, 522, 555, 618 and 637 cm−1 (ν4 ). Cornetite vibrational modes occur at 960 cm−1 (ν1 ), 415 and 464 cm−1 (ν2 ), 1000, 1015 and 1070 cm−1 (ν3 ) and 510, 527, 558, 582, 623 and 647 cm−1 (ν4 ). Fundamentally, there have been no comprehensive
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R.L. Frost / Spectrochimica Acta Part A 60 (2004) 1439–1445
studies of the vibrational spectroscopy of the zinc phosphate minerals.
with squared regression coefficient of R2 greater than 0.995.
2. Experimental
3. Results and discussion
2.1. Minerals
The Raman and infrared spectra of hopeite are shown in Fig. 1. For hopeite, two Raman bands in the hydroxyl stretching region are observed at 3456 and 3247 cm−1 . No Raman bands of the hydroxyl stretching region of hopeite have been reported to date. In contrast four infrared bands are observed at 3542, 3473, 3338 and 3149 cm−1 . Pawlig et al. reported the infrared spectrum of hopeite and its deuteron-analogs [29]. They reported infrared bands at 3537, 3410, 3263 and 3181 cm−1 [29]. The structure of hopeite consists of ZnO2 (H2 O)4 octahedra, ZnO4 tetrahedra, and PO4 tetrahedra, none of which are regular; these polyhedra share corners and edges [30]. It is likely that the two water molecules are non-equivalent, thus giving rise to in-phase and out-of-phase behaviour. Hence, two Raman bands and two infrared bands would be expected. The crystal structure of parahopeite, Zn3 (PO4 )2 ·4H2 O, is similar to those of phosphophyllite and hopeite in that one of the two Zn atoms is 6-coordinated and the other is
G3566: Hopeite, Broken Hill, Zambia. G14170: Parahopeite, Reaphook Hill, SA. G14784: Tarbutite, Broken Hill, Zambia. G5847: Spencerite, Salmo British Columbia, Canada. G8448: Scholzite, Reaphook Hill, SA. Parascholzite was supplied by the Mineral Research Company. 2.2. Raman microprobe spectroscopy The crystals of the zinc phosphate minerals were placed and oriented on a polished metal surface on the stage of an Olympus BHSM microscope, which is equipped with 10× and 50× objectives. The microscope is part of a Renishaw 1000 Raman microscope system, which also includes a monochromator, a notch filter system and a thermo-electrically cooled charge coupled device (CCD) detector. Raman spectra were excited by a Spectra-Physics model 127 He–Ne laser (633 nm) and acquired at a nominal resolution of 2 cm−1 in the range between 100 and 4000 cm−1 . The crystals were oriented to provide maximum intensity. All crystal orientations were used to obtain the spectra. Power at the sample was measured as 1 mW. The incident radiation was scrambled to avoid polarisation effects.
Infrared Intensity
The following minerals were obtained from South Australian Museum and have been characterised. The minerals were checked for phase composition using X-ray diffraction and for chemical composition using the electron probe.
Hopeite
2.3. Infrared spectroscopy Infrared spectra were obtained using a Nicolet Nexus 870 FTIR spectrometer with a smart endurance single bounce diamond ATR cell. Spectra over the 4000–525 cm−1 range were obtained by the co-addition of 64 scans with a resolution of 4 cm−1 and a mirror velocity of 0.6329 cm s−1 . Spectracalc software package GRAMS. Band component analysis was undertaken using the Jandel ‘Peakfit’ software package, which enabled the type of fitting function to be selected and allows specific parameters to be fixed or varied accordingly. Band fitting was done using a Gauss–Lorentz cross-product function with the minimum number of component bands used for the fitting process. The Gauss–Lorentz ratio was maintained at values greater than 0.7 and fitting was undertaken until reproducible results were obtained
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Fig. 1. Infrared and Raman spectra of the hydroxyl stretching region of hopeite.
R.L. Frost / Spectrochimica Acta Part A 60 (2004) 1439–1445
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Fig. 2. Infrared and Raman spectra of the hydroxyl stretching region of parahopeite.
4-coordinated. Parahopeite differs from the other two minerals because one of the P–O tetrahedral O atoms is bonded to both the 6- and 4-coordinated cations. All four tetrahedral O atoms are bonded to the 4-coordinated Zn in parahopeite. In phosphophyllite and hopeite, only three of the O atoms are so bonded. Consequently, the vibrational spectra of parahopeite would be predicted to be different from that of hopeite. Fig. 2 shows that this is the case. The Raman spectra of parahopeite in the hydroxyl stretching region shows four bands centred at 3439, 3293, 3163 and 3027 cm−1 (Fig. 3). Four bands are observed in the infrared spectrum at 3451, 3311, 3143 and 3043 cm−1 . It is predicted that there are two non-equivalent water molecules in the unit cell of parahopeite. These water OH stretching vibrations will show in-phase and out-of-phase behaviour resulting in the prediction of four bands in the Raman and infrared spectrum. Spencerite differs from both hopeite and parahopeite in that phosphate has been replaced by OH units. A similar structure exists for spencerite as for parahopeite [31]. The atomic arrangement in spencerite consists of complex sheets of co-ordination octahedra and tetrahedra around Zn and P atoms connected by layers of water molecules. Four Raman
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Fig. 3. Infrared and Raman spectra of the hydroxyl stretching region of spencerite.
