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Raman spectroscopy of selected tsumcorite Pb(Zn,Fe3+)2(AsO4)2(OH,H2O) minerals—Implications for arsenate accumulation
Spectrochimica Acta Part A 86 (2012) 224–230
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Raman spectroscopy of selected tsumcorite Pb(Zn,Fe3+ )2 (AsO4 )2 (OH,H2 O) minerals—Implications for arsenate accumulation Ray L. Frost ∗ , Yunfei Xi Chemistry Discipline, Faculty of Science and Technology, Queensland University of Technology, GPO Box 2434, Brisbane, Queensland 4001, Australia
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Article history: Received 15 August 2011 Received in revised form 3 October 2011 Accepted 14 October 2011 Keywords: Raman spectroscopy Gartrellite Tsumcorite Thometzekite Arsenate Sulphate
a b s t r a c t The presence of arsenic in the environment is a hazard. The accumulation of arsenate by a range of cations in the formation of minerals provides a mechanism for the accumulation of arsenate. The formation of the tsumcorite minerals is an example of a series of minerals which accumulate arsenate. There are about twelve examples in this mineral group. Raman spectroscopy offers a method for the analysis of these minerals. The structure of selected tsumcorite minerals with arsenate and sulphate anions were analysed by Raman spectroscopy. Isomorphic substitution of sulphate for arsenate is observed for gartrellite and thometzekite. A comparison is made with the sulphate bearing mineral natrochalcite. The position of the hydroxyl and water stretching vibrations are related to the strength of the hydrogen bond formed between the OH unit and the AsO4 3− anion. Characteristic Raman spectra of the minerals enable the assignment of the bands to specific vibrational modes. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The tsumcorite mineral group are a set of minerals which in their formation accumulate arsenic as arsenate. Tsumcorite was named after the TSUMeb CORporation mine at Tsumeb, in Namibia, in recognition of the Corporation’s support for mineralogical investigations of the orebody at its Mineral Research Laboratory. The tsumcorite mineral group are well known [1–5]. Many of the minerals were found in the oxidised zones of the famous Tsumeb ore deposit [6]. At the Puttapa Mine in Australia it occurs with adamite, mimetite, smithsonite, goethite and quartz. At the Kintore Open Cut, Broken Hill, Australia it occurs with segnitite, beudantite, carminite and mawbyite. Many new minerals have been discovered in these oxidised zones in mineral deposits in other parts of Australia [7–11]. Many of these minerals are based upon arsenates of lead and zinc in combination with other cations. The tsumcorite group of minerals are a mineral group based upon monoclinic and triclinic arsenates, phosphates, vanadates and sulphates of the general formulae (M1)(M2)2 (XO4 )2 (OH,H2 O)2 where M1 is Pb, Ca or Na, M2 is Cu, Zn, Fe3+ , Co or Mn and X is As, P, V and/or S. The minerals gartrellite Pb[(Cu,Zn)(Fe3+ , Zn, Cu)] (AsO4 )(OH,H2 O)2 , helmutwinklerite Pb(Zn,Cu)2 (AsO4 )2 2H2 O and thometzekite [12] are triclinic. The minerals ferrilotharmeyerite [13] Ca(Fe3+ ,Zn)2 (AsO4 )2 (OH,H2 O)2 , lotharmeyerite Ca(Mn3+ ,Zn)2 (AsO4 )2 (OH,H2 O)2 , maw-
∗ 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.2011.10.028
byite [10] Pb(Fe3+ ,Zn)2 (AsO4 )2 (OH,H2 O)2 , mounanaite 3+ Pb(Fe )2(VO4 )2 (OH)2 , natrochalcite [14] NaCu2 (SO4 )2 (OH,H2 O)2 and tsumcorite [15] Pb(Zn,Fe3+ )2 (AsO4 )2 (OH,H2 O) are monoclinic [15]. Tsumcorite belongs to the monoclinic crystal class 2/m. This means that it has a twofold axis of symmetry along the b axis and a mirror plane perpendicular to this, in the plane containing the a and c axes. The a and c axes are inclined to each other at angle ˇ = 115.3◦ . The unit cell parameters are ˚ b = 6.326–6.329 A˚ and c = 7.577–7.583 A. ˚ There a = 9.124–9.131 A, are two formula units per unit cell (Z = 2), and the space group is C2/m This means the cell is a C-face centered lattice, with lattice points in the center of the C face as well as at the corners of the cell. There are some problems associated with writing the mineral formula in that the formula may change as a function of the degree of solid solution formation and the amount of isomorphic substitution. Both anion and cation substitution may occur. Sulphate, phosphate and carbonate may replace the arsenate. For example it is quite comprehensible that a formula such as PbCu(Fe3+ ,Cu)(AsO4 )2 (OH,H2 O)2 is found. Variation in mineral composition is expected for gartrellites from different origins. The formula of gartrellite may be written as Pb[(Cu,Fe2+ )(Fe3+ , Zn, Cu)] (AsO4 )(CO3 ,H2 O)2 . For example the gartrellite found at Ashburton Downs, Western Australia has a calculated formula of PbCu1.5 Fe2+ 0.5 As1.5 (SO4 )0.5 (CO3 )0.5 (H2 O)0.2 . Of course Raman spectroscopy will readily determine the presence of carbonate in the mineral. The presence or absence of two moles of water is the determining factor as to whether the mineral is triclinic or monoclinic.