bands are observed at 3548, 3516, 3469 and 3146 cm−1 . Four infrared bands are observed at 3520, 3395, 3180 and 2996 cm−1 . Spencerite has one hydroxyl unit and consequently the higher wavenumber band (at 3548 cm−1 in the Raman spectrum and at 3520 cm−1 in the infrared spectrum) is assigned to the symmetric stretching vibration. The other three lower wavenumber bands are assigned to water stretching bands. The Raman spectrum of the hydroxyl stretching region of tarbuttite consists of a single sharp band centred at 3446 cm−1 and a bandwidth of 8.5 cm−1 (Fig. 4). The infrared spectrum of the hydroxyl stretching region shows a single intense band at 3428 cm−1 with a tail extending to lower wavenumbers. Scholzites have apparent stacking disorders. Thus, several polytypes can exist and one of these is parascholzite [20]. It is no doubt caused by variation in the moles of water of crystallisation [21].The Raman spectrum of scholzite in the hydroxyl stretching region shows four bands at 3437, 3343, 3283 and 3185 cm−1 (Fig. 5). Three bands were observed in the infrared spectrum of the hydroxyl stretching region at 3425, 3310 and 3204 cm−1 . Bands are observed in similar positions in the Raman and infrared spectrum of parascholzite.
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Fig. 5. Infrared and Raman spectra of the hydroxyl stretching region of scholzite.
Fig. 4. Infrared and Raman spectra of the hydroxyl stretching region of tarbuttite.
3.1. PO4 stretching vibrations
Infrared Intensity
The Raman and infrared spectra of hopeite in the PO4 stretching region are compared in Fig. 6. The infrared spectra consist of a complex band profile composed of overlapping bands whereas the Raman spectra show good band separation with clearly resolved bands. The most intense band in the Raman spectrum is observed at 940 cm−1 and is assigned to the ν1 PO4 symmetric stretching mode. A previous study reported a band at 941 cm−1 which is in excellent agreement with these results [32]. Other Raman bands are observed at 1150, 1059, 1000 and 995 cm−1 and are assigned to the ν3 PO4 antisymmetric stretching modes. In the infrared spectra component bands are curve resolved at 1137, 1096, 1059, 1019 and 995 cm−1 . These bands are assigned to the ν3 PO4 antisymmetric stretching modes. In addition two bands are observed at 945 and 922 cm−1 . these bands are ascribed to the ν1 symmetric stretching modes. It is difficult to find Raman data to compare with these results [26]. Bands were found for an exchanged zinc phosphate coating at 1113, 1098, 1010, 983 and 958 cm−1 . However, these bands differ from the band positions reported for hopeite in this work. The infrared and Raman spectra of the PO4 stretching region for parahopeite are shown in Fig. 7. Clearly, the Raman and infrared spectra of parahopeite is different from that of hopeite. This is related to the differences in the crystal
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Fig. 6. Infrared and Raman spectra of the PO4 stretching region of hopeite.
R.L. Frost / Spectrochimica Acta Part A 60 (2004) 1439–1445
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structures of hopeite and parahopeite. Hopeite is orthorhombic and crystallises in the space group Pnma = D2h 16 and is centrosymmetric with four formula units per unit cell. The structure of hopeite consists of alternating sheets of tetrahedral ZnO4 and octahedral ZnO2 (H2 O)4 octahedra. The phosphate anions occupy 8d with C1 site symmetry and connect the ZnO4 tetrahedra and the ZnO2 (H2 O)4 octahedra. The water molecules divide equally between the 4c position with site symmetry Cs and the 8d position. The zinc cations divide between the tetrahedral 8d and the octahedra 4c position. The crystal structure of parahopeite, Zn3 (PO4 )2 ·4H2 O, is similar to those of phosphophyllite and hopeite in that one of the two Zn atoms is 6-coordinated and the other is 4-coordinated. Parahopeite differs from hopeite because one of the P–O tetrahedral O atoms is bonded to both the 6- and 4-coordinated cations. All four tetrahedral O atoms are bonded to the 4-coordinated Zn in parahopeite. In phosphophyllite and hopeite, only three of the O atoms are so bonded. Consequently the vibrational spectra of parahopeite would be predicted to be different from that of hopeite. The Raman spectrum of parahopeite shows a band centred upon 959 cm−1 which is assigned to the symmetric stretching mode. Three antisymmetric stretching bands are observed at 1053, 1033 and 1003 cm−1 . The infrared spectrum of parahopeite shows two bands at 951 and 919 cm−1 . Three bands are observed at 1106, 1045 and 1002 cm−1 . To our
Fig. 8. Infrared and Raman spectra of the PO4 stretching region of spencerite.
knowledge no vibrational spectra of parahopeite have been reported. Castagnola and Dutta showed some Raman spectra of ion exchanged zinc phosphates and some resemblance of the spectra of the films corresponds to the spectrum of parahopeite [26]. The Raman and infrared spectra of the PO4 stretching region of spencerite are shown in Fig. 8. The Raman spectrum shows an intense band 952 cm−1 , attributed to the ν1 PO4 symmetric stretching mode, and low intensity bands at 1095, 1019, 999 and 989 cm−1 , attributed to the ν3 PO4 antisymmetric stretching modes. In the infrared spectrum, the equivalent bands are found at 940 cm−1 (ν1 ) and 1048, 1010 and 987 cm−1 (ν3 ). An infrared band is observed at 842 cm−1 and may be attributed to a water librational mode. No band is observed in this position in the Raman spectrum. The Raman and infrared spectra of the PO4 stretching region of tarbuttite are shown in Fig. 9. The Raman spectrum of tarbuttite shows an intense band centred at 965 cm−1 assigned to the ν1 symmetric stretching mode and at 1069, 1051 and 1011 cm−1 assigned to the ν3 antisymmetric stretching modes. The infrared spectrum of tarbuttite shows two bands at 954 and 902 cm−1 and also at 1088, 1056 and 990 cm−1 . The Raman and infrared spectra of the PO4 stretching region of scholzite are shown in Fig. 10. The Raman spectrum of scholzite is the most complex of any of the spectra
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Fig. 9. Infrared and Raman spectra of the PO4 stretching region of tarbuttite.
of the zinc phosphate minerals. Scholzites have apparent stacking disorders. Thus, several polytypes can exist and one of these is parascholzite [20]. It is no doubt caused by variation in the moles of water of crystallisation [21]. The Raman spectrum of scholzite shows an intense band at 1000 cm−1 with other bands at 1171, 1115, 1088, 1053 and 1026 cm−1 . All of these bands are assigned to the ν3 antisymmetric stretching modes of the PO4 . A band is observed at 923 cm−1 with a second band at 935 cm−1 . These bands are assigned to the ν1 PO4 symmetric stretching modes. The pattern of the infrared spectrum of scholzite shows a complex profile of overlapping bands. The infrared spectrum shows bands at 1107, 1047 and 999 cm−1 assigned to the ν3 PO4 antisymmetric stretching modes and at 956 and 929 cm−1 assigned to the ν1 PO4 symmetric stretching modes. The Raman spectrum of parascholzite displays an intense Raman band at 925 cm−1 attributed to the ν1 PO4 symmetric stretching mode and bands at 1170, 1115, 1086 and 999 cm−1 assigned to the ν3 antisymmetric stretching modes. What is clearly evident is that the Raman spectra of these minerals characterises the minerals and that the Raman spectrum is more definitive than the infrared spectrum. This observation has implications for the study of zinc phosphate films on metallic surfaces in that Raman spectroscopy may be more appropriate for the study of the evolution of the films.
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Fig. 10. Infrared and Raman spectra of the PO4 stretching region of scholzite.
4. Conclusion A combination of Raman and infrared spectroscopy has been used to characterise the zinc phosphate minerals including hopeite, parahopeite, spencerite, tarbuttite, scholzite and parascholzite. Each mineral may be identified by its own characteristic Raman spectrum. The use of Raman spectroscopy has serious implications for the study of zinc phosphate coatings, secondary mineral formation involving zinc phosphates and the use of zinc phosphates in dental applications. In secondary mineral formation from bones, it is likely that the starting material is hydroxyapatite which is converted to hopeite and later to tarbuttite and finally to scholzite or one of its polytypes.