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Table 1 Table of the tsumcorite minerals studied. Mineral
Type number
Formulae
Origin
Gartrellite Ferrilotharmeyerite Natrochalcite Tsumcorite Thometzekite
M39987 M36822 M32894 M37949 M43672
Pb[(Cu,Zn)(Fe3+ , Zn, Cu)](AsO4 )2 (OH,H2 O) Ca(Fe3+ ,Zn)2 (AsO4 )2 (OH,H2 O)2 NaCu2 (SO4 )2 (OH,H2 O)2 Pb(Zn,Fe3+ )2 (AsO4 )2 (OH,H2 O) Pb(Cu,Zn)2 (AsO4 )2 2H2 O
Anticline deposit, Ashburton Downs, Western Australia Tsumeb, Namibia Chuquicamata, Chile Tsumeb, Namibia Tsumeb, Namibia
Tsumcorite crystallizes in the monoclinic space group C2/m with a 9.124 (4), b 6.329 (2), c 7.577 (2), ˇ = 115◦ 17 (2) , Z = 2 [15]. Crystal symmetry is either triclinic in the case of an ordered occupation of two cationic sites, triclinic due to ordering of the H bonds in the case of species with 2 water molecules per formula unit, or monoclinic in the other cases. Crystals of ferrilotharmeyerite, tsumcorite, thometzekite (sulfatian), and mounanaite have monoclinic symmetry, space group C2/m. The triclinic members of the tsumcorite group are gartrellite, zincian gartrellite, phosphogartrellite, helmutwinklerite, and probably (sulphate-free) thometzekite; the space group is P/1, with a pronounced monoclinic C-centered pseudocell. The triclinic distortion is caused by an ordered arrangement of Fe[6]O6 octahedra and tetragonal bi-pyramidal Cu[4+2]O6 polyhedra [13]. Of course Raman spectroscopy will readily determine the presence of carbonate in the mineral. The presence or absence of two moles of water is the determining factor in whether the mineral is triclinic or not. It has been shown that crystals of ferrilotharmeyerite, tsumcorite, thometzekite (sulfatian), and mounanaite have monoclinic symmetry, space group C2/m [13]. The triclinic members of the tsumcorite group have the space group is P1, with a pronounced monoclinic C-centered pseudocell [13]. The tsumcorite minerals are often formed in the oxidised zones of arsenic bearing Pb–Zn deposits. The particular mineral formed depends upon the composition of the polymetallic ore deposit. The minerals are of a rare nature. Complex solution chemistry involving mixtures of the cations of lead, zinc, and ferric iron may result in the formation of the tsumcorite group of minerals. The type of mineral formed is a function of concentration, pH, temperature and the available anion present in the mother solution. The complex set of variable requires a multidimensional phase diagram [16]. Raman spectroscopy has proven an excellent technique for the study of oxyanions in both solution and in secondary mineral formation [17–26]. In this work we extend our studies to the arsenates of the tsumcorite mineral group. 2. Experimental 2.1. Minerals The tsumcorite minerals were obtained from Museum Victoria. The selected minerals and their museum number and place of origin are reported in Table 1. Details of the tsumcorite minerals have been published (page 207) [27]. 2.2. Raman spectroscopy Crystals of the individual members of the tsumcorite mineral group 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. 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 128 scans with a resolution of 4 cm−1 and a mirror velocity of 0.6329 cm/s. Spectra were co-added to improve the signal to noise ratio. 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. Arsenate vibrations According to Myneni et al. [28,29] and Nakamoto [30], (AsO4 )3− is a tetrahedral unit, which exhibits four fundamental vibrations: the Raman active 1 symmetric stretching vibration (A1 ) at 818 cm−1 ; the Raman active doubly degenerate 2 symmetric bending vibration (E) observed at 350 cm−1 , the infrared and Raman active triply degenerate 3 antisymmetric stretching vibration (F2 ) found around 786 cm−1 , and the infrared and Raman active triply degenerate 4 bending vibration (F2 ) observed at 405 cm−1 . Protonation, metal complexation, and/or adsorption on a mineral surface will cause the change in (AsO4 )3− symmetry from Td to lower symmetries, such as C3v , C2v or even C1 . This loss of degeneracy causes splitting of degenerate vibrations of AsO4 3− and the shifting of the As–OH stretching vibrations to different wavenumbers. Sulphates as with other oxyanions lend themselves to analysis by Raman spectroscopy [31]. In aqueous systems, the sulphate anion is of Td symmetry and is characterised by Raman bands at 981 cm−1 (1 ), 451 cm−1 (2 ), 1104 cm−1 (3 ) and 613 cm−1 (4 ). Reduction in symmetry in the crystal structure of sulphates such as in a number of minerals will cause the splitting of these vibrational modes. Such chemical interactions reduce AsO4 3− tetrahedral symmetry, as mentioned above, to either C3v /C3 (corner-sharing), C2v /C2 (edge-sharing, bidentate binuclear), or C1 /Cs (corner-sharing, edgesharing, bidentate binuclear, multidentate) [28,29]. In association with AsO4 3− symmetry and coordination changes, the A1 band may shift to different wavenumbers and the doubly degenerate E and triply degenerate F modes may give rise to several new A1 ,
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Fig. 1. Raman spectrum of (a) ferrilothmeyerite, (b) natrochalcite, (c) thometzekite, and (d) tsumcorite in the 600–1100 cm−1 region.