Acknowledgements The financial and infra-structure support of the Queensland University of Technology Inorganic Materials Research Program of the School of Physical and Chemical Sciences is gratefully acknowledged. The Australian Research Council (ARC) is thanked for funding. Mr. M. Weier is thanked for collecting some of the spectral data. Professor Alan Pring
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of the South Australian Museum is thanked for a loan of the phosphate minerals. References [1] Y. Arnaud, E. Sahakian, J. Lenoir, A. Roche, J.C. Charbonnier, Appl. Surf. Sci. 32 (1988) 296. [2] Y. Arnaud, E. Sahakian, M. Romand, J.C. Charbonnier, Appl. Surf. Sci. 32 (1988) 281. [3] M. Clara, F. Magalhaes, J.P. De Jesus, P.A. Williams, Mineral. Mag. 50 (1986) 33. [4] S. Crisp, I.K. O’neill, H.J. Prosser, B. Stuart, A.D. Wilson, J. Dental Res. 57 (1978) 245. [5] B. Gade, A. Heindl, H. Westermann, H. Pollmann, Applied mineralogy, in: Research, Economy, Technology, Ecology and Culture, Proceedings of the 6th International Congress on Applied Mineralogy, vol. 2, Goettingen, Germany, 17–19 July 2000, p. 539. [6] W. Martens, R.L. Frost, Am. Mineralogist 88 (2003) 37. [7] R.L. Frost, W. Martens, P.A. Williams, J.T. Kloprogge, Mineral. Mag. 66 (2002) 1063. [8] R.L. Frost, T. Kloprogge, P.A. Williams, W. Martens, T.E. Johnson, P. Leverett, Spectrochim. Acta, Part A: Mol. Biomol. Spectrosc. 58A (2002) 2861. [9] R.J. Hill, J.E. Johnson, J.B. Jones, Neues Jahrbuch fuer Mineralogie, Monatshefte (1973) 1. [10] R.J. Hill, J.B. Jones, Am. Mineralogist 61 (1976) 987. [11] I. Kumbasar, J.J. Finney, Mineral. Mag. J. Mineral. Soc. 36 (1876–1968) 621.
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[12] A. Ledoux, T.L. Walker, A.C. Wheatley, Mineral. Mag. 18 (1917) 101. [13] L. Fanfani, P.F. Zanazzi, Period. Mineral. 34 (1965) 539. [14] J.J. Finney, Am. Mineralogist 51 (1966) 1218. [15] T.L. Walker, J. Wash, Acad. Sci. 7 (1917) 456. [16] T.L. Walker, Mineral. Mag. 18 (1916) 76. [17] H. Strunz, Fortschr. Mineral. 27 (1950) 31. [18] H. Strunz, C. Tennyson, Z. Krist. 107 (1956) 318. [19] B.D. Sturman, R.C. Rouse, P.J. Dunn, Am. Mineralogist 66 (1981) 843. [20] K. Taxer, Z. Kristallogr. 198 (1992) 239. [21] K. Taxer, H. Bartl, Z. Kristallogr. 212 (1997) 197. [22] S.V. Gevork’yan, A.S. Povarennykh, Mineral. Z. 2 (1980) 29. [23] S.V. Gevork’yan, A.S. Povarennykh, Konstitutsiya i Svoistva Mineralov 9 (1975) 73. [24] C. Paluszkiewicz, A. Stoch, A. Brozek, E. Dlugon, J. Mol. Struct. 293 (1993) 291. [25] N. Satoh, Hyomen Gijutsu 40 (1989) 933. [26] M.J. Castagnola, P.K. Dutta, Microporous Mesoporous Mater. 42 (2001) 235. [27] R.L. Frost, W.N. Martens, T. Kloprogge, P.A. Williams, Neues Jahrbuch fuer Mineralogie, Monatshefte (2002) 481. [28] R.L. Frost, P.A. Williams, W. Martens, J.T. Kloprogge, P. Leverett, J. Raman Spectrosc. 33 (2002) 260. [29] O. Pawlig, V. Schellenschlager, H.D. Lutz, R. Trettin, Spectrochim. Acta, Part A: Mol. Biomol. Spectrosc. 57A (2001) 581. [30] A. Whitaker, Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. B31 (1975) 2026. [31] L. Fanfani, A. Nunzi, P.F. Zanazzi, Mineral. Mag. 38 (1972) 687. [32] O. Pawlig, R. Trettin, Mater. Res. Bull. 34 (1999) 1959.