B1 , and/or E vibrations [28,29]. In the absence of symmetry deviations, AsO3 OH2− in C3v symmetry exhibit the s As–OH and as and s AsO3 OH2− vibrations together with corresponding the ı As-OH in-plane bending vibration, ı As-OH out-of-plane bending vibration, s AsO3 OH2− stretching vibration and ıas (AsO3 OH)2− bending vibration [32–34]. Keller [32] assigned observed the following infrared bands in Na2 (AsO3 OH)·7H2 O 450 and 360 cm−1 to the ıas (4 ) (AsO3 OH)2− bend (E), 580 cm−1 to the ı As–OH outof-plane bend, 715 cm−1 to the As–OH stretch (A1 ), 830 cm−1 to the as AsO3 OH2− stretch (E), and 1165 cm−1 to the ı As–OH in plane bend. In the Raman spectrum of Na2 (AsO3 OH)·7H2 O, Vansant
et al. [33] attributed observed Raman bands to the following vibrations 55, 94, 116 and 155 cm−1 to lattice modes, 210 cm−1 to (OH. . .O) stretch, 315 cm−1 to (AsO3 OH)2− rocking, 338 cm−1 to the ıs (AsO3 )2− bend, 381 cm−1 to the ıas (AsO3 OH)2− bend, 737 cm−1 to the s As–OH stretch (A1 ), 866 cm−1 to the as (AsO3 OH)2− stretch (E). 3.2. Raman and infrared spectroscopy The Raman spectrum of ferrilotharmeyerite, natrochalcite, thometzekite and tsumcorite in the 600–1100 cm−1
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Fig. 2. Raman spectrum of (a) ferrilothmeyerite, (b) natrochalcite, (c) thometzekite, and (d) tsumcorite in the 200–600 cm−1 region.
region are shown in Fig. 1a, b, c and d, respectively. This spectral region is the region of the symmetric stretching region of sulphates and arsenates. Minerals of the tsumcorite mineral group depend upon arsenate or sulphate anions in the mineral structure, or at least a mixture of these anions. The most intense Raman bands for ferrilotharmeyerite are observed at 765, 814, 830 and 880 cm−1 . One possible assignment is that the band at 880 cm−1 is attributable to the AsO4 3−
symmetric stretching vibration and the two bands at 830 and 814 cm−1 to the AsO4 3− antisymmetric stretching vibrations. Previous studies by the authors have suggested that the band at 763 cm−1 may be attributed to a water librational mode. This mineral is monoclinic and consequently, two AsO4 3− stretching vibrations are predicted. A comparison can be made with the Raman spectrum of gartrellite. Five Raman bands for gartrellite are observed at 799, 811, 830, 866 and 891 cm−1 . The last two bands are assigned to the Raman active AsO4 3− 1 symmetric stretching
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Fig. 3. Raman spectrum of (a) ferrilothmeyerite, (b) natrochalcite, (c) thometzekite, and (d) tsumcorite in the 2000–4000 cm−1 region.
vibration (A1 ). The first three bands are assigned to the infrared and Raman active triply degenerate AsO4 3− 3 antisymmetric stretching vibration (F2 ). In contrast, the spectrum of natrochalcite depends on the sulphate anion and perhaps are less complex than that of gartrellite and ferrilothmeyerite as natrochalcite (NaCu2 (SO4 )2 (OH,H2 O)2 ) is a mineral based upon sulphate. The band at 997 cm−1 in the spectrum is assigned to the SO4 2− 1 symmetric stretching vibration. The two bands at 1046 and 1023 cm−1 are attributed to the SO4 2− 3 antisymmetric stretching modes. The infrared
spectrum shows two bands at 993 and 1041 cm−1 which are attributed to the SO4 2− symmetric and antisymmetric stretching vibrations. An intense band is observed at 840 cm−1 is assigned to the AsO4 3− symmetric stretching vibration. Additional bands observed at 725 and 787 cm−1 are assigned to the AsO4 3− antisymmetric stretching vibrational mode. In addition, a Raman band is observed at 978 cm−1 and is assigned to the SO4 2− symmetric stretching vibration. Infrared bands for thometzekite are observed at 1174, 1085, 1049 cm−1 attributed to the SO4 2− antisymmetric
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stretching vibrations; 987 cm−1 attributed to the SO4 2− symmetric stretching vibration; and 863 cm−1 the AsO4 3− antisymmetric stretching vibration. The Raman spectrum of tsumcorite shows a pattern similar to that observed for thometzekite with bands observed at 834 and 746 cm−1 . These two bands are assigned to the symmetric and antisymmetric stretching vibrations of the AsO4 3− units. A low intensity band is observed at 927 cm−1 is assigned to the symmetric stretching bands of the SO4 2− units. The infrared spectrum of tsumcorite shows bands at 892, 800 and 784 cm−1 . The Raman spectrum of ferrilotharmeyerite, natrochalcite, thometzekite and tsumcorite in the 100–600 cm−1 region are shown in Fig. 2a, b, c and d, respectively. The difficulty with the low wavenumber region of the Raman spectrum of the tsumcorite mineral group is the overlap of the sulphate and arsenate bands attributable to the bending modes. The Raman spectrum of ferrilotharmeyerite shows two bands at 485 and 510 cm−1 . These bands are assigned to the out-of-plane bending modes of the AsO4 unit. Three bands are observed at 323, 368 and 419 cm−1 and are attributed to the AsO4 3− in-plane bending modes. An intense band is observed at 230 cm−1 . It is not known what this band may be assigned to but one possibility is the MO stretching vibration. A comparison may be made with the Raman spectrum of gartrellite. Raman bands are observed at 499, 474 and 438 cm−1 . These may be ascribed to the AsO4 3− 4 bending modes. A set of Raman bands for gartrellite are observed at 357, 331 and 304 cm−1 and are attributed to the 2 bending modes of the AsO4 3− unit. An elegant contrast can be made with the Raman spectrum of natrochalcite in this spectral region. Two sets of bands are observed. Firstly bands at 606 and 635 cm−1 and secondly bands at 402, 429, 443 and 465 cm−1 . The first set of bands