An infrared and Raman spectroscopic study of natural zinc phosphates Ray L. Frost∗ Inorganic Materials Research Program, School of Physical and Chemical Sciences, Queensland University of Technology, GPO Box 2434, Brisbane Qld 4001, Australia Received 17 July 2003; received in revised form 17 July 2003; accepted 20 August 2003
Abstract Zinc phosphates are important in the study of the phosphatisation of metals. Raman spectroscopy in combination with infrared spectroscopy has been used to characterise the zinc phosphate minerals. The minerals may be characterised by the patterns of the hydroxyl stretching vibrations in both the Raman and infrared spectra. Spencerite is characterised by a sharp Raman band at 3516 cm−1 and tarbuttite by a single band at 3446 cm−1 . The patterns of the Raman spectra of the hydroxyl stretching region of hopeite and parahopeite are different in line with their differing crystal structures. The Raman spectrum of the PO4 stretching region shows better band separated peaks than the infrared spectra which consist of a complex set of overlapping bands. The position of the PO4 symmetric stretching mode can be used to identify the zinc phosphate mineral. It is apparent that Raman spectroscopy lends itself to the fundamental study of the evolution of zinc phosphate films. © 2003 Elsevier B.V. All rights reserved. Keywords: Hopeite; Parahopeite; Spencerite; Scholzite; Tarbuttite; Phosphate; Raman spectroscopy
1. Introduction The study of zinc phosphates is important for a number of reasons. Firstly, because the phosphatisation of metals can result in the formation of zinc and other metal phosphates [1–3]. Secondly, the use of these types of minerals has application in dental cements [4]. Thirdly, the formation of such minerals is important for the fundamental understanding of the oxidised zones of metal deposits [3]. Further many of these minerals may form in landfills and their stability means that there presence is permanent [5]. It is also probable that many of the zinc and calcium phosphate minerals also occur when replacing human and animal bones. Whereas the copper phosphate minerals are many and common [6–8], the phosphates of the divalent cations are not. The zinc phosphate minerals are confined to the dimorphs of hopeite [9,10] and parahopeite (Zn3 (PO4 )2 ·4H2 O) [11,12], tarbuttite (Zn2 PO4 (OH)) [13,14], spencerite (Zn2 PO4 (OH)·1.5H2 O) [15,16], together with the mixed
∗
Tel.: +61-7-3864-2407; fax: +61-7-3864-1804. E-mail address: [email protected] (R.L. Frost).
1386-1425/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2003.08.009
cationic minerals scholzite (CaZn2 (PO4 )2 ·3H2 O) [17,18], and parascholzite (CaZn2 (PO4 )2 ·2H2 O) [19–21]. Infrared spectroscopy has proven most useful for the study of these phosphates particularly for phosphate coatings [4,22–24]. Raman spectroscopy has been used to a limited extent [25,26]. In aqueous systems, Raman spectra of phosphate oxyanions show a symmetric stretching mode (ν1 ) at 938 cm−1 , the antisymmetric stretching mode (ν3 ) at 1017 cm−1 , the symmetric bending mode (ν2 ) at 420 cm−1 and the ν4 mode at 567 cm−1 [7,27,28]. The Raman spectroscopy of some phosphate minerals have been studied [7,8,27]. The vibrational spectra of related minerals such as pseudo-malachite and reichenbachite are different in line with the differences in the crystal structures. The pseudo-malachite vibrational spectrum consists of ν1 at 953 cm−1 , ν2 at 422 and 450 cm−1 , ν3 at 1025 and 1096 cm−1 and ν4 at 482, 530, 555 and 615 cm−1 . Libethenite vibrational modes occur at 960 cm−1 (ν1 ), 445 cm−1 (ν2 ), 1050 cm−1 (ν3 ) and 480, 522, 555, 618 and 637 cm−1 (ν4 ). Cornetite vibrational modes occur at 960 cm−1 (ν1 ), 415 and 464 cm−1 (ν2 ), 1000, 1015 and 1070 cm−1 (ν3 ) and 510, 527, 558, 582, 623 and 647 cm−1 (ν4 ). Fundamentally, there have been no comprehensive
1440
R.L. Frost / Spectrochimica Acta Part A 60 (2004) 1439–1445
studies of the vibrational spectroscopy of the zinc phosphate minerals.
with squared regression coefficient of R2 greater than 0.995.
2. Experimental
3. Results and discussion
2.1. Minerals
The Raman and infrared spectra of hopeite are shown in Fig. 1. For hopeite, two Raman bands in the hydroxyl stretching region are observed at 3456 and 3247 cm−1 . No Raman bands of the hydroxyl stretching region of hopeite have been reported to date. In contrast four infrared bands are observed at 3542, 3473, 3338 and 3149 cm−1 . Pawlig et al. reported the infrared spectrum of hopeite and its deuteron-analogs [29]. They reported infrared bands at 3537, 3410, 3263 and 3181 cm−1 [29]. The structure of hopeite consists of ZnO2 (H2 O)4 octahedra, ZnO4 tetrahedra, and PO4 tetrahedra, none of which are regular; these polyhedra share corners and edges [30]. It is likely that the two water molecules are non-equivalent, thus giving rise to in-phase and out-of-phase behaviour. Hence, two Raman bands and two infrared bands would be expected. The crystal structure of parahopeite, Zn3 (PO4 )2 ·4H2 O, is similar to those of phosphophyllite and hopeite in that one of the two Zn atoms is 6-coordinated and the other is
G3566: Hopeite, Broken Hill, Zambia. G14170: Parahopeite, Reaphook Hill, SA. G14784: Tarbutite, Broken Hill, Zambia. G5847: Spencerite, Salmo British Columbia, Canada. G8448: Scholzite, Reaphook Hill, SA. Parascholzite was supplied by the Mineral Research Company. 2.2. Raman microprobe spectroscopy The crystals of the zinc phosphate minerals were placed and oriented on a polished metal surface on the stage of an Olympus BHSM microscope, which is equipped with 10× and 50× objectives. The microscope is part of a Renishaw 1000 Raman microscope system, which also includes a monochromator, a notch filter system and a thermo-electrically cooled charge coupled device (CCD) detector. Raman spectra were excited by a Spectra-Physics model 127 He–Ne laser (633 nm) and acquired at a nominal resolution of 2 cm−1 in the range between 100 and 4000 cm−1 . The crystals were oriented to provide maximum intensity. All crystal orientations were used to obtain the spectra. Power at the sample was measured as 1 mW. The incident radiation was scrambled to avoid polarisation effects.