is attributed to the SO4 2− out-of-plane bending modes whilst the second set to the SO4 2− in-plane bending modes. The Raman spectrum of thometzekite and tsumcorite are similar. Bands for thometzekite are observed at 499, 428 and 401 cm−1 . These bands are attributed to the 4 AsO4 3− bending modes. A second set observed at 356 and 322 cm−1 is assigned to the 2 AsO4 3− bending modes. For tsumcorite the first set of bands are observed at 523, 494, 428 and 401 cm−1 . The second set consists of bands at 362, 341 and 294 cm−1 . Much information can be obtained from the study of the hydroxyl stretching region of the tsumcorites. The minerals may contain hydroxyl units or water molecules in the structure or both. The Raman spectrum of ferrilotharmeyerite, natrochalcite, thometzekite and tsumcorite in the ∼2000–4000 cm−1 region are shown in Fig. 3a, b, c and d, respectively. The Raman spectrum of ferrilotharmeyerite shows a low intensity band at 3440 cm−1 and two more intense bands at 2973 and 2636 cm−1 . The first band is assigned to the hydroxyl stretching vibration of the OH unit and the latter two bands to the OH stretching of the water units. The FGA (factor group analysis) suggests that there should be two Raman active modes; one for the water and one for the OH unit. The position of the bands suggests that the OH units are strongly hydrogen bonded and the OH–OAs distances are short. A comparison can be made with the Raman spectrum of gartrellite. The Raman spectrum of gartrellite shows three bands at 3404, 3229 and 2999 cm−1 . Gartrellite is said to have a triclinic structure which means the FGA would predict two Raman bands in the OH stretching region. The mineral gartrellite Pb[(Cu,Zn)(Fe3+ , Zn, Cu)] (AsO4 )(OH,H2 O)2 has both OH and H2 O units. It is probable that the high wavenumber band which is comparatively sharp may be assigned to the symmetric stretching vibration of the OH unit. Here we are comparing the position of the band at 3440 with a band at 3404 cm−1 . The intensity of the band at 3404 cm−1 is much stronger than the band at 3440 cm−1 for ferrilotharmeyerite. The band at 3440 is
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broad and of low intensity as a consequence the defining of its position is more difficult. This implies that the degree of substitution of hydroxyls is much greater in gartrellite as compared with ferrilotharmeyerite. The Raman spectrum of natrochalcite appears simpler. Two bands are observed in the spectrum at 3153 and 3189 cm−1 . The bands appear to be band separated in the 77 K spectrum and three bands are observed at 3199, 3138 and 3049 cm−1 . Natrochalcite (NaCu2 (SO4 )2 (OH,H2 O)2 ) contains both hydroxyl and water units. The positions of the hydroxyl stretching vibration of these two units appear to be similar. In the infrared spectrum bands for natrochalcite are observed at 3349, 3158 and 2906 cm−1 . The Raman spectrum of thometzekite more closely resembles that of ferrilotharmeyerite. The spectra in the hydroxyl stretching region of this mineral are more complex. Raman bands are observed at 2332, 3808, 3268 and 3511 cm−1 . Infrared bands for ferrilotharmeyerite are observed at 3538, 3469, 3286, 2929 and 2549 cm−1 . The mineral thometzekite does not have any hydroxyl units in the formula. Thus all of the bands are due to OH stretching vibrations of water units. 4. Conclusions The existence of free arsenic compounds in the environment is a soil contaminant and is a health hazard. The removal of arsenate from the environment is of great importance. One method of removing arsenate is the formation of minerals in the natural environment. Among the many arsenate minerals are the tsumcorite mineral group. Such minerals are commonly found as secondary minerals in the mineral deposits of Australia. It is apparent that the formation of the arsenates of the tsumcorite mineral group offers a mechanism of arsenate removal. The analysis of minerals containing arsenate units is readily characterised by Raman spectroscopy. Among the many arsenate containing minerals is the tsumcorite mineral group. The tsumcorite arsenate minerals are characterised by typical spectra of the AsO4 3− units. The symmetric stretching modes are observed in the 840–880 cm−1 region; the antisymmetric stretching modes are observed in the 812–840 cm−1 region. Some bands are observed around 765 cm−1 region and are attributed to water librational modes. The 4 bending modes are observed around 499 cm−1 and the 2 bending modes in the 300–360 cm−1 region. Multiple bands are observed in these regions indicating a loss of symmetry of the AsO4 unit. Extensive isomorphic substitution of sulphate for arsenate was observed. The fundamentals of the spectra are related to the structure of the minerals. A comparison is made with the Raman spectrum of the sulphate based mineral, natrochalcite. This comparison proves the presence of sulphate in the two minerals gartrellite and thometzekite. Gartrellite shows a much greater sulphate isomorphic substitution than thometzekite. The range of OH stretching wavenumbers shows a range of hydrogen bond strengths based upon the range of calculated hydrogen bond distances. High wavenumber bands around 3400–3500 cm−1 indicate the presence of OH units in the tsumcorite minerals. Acknowledgements The financial and infra-structure support of the Queensland University of Technology, Chemistry discipline is gratefully acknowledged. The Australian Research Council (ARC) is thanked for funding the instrumentation. Mr. Dermot Henry of Museum Victoria is especially thanked for the loan of the tsumcorite minerals for Raman and infrared spectroscopic analysis.
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References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
F. Cesbron, J. Fritsche, Bull. Soc. Fr. Mineral. Crystallogr. 92 (1969) 196. G. Cocco, Periodico Mineral. (Rome) 21 (1952) 103. C. Palache, C.H. Warren, Camb. Am. J. Sci. 26 (1908) 342. C. Palache, Am. J. Sci. 237 (1939) 447. S. Skramovsky, J. Stepan, Cas. Cesk. Lek. 19 (1939) 4. B.H. Geier, K. Kautz, G. Mueller, Neues Jahrb. Mineral. Monatsh. (1971) 305. P. Keller, H. Hess, P.J. Dunn, Neues Jahrb. Mineral. Monatsh. (1979) 389. P. Keller, H. Hess, P.J. Dunn, Chem. Erde 40 (1981) 105. P. Keller, J. Innes, P.J. Dunn, Neues Jahrb. Mineral. Monatsh. (1986) 523. Kharisun, M.R. Taylor, D.J.M. Bevan, A.D. Rae, A. Pring, Mineral. Mag. 61 (1997) 685. W. Krause, K. Belendorff, H.J. Bernhardt, K. Petitjean, Neues Jahrb. Mineral. Monatsh. (1998) 111. K. Schmetzer, B. Nuber, O. Medenbach, Neues Jahrb. Mineral. Monatsh. (1985) 446. W. Krause, K. Belendorff, H.J. Bernhardt, C. McCammon, H. Effenberger, W. Mikenda, Eur. J. Mineral. 10 (1998) 179. I.M. Rumanova, G.F. Volodina, Doklady Akad. Nauk S. S. S. R. 123 (1958) 78. E. Tillmanns, W. Gebert, Acta Crystallogr. B 29 (1973) 2789. P.A. Williams, Oxide Zone Geochemistry, Ellis Horwood Ltd., Chichester, West Sussex, England, 1990.
[17] R.L. Frost, J. Cejka, J. Sejkora, J. Plasil, B.J. Reddy, E.C. Keeffe, Spectrochim. Acta A 78 (2011) 494. [18] R.L. Frost, S.J. Palmer, Spectrochim. Acta A 78 (2011) 248. [19] R.L. Frost, S.J. Palmer, Spectrochim. Acta A 78 (2011) 1255. [20] R.L. Frost, S.J. Palmer, Spectrochim. Acta A 78 (2011) 1250. [21] R.L. Frost, S.J. Palmer, Spectrochim. Acta A 79 (2011) 1794. [22] R.L. Frost, S.J. Palmer, Spectrochim. Acta A 79 (2011) 1215. [23] R.L. Frost, S.J. Palmer, Spectrochim. Acta A 79 (2011) 1210. [24] R.L. Frost, S.J. Palmer, S. Bahfenne, Spectrochim. Acta A 78 (2011) 1302. [25] R.L. Frost, S.J. Palmer, R.E. Pogson, Spectrochim. Acta A 79 (2011) 1149. [26] R.L. Frost, B.J. Reddy, S.J. Palmer, E.C. Keeffe, Spectrochim. Acta A 78 (2011) 996. [27] J.W. Anthony, R.A. Bideaux, K.W. Bladh, M.C. Nichols, Handbook of mineralogy, in: Arsenates, Phosphates, Vanadates, vol. IV, Mineral Data Publishing, Tucson, Arizona, 2000. [28] S.C.B. Myneni, S.J. Traina, G.A. Waychunas, T.J. Logan, Geochim. Cosmochim. Acta 62 (1998) 3285. [29] S.C.B. Myneni, S.J. Traina, G.A. Waychunas, T.J. Logan, Geochim. Cosmochim. Acta 62 (1998) 3499. [30] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, Wiley, New York, 1986. [31] R.L. Frost, P.A. Williams, W. Martens, P. Leverett, J.T. Kloprogge, Am. Mineral. 89 (2004) 1130. [32] P. Keller, Neues Jahrb. Mineral. Monatsh. (1971) 491. [33] F.K. Vansant, B.J.V.D. Veken, J. Mol. Struct. 15 (1973) 439. [34] F.K. Vansant, B.J.V.D. Veken, H.O. Desseyn, J. Mol. Struct. (1973) 425.