Infrared Intensity
The following minerals were obtained from South Australian Museum and have been characterised. The minerals were checked for phase composition using X-ray diffraction and for chemical composition using the electron probe.
Hopeite
2.3. Infrared spectroscopy Infrared spectra were obtained using a Nicolet Nexus 870 FTIR spectrometer with a smart endurance single bounce diamond ATR cell. Spectra over the 4000–525 cm−1 range were obtained by the co-addition of 64 scans with a resolution of 4 cm−1 and a mirror velocity of 0.6329 cm s−1 . Spectracalc software package GRAMS. Band component analysis was undertaken using the Jandel ‘Peakfit’ software package, which enabled the type of fitting function to be selected and allows specific parameters to be fixed or varied accordingly. Band fitting was done using a Gauss–Lorentz cross-product function with the minimum number of component bands used for the fitting process. The Gauss–Lorentz ratio was maintained at values greater than 0.7 and fitting was undertaken until reproducible results were obtained
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Fig. 1. Infrared and Raman spectra of the hydroxyl stretching region of hopeite.
R.L. Frost / Spectrochimica Acta Part A 60 (2004) 1439–1445
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Fig. 2. Infrared and Raman spectra of the hydroxyl stretching region of parahopeite.
4-coordinated. Parahopeite differs from the other two minerals because one of the P–O tetrahedral O atoms is bonded to both the 6- and 4-coordinated cations. All four tetrahedral O atoms are bonded to the 4-coordinated Zn in parahopeite. In phosphophyllite and hopeite, only three of the O atoms are so bonded. Consequently, the vibrational spectra of parahopeite would be predicted to be different from that of hopeite. Fig. 2 shows that this is the case. The Raman spectra of parahopeite in the hydroxyl stretching region shows four bands centred at 3439, 3293, 3163 and 3027 cm−1 (Fig. 3). Four bands are observed in the infrared spectrum at 3451, 3311, 3143 and 3043 cm−1 . It is predicted that there are two non-equivalent water molecules in the unit cell of parahopeite. These water OH stretching vibrations will show in-phase and out-of-phase behaviour resulting in the prediction of four bands in the Raman and infrared spectrum. Spencerite differs from both hopeite and parahopeite in that phosphate has been replaced by OH units. A similar structure exists for spencerite as for parahopeite [31]. The atomic arrangement in spencerite consists of complex sheets of co-ordination octahedra and tetrahedra around Zn and P atoms connected by layers of water molecules. Four Raman
3700
3500
3300
3100
2900
Wavenumber/cm
2700
2500
-1
Fig. 3. Infrared and Raman spectra of the hydroxyl stretching region of spencerite.
bands are observed at 3548, 3516, 3469 and 3146 cm−1 . Four infrared bands are observed at 3520, 3395, 3180 and 2996 cm−1 . Spencerite has one hydroxyl unit and consequently the higher wavenumber band (at 3548 cm−1 in the Raman spectrum and at 3520 cm−1 in the infrared spectrum) is assigned to the symmetric stretching vibration. The other three lower wavenumber bands are assigned to water stretching bands. The Raman spectrum of the hydroxyl stretching region of tarbuttite consists of a single sharp band centred at 3446 cm−1 and a bandwidth of 8.5 cm−1 (Fig. 4). The infrared spectrum of the hydroxyl stretching region shows a single intense band at 3428 cm−1 with a tail extending to lower wavenumbers. Scholzites have apparent stacking disorders. Thus, several polytypes can exist and one of these is parascholzite [20]. It is no doubt caused by variation in the moles of water of crystallisation [21].The Raman spectrum of scholzite in the hydroxyl stretching region shows four bands at 3437, 3343, 3283 and 3185 cm−1 (Fig. 5). Three bands were observed in the infrared spectrum of the hydroxyl stretching region at 3425, 3310 and 3204 cm−1 . Bands are observed in similar positions in the Raman and infrared spectrum of parascholzite.
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Infrared
Intensity
Intensity
Infrared
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Tarbutite
Raman
Raman 3700
3700
3500
3300
3100
3500
Wavenumber/cm
3100
2900
2700
2500
Wavenumber/cm-1
2900
-1
3300
Fig. 5. Infrared and Raman spectra of the hydroxyl stretching region of scholzite.
Fig. 4. Infrared and Raman spectra of the hydroxyl stretching region of tarbuttite.