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Raman spectroscopy of selected tsumcorite Pb(Zn,Fe3+ )2 (AsO4 )2 (OH,H2 O) minerals—Implications for arsenate accumulation Ray L. Frost ∗ , Yunfei Xi 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
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Article history: Received 15 August 2011 Received in revised form 3 October 2011 Accepted 14 October 2011 Keywords: Raman spectroscopy Gartrellite Tsumcorite Thometzekite Arsenate Sulphate
a b s t r a c t The presence of arsenic in the environment is a hazard. The accumulation of arsenate by a range of cations in the formation of minerals provides a mechanism for the accumulation of arsenate. The formation of the tsumcorite minerals is an example of a series of minerals which accumulate arsenate. There are about twelve examples in this mineral group. Raman spectroscopy offers a method for the analysis of these minerals. The structure of selected tsumcorite minerals with arsenate and sulphate anions were analysed by Raman spectroscopy. Isomorphic substitution of sulphate for arsenate is observed for gartrellite and thometzekite. A comparison is made with the sulphate bearing mineral natrochalcite. The position of the hydroxyl and water stretching vibrations are related to the strength of the hydrogen bond formed between the OH unit and the AsO4 3− anion. Characteristic Raman spectra of the minerals enable the assignment of the bands to specific vibrational modes. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The tsumcorite mineral group are a set of minerals which in their formation accumulate arsenic as arsenate. Tsumcorite was named after the TSUMeb CORporation mine at Tsumeb, in Namibia, in recognition of the Corporation’s support for mineralogical investigations of the orebody at its Mineral Research Laboratory. The tsumcorite mineral group are well known [1–5]. Many of the minerals were found in the oxidised zones of the famous Tsumeb ore deposit [6]. At the Puttapa Mine in Australia it occurs with adamite, mimetite, smithsonite, goethite and quartz. At the Kintore Open Cut, Broken Hill, Australia it occurs with segnitite, beudantite, carminite and mawbyite. Many new minerals have been discovered in these oxidised zones in mineral deposits in other parts of Australia [7–11]. Many of these minerals are based upon arsenates of lead and zinc in combination with other cations. The tsumcorite group of minerals are a mineral group based upon monoclinic and triclinic arsenates, phosphates, vanadates and sulphates of the general formulae (M1)(M2)2 (XO4 )2 (OH,H2 O)2 where M1 is Pb, Ca or Na, M2 is Cu, Zn, Fe3+ , Co or Mn and X is As, P, V and/or S. The minerals gartrellite Pb[(Cu,Zn)(Fe3+ , Zn, Cu)] (AsO4 )(OH,H2 O)2 , helmutwinklerite Pb(Zn,Cu)2 (AsO4 )2 2H2 O and thometzekite [12] are triclinic. The minerals ferrilotharmeyerite [13] Ca(Fe3+ ,Zn)2 (AsO4 )2 (OH,H2 O)2 , lotharmeyerite Ca(Mn3+ ,Zn)2 (AsO4 )2 (OH,H2 O)2 , maw-
∗ 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.2011.10.028
byite [10] Pb(Fe3+ ,Zn)2 (AsO4 )2 (OH,H2 O)2 , mounanaite 3+ Pb(Fe )2(VO4 )2 (OH)2 , natrochalcite [14] NaCu2 (SO4 )2 (OH,H2 O)2 and tsumcorite [15] Pb(Zn,Fe3+ )2 (AsO4 )2 (OH,H2 O) are monoclinic [15]. Tsumcorite belongs to the monoclinic crystal class 2/m. This means that it has a twofold axis of symmetry along the b axis and a mirror plane perpendicular to this, in the plane containing the a and c axes. The a and c axes are inclined to each other at angle ˇ = 115.3◦ . The unit cell parameters are ˚ b = 6.326–6.329 A˚ and c = 7.577–7.583 A. ˚ There a = 9.124–9.131 A, are two formula units per unit cell (Z = 2), and the space group is C2/m This means the cell is a C-face centered lattice, with lattice points in the center of the C face as well as at the corners of the cell. There are some problems associated with writing the mineral formula in that the formula may change as a function of the degree of solid solution formation and the amount of isomorphic substitution. Both anion and cation substitution may occur. Sulphate, phosphate and carbonate may replace the arsenate. For example it is quite comprehensible that a formula such as PbCu(Fe3+ ,Cu)(AsO4 )2 (OH,H2 O)2 is found. Variation in mineral composition is expected for gartrellites from different origins. The formula of gartrellite may be written as Pb[(Cu,Fe2+ )(Fe3+ , Zn, Cu)] (AsO4 )(CO3 ,H2 O)2 . For example the gartrellite found at Ashburton Downs, Western Australia has a calculated formula of PbCu1.5 Fe2+ 0.5 As1.5 (SO4 )0.5 (CO3 )0.5 (H2 O)0.2 . Of course Raman spectroscopy will readily determine the presence of carbonate in the mineral. The presence or absence of two moles of water is the determining factor as to whether the mineral is triclinic or monoclinic.