3.1. PO4 stretching vibrations
Infrared Intensity
The Raman and infrared spectra of hopeite in the PO4 stretching region are compared in Fig. 6. The infrared spectra consist of a complex band profile composed of overlapping bands whereas the Raman spectra show good band separation with clearly resolved bands. The most intense band in the Raman spectrum is observed at 940 cm−1 and is assigned to the ν1 PO4 symmetric stretching mode. A previous study reported a band at 941 cm−1 which is in excellent agreement with these results [32]. Other Raman bands are observed at 1150, 1059, 1000 and 995 cm−1 and are assigned to the ν3 PO4 antisymmetric stretching modes. In the infrared spectra component bands are curve resolved at 1137, 1096, 1059, 1019 and 995 cm−1 . These bands are assigned to the ν3 PO4 antisymmetric stretching modes. In addition two bands are observed at 945 and 922 cm−1 . these bands are ascribed to the ν1 symmetric stretching modes. It is difficult to find Raman data to compare with these results [26]. Bands were found for an exchanged zinc phosphate coating at 1113, 1098, 1010, 983 and 958 cm−1 . However, these bands differ from the band positions reported for hopeite in this work. The infrared and Raman spectra of the PO4 stretching region for parahopeite are shown in Fig. 7. Clearly, the Raman and infrared spectra of parahopeite is different from that of hopeite. This is related to the differences in the crystal
Hopeite
Raman 1200
1100
1000
900
Wavenumber/cm
800
700
-1
Fig. 6. Infrared and Raman spectra of the PO4 stretching region of hopeite.
R.L. Frost / Spectrochimica Acta Part A 60 (2004) 1439–1445
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Infrared
Intensity
Intensity
Infrared
Parahopeite
Spencerite
Raman
Raman 1200
1100
1000
900
800
700
-1
Wavenumber/cm
1200
1100
1000
900
800
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Wavenumber/cm Fig. 7. Infrared and Raman spectra of the PO4 stretching region of parahopeite.
structures of hopeite and parahopeite. Hopeite is orthorhombic and crystallises in the space group Pnma = D2h 16 and is centrosymmetric with four formula units per unit cell. The structure of hopeite consists of alternating sheets of tetrahedral ZnO4 and octahedral ZnO2 (H2 O)4 octahedra. The phosphate anions occupy 8d with C1 site symmetry and connect the ZnO4 tetrahedra and the ZnO2 (H2 O)4 octahedra. The water molecules divide equally between the 4c position with site symmetry Cs and the 8d position. The zinc cations divide between the tetrahedral 8d and the octahedra 4c position. The crystal structure of parahopeite, Zn3 (PO4 )2 ·4H2 O, is similar to those of phosphophyllite and hopeite in that one of the two Zn atoms is 6-coordinated and the other is 4-coordinated. Parahopeite differs from hopeite because one of the P–O tetrahedral O atoms is bonded to both the 6- and 4-coordinated cations. All four tetrahedral O atoms are bonded to the 4-coordinated Zn in parahopeite. In phosphophyllite and hopeite, only three of the O atoms are so bonded. Consequently the vibrational spectra of parahopeite would be predicted to be different from that of hopeite. The Raman spectrum of parahopeite shows a band centred upon 959 cm−1 which is assigned to the symmetric stretching mode. Three antisymmetric stretching bands are observed at 1053, 1033 and 1003 cm−1 . The infrared spectrum of parahopeite shows two bands at 951 and 919 cm−1 . Three bands are observed at 1106, 1045 and 1002 cm−1 . To our
Fig. 8. Infrared and Raman spectra of the PO4 stretching region of spencerite.
knowledge no vibrational spectra of parahopeite have been reported. Castagnola and Dutta showed some Raman spectra of ion exchanged zinc phosphates and some resemblance of the spectra of the films corresponds to the spectrum of parahopeite [26]. The Raman and infrared spectra of the PO4 stretching region of spencerite are shown in Fig. 8. The Raman spectrum shows an intense band 952 cm−1 , attributed to the ν1 PO4 symmetric stretching mode, and low intensity bands at 1095, 1019, 999 and 989 cm−1 , attributed to the ν3 PO4 antisymmetric stretching modes. In the infrared spectrum, the equivalent bands are found at 940 cm−1 (ν1 ) and 1048, 1010 and 987 cm−1 (ν3 ). An infrared band is observed at 842 cm−1 and may be attributed to a water librational mode. No band is observed in this position in the Raman spectrum. The Raman and infrared spectra of the PO4 stretching region of tarbuttite are shown in Fig. 9. The Raman spectrum of tarbuttite shows an intense band centred at 965 cm−1 assigned to the ν1 symmetric stretching mode and at 1069, 1051 and 1011 cm−1 assigned to the ν3 antisymmetric stretching modes. The infrared spectrum of tarbuttite shows two bands at 954 and 902 cm−1 and also at 1088, 1056 and 990 cm−1 . The Raman and infrared spectra of the PO4 stretching region of scholzite are shown in Fig. 10. The Raman spectrum of scholzite is the most complex of any of the spectra
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Infrared
Intensity
Intensity
Infrared
Tarbutite
0.015
Scholzite
Raman .