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Table 1 Table of the tsumcorite minerals studied. Mineral
Type number
Formulae
Origin
Gartrellite Ferrilotharmeyerite Natrochalcite Tsumcorite Thometzekite
M39987 M36822 M32894 M37949 M43672
Pb[(Cu,Zn)(Fe3+ , Zn, Cu)](AsO4 )2 (OH,H2 O) Ca(Fe3+ ,Zn)2 (AsO4 )2 (OH,H2 O)2 NaCu2 (SO4 )2 (OH,H2 O)2 Pb(Zn,Fe3+ )2 (AsO4 )2 (OH,H2 O) Pb(Cu,Zn)2 (AsO4 )2 2H2 O
Anticline deposit, Ashburton Downs, Western Australia Tsumeb, Namibia Chuquicamata, Chile Tsumeb, Namibia Tsumeb, Namibia
Tsumcorite crystallizes in the monoclinic space group C2/m with a 9.124 (4), b 6.329 (2), c 7.577 (2), ˇ = 115◦ 17 (2) , Z = 2 [15]. Crystal symmetry is either triclinic in the case of an ordered occupation of two cationic sites, triclinic due to ordering of the H bonds in the case of species with 2 water molecules per formula unit, or monoclinic in the other cases. Crystals of ferrilotharmeyerite, tsumcorite, thometzekite (sulfatian), and mounanaite have monoclinic symmetry, space group C2/m. The triclinic members of the tsumcorite group are gartrellite, zincian gartrellite, phosphogartrellite, helmutwinklerite, and probably (sulphate-free) thometzekite; the space group is P/1, with a pronounced monoclinic C-centered pseudocell. The triclinic distortion is caused by an ordered arrangement of Fe[6]O6 octahedra and tetragonal bi-pyramidal Cu[4+2]O6 polyhedra [13]. Of course Raman spectroscopy will readily determine the presence of carbonate in the mineral. The presence or absence of two moles of water is the determining factor in whether the mineral is triclinic or not. It has been shown that crystals of ferrilotharmeyerite, tsumcorite, thometzekite (sulfatian), and mounanaite have monoclinic symmetry, space group C2/m [13]. The triclinic members of the tsumcorite group have the space group is P1, with a pronounced monoclinic C-centered pseudocell [13]. The tsumcorite minerals are often formed in the oxidised zones of arsenic bearing Pb–Zn deposits. The particular mineral formed depends upon the composition of the polymetallic ore deposit. The minerals are of a rare nature. Complex solution chemistry involving mixtures of the cations of lead, zinc, and ferric iron may result in the formation of the tsumcorite group of minerals. The type of mineral formed is a function of concentration, pH, temperature and the available anion present in the mother solution. The complex set of variable requires a multidimensional phase diagram [16]. Raman spectroscopy has proven an excellent technique for the study of oxyanions in both solution and in secondary mineral formation [17–26]. In this work we extend our studies to the arsenates of the tsumcorite mineral group. 2. Experimental 2.1. Minerals The tsumcorite minerals were obtained from Museum Victoria. The selected minerals and their museum number and place of origin are reported in Table 1. Details of the tsumcorite minerals have been published (page 207) [27]. 2.2. Raman spectroscopy Crystals of the individual members of the tsumcorite mineral group 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. 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 128 scans with a resolution of 4 cm−1 and a mirror velocity of 0.6329 cm/s. Spectra were co-added to improve the signal to noise ratio. 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. Arsenate vibrations According to Myneni et al. [28,29] and Nakamoto [30], (AsO4 )3− is a tetrahedral unit, which exhibits four fundamental vibrations: the Raman active 1 symmetric stretching vibration (A1 ) at 818 cm−1 ; the Raman active doubly degenerate 2 symmetric bending vibration (E) observed at 350 cm−1 , the infrared and Raman active triply degenerate 3 antisymmetric stretching vibration (F2 ) found around 786 cm−1 , and the infrared and Raman active triply degenerate 4 bending vibration (F2 ) observed at 405 cm−1 . Protonation, metal complexation, and/or adsorption on a mineral surface will cause the change in (AsO4 )3− symmetry from Td to lower symmetries, such as C3v , C2v or even C1 . This loss of degeneracy causes splitting of degenerate vibrations of AsO4 3− and the shifting of the As–OH stretching vibrations to different wavenumbers. Sulphates as with other oxyanions lend themselves to analysis by Raman spectroscopy [31]. In aqueous systems, the sulphate anion is of Td symmetry and is characterised by Raman bands at 981 cm−1 (1 ), 451 cm−1 (2 ), 1104 cm−1 (3 ) and 613 cm−1 (4 ). Reduction in symmetry in the crystal structure of sulphates such as in a number of minerals will cause the splitting of these vibrational modes. Such chemical interactions reduce AsO4 3− tetrahedral symmetry, as mentioned above, to either C3v /C3 (corner-sharing), C2v /C2 (edge-sharing, bidentate binuclear), or C1 /Cs (corner-sharing, edgesharing, bidentate binuclear, multidentate) [28,29]. In association with AsO4 3− symmetry and coordination changes, the A1 band may shift to different wavenumbers and the doubly degenerate E and triply degenerate F modes may give rise to several new A1 ,
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Fig. 1. Raman spectrum of (a) ferrilothmeyerite, (b) natrochalcite, (c) thometzekite, and (d) tsumcorite in the 600–1100 cm−1 region.
B1 , and/or E vibrations [28,29]. In the absence of symmetry deviations, AsO3 OH2− in C3v symmetry exhibit the s As–OH and as and s AsO3 OH2− vibrations together with corresponding the ı As-OH in-plane bending vibration, ı As-OH out-of-plane bending vibration, s AsO3 OH2− stretching vibration and ıas (AsO3 OH)2− bending vibration [32–34]. Keller [32] assigned observed the following infrared bands in Na2 (AsO3 OH)·7H2 O 450 and 360 cm−1 to the ıas (4 ) (AsO3 OH)2− bend (E), 580 cm−1 to the ı As–OH outof-plane bend, 715 cm−1 to the As–OH stretch (A1 ), 830 cm−1 to the as AsO3 OH2− stretch (E), and 1165 cm−1 to the ı As–OH in plane bend. In the Raman spectrum of Na2 (AsO3 OH)·7H2 O, Vansant
et al. [33] attributed observed Raman bands to the following vibrations 55, 94, 116 and 155 cm−1 to lattice modes, 210 cm−1 to (OH. . .O) stretch, 315 cm−1 to (AsO3 OH)2− rocking, 338 cm−1 to the ıs (AsO3 )2− bend, 381 cm−1 to the ıas (AsO3 OH)2− bend, 737 cm−1 to the s As–OH stretch (A1 ), 866 cm−1 to the as (AsO3 OH)2− stretch (E). 3.2. Raman and infrared spectroscopy The Raman spectrum of ferrilotharmeyerite, natrochalcite, thometzekite and tsumcorite in the 600–1100 cm−1
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Fig. 2. Raman spectrum of (a) ferrilothmeyerite, (b) natrochalcite, (c) thometzekite, and (d) tsumcorite in the 200–600 cm−1 region.