Raman 1200
1100
1000
900
800
700
-1
Wavenumber/cm
1200
1100
1000
900
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Fig. 9. Infrared and Raman spectra of the PO4 stretching region of tarbuttite.
of the zinc phosphate minerals. Scholzites have apparent stacking disorders. Thus, several polytypes can exist and one of these is parascholzite [20]. It is no doubt caused by variation in the moles of water of crystallisation [21]. The Raman spectrum of scholzite shows an intense band at 1000 cm−1 with other bands at 1171, 1115, 1088, 1053 and 1026 cm−1 . All of these bands are assigned to the ν3 antisymmetric stretching modes of the PO4 . A band is observed at 923 cm−1 with a second band at 935 cm−1 . These bands are assigned to the ν1 PO4 symmetric stretching modes. The pattern of the infrared spectrum of scholzite shows a complex profile of overlapping bands. The infrared spectrum shows bands at 1107, 1047 and 999 cm−1 assigned to the ν3 PO4 antisymmetric stretching modes and at 956 and 929 cm−1 assigned to the ν1 PO4 symmetric stretching modes. The Raman spectrum of parascholzite displays an intense Raman band at 925 cm−1 attributed to the ν1 PO4 symmetric stretching mode and bands at 1170, 1115, 1086 and 999 cm−1 assigned to the ν3 antisymmetric stretching modes. What is clearly evident is that the Raman spectra of these minerals characterises the minerals and that the Raman spectrum is more definitive than the infrared spectrum. This observation has implications for the study of zinc phosphate films on metallic surfaces in that Raman spectroscopy may be more appropriate for the study of the evolution of the films.
Wavenumber/cm
Fig. 10. Infrared and Raman spectra of the PO4 stretching region of scholzite.
4. Conclusion A combination of Raman and infrared spectroscopy has been used to characterise the zinc phosphate minerals including hopeite, parahopeite, spencerite, tarbuttite, scholzite and parascholzite. Each mineral may be identified by its own characteristic Raman spectrum. The use of Raman spectroscopy has serious implications for the study of zinc phosphate coatings, secondary mineral formation involving zinc phosphates and the use of zinc phosphates in dental applications. In secondary mineral formation from bones, it is likely that the starting material is hydroxyapatite which is converted to hopeite and later to tarbuttite and finally to scholzite or one of its polytypes.
Acknowledgements The financial and infra-structure support of the Queensland University of Technology Inorganic Materials Research Program of the School of Physical and Chemical Sciences is gratefully acknowledged. The Australian Research Council (ARC) is thanked for funding. Mr. M. Weier is thanked for collecting some of the spectral data. Professor Alan Pring
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of the South Australian Museum is thanked for a loan of the phosphate minerals. References [1] Y. Arnaud, E. Sahakian, J. Lenoir, A. Roche, J.C. Charbonnier, Appl. Surf. Sci. 32 (1988) 296. [2] Y. Arnaud, E. Sahakian, M. Romand, J.C. Charbonnier, Appl. Surf. Sci. 32 (1988) 281. [3] M. Clara, F. Magalhaes, J.P. De Jesus, P.A. Williams, Mineral. Mag. 50 (1986) 33. [4] S. Crisp, I.K. O’neill, H.J. Prosser, B. Stuart, A.D. Wilson, J. Dental Res. 57 (1978) 245. [5] B. Gade, A. Heindl, H. Westermann, H. Pollmann, Applied mineralogy, in: Research, Economy, Technology, Ecology and Culture, Proceedings of the 6th International Congress on Applied Mineralogy, vol. 2, Goettingen, Germany, 17–19 July 2000, p. 539. [6] W. Martens, R.L. Frost, Am. Mineralogist 88 (2003) 37. [7] R.L. Frost, W. Martens, P.A. Williams, J.T. Kloprogge, Mineral. Mag. 66 (2002) 1063. [8] R.L. Frost, T. Kloprogge, P.A. Williams, W. Martens, T.E. Johnson, P. Leverett, Spectrochim. Acta, Part A: Mol. Biomol. Spectrosc. 58A (2002) 2861. [9] R.J. Hill, J.E. Johnson, J.B. Jones, Neues Jahrbuch fuer Mineralogie, Monatshefte (1973) 1. [10] R.J. Hill, J.B. Jones, Am. Mineralogist 61 (1976) 987. [11] I. Kumbasar, J.J. Finney, Mineral. Mag. J. Mineral. Soc. 36 (1876–1968) 621.
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