region are shown in Fig. 1a, b, c and d, respectively. This spectral region is the region of the symmetric stretching region of sulphates and arsenates. Minerals of the tsumcorite mineral group depend upon arsenate or sulphate anions in the mineral structure, or at least a mixture of these anions. The most intense Raman bands for ferrilotharmeyerite are observed at 765, 814, 830 and 880 cm−1 . One possible assignment is that the band at 880 cm−1 is attributable to the AsO4 3−
symmetric stretching vibration and the two bands at 830 and 814 cm−1 to the AsO4 3− antisymmetric stretching vibrations. Previous studies by the authors have suggested that the band at 763 cm−1 may be attributed to a water librational mode. This mineral is monoclinic and consequently, two AsO4 3− stretching vibrations are predicted. A comparison can be made with the Raman spectrum of gartrellite. Five Raman bands for gartrellite are observed at 799, 811, 830, 866 and 891 cm−1 . The last two bands are assigned to the Raman active AsO4 3− 1 symmetric stretching
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Fig. 3. Raman spectrum of (a) ferrilothmeyerite, (b) natrochalcite, (c) thometzekite, and (d) tsumcorite in the 2000–4000 cm−1 region.
vibration (A1 ). The first three bands are assigned to the infrared and Raman active triply degenerate AsO4 3− 3 antisymmetric stretching vibration (F2 ). In contrast, the spectrum of natrochalcite depends on the sulphate anion and perhaps are less complex than that of gartrellite and ferrilothmeyerite as natrochalcite (NaCu2 (SO4 )2 (OH,H2 O)2 ) is a mineral based upon sulphate. The band at 997 cm−1 in the spectrum is assigned to the SO4 2− 1 symmetric stretching vibration. The two bands at 1046 and 1023 cm−1 are attributed to the SO4 2− 3 antisymmetric stretching modes. The infrared
spectrum shows two bands at 993 and 1041 cm−1 which are attributed to the SO4 2− symmetric and antisymmetric stretching vibrations. An intense band is observed at 840 cm−1 is assigned to the AsO4 3− symmetric stretching vibration. Additional bands observed at 725 and 787 cm−1 are assigned to the AsO4 3− antisymmetric stretching vibrational mode. In addition, a Raman band is observed at 978 cm−1 and is assigned to the SO4 2− symmetric stretching vibration. Infrared bands for thometzekite are observed at 1174, 1085, 1049 cm−1 attributed to the SO4 2− antisymmetric
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stretching vibrations; 987 cm−1 attributed to the SO4 2− symmetric stretching vibration; and 863 cm−1 the AsO4 3− antisymmetric stretching vibration. The Raman spectrum of tsumcorite shows a pattern similar to that observed for thometzekite with bands observed at 834 and 746 cm−1 . These two bands are assigned to the symmetric and antisymmetric stretching vibrations of the AsO4 3− units. A low intensity band is observed at 927 cm−1 is assigned to the symmetric stretching bands of the SO4 2− units. The infrared spectrum of tsumcorite shows bands at 892, 800 and 784 cm−1 . The Raman spectrum of ferrilotharmeyerite, natrochalcite, thometzekite and tsumcorite in the 100–600 cm−1 region are shown in Fig. 2a, b, c and d, respectively. The difficulty with the low wavenumber region of the Raman spectrum of the tsumcorite mineral group is the overlap of the sulphate and arsenate bands attributable to the bending modes. The Raman spectrum of ferrilotharmeyerite shows two bands at 485 and 510 cm−1 . These bands are assigned to the out-of-plane bending modes of the AsO4 unit. Three bands are observed at 323, 368 and 419 cm−1 and are attributed to the AsO4 3− in-plane bending modes. An intense band is observed at 230 cm−1 . It is not known what this band may be assigned to but one possibility is the MO stretching vibration. A comparison may be made with the Raman spectrum of gartrellite. Raman bands are observed at 499, 474 and 438 cm−1 . These may be ascribed to the AsO4 3− 4 bending modes. A set of Raman bands for gartrellite are observed at 357, 331 and 304 cm−1 and are attributed to the 2 bending modes of the AsO4 3− unit. An elegant contrast can be made with the Raman spectrum of natrochalcite in this spectral region. Two sets of bands are observed. Firstly bands at 606 and 635 cm−1 and secondly bands at 402, 429, 443 and 465 cm−1 . The first set of bands is attributed to the SO4 2− out-of-plane bending modes whilst the second set to the SO4 2− in-plane bending modes. The Raman spectrum of thometzekite and tsumcorite are similar. Bands for thometzekite are observed at 499, 428 and 401 cm−1 . These bands are attributed to the 4 AsO4 3− bending modes. A second set observed at 356 and 322 cm−1 is assigned to the 2 AsO4 3− bending modes. For tsumcorite the first set of bands are observed at 523, 494, 428 and 401 cm−1 . The second set consists of bands at 362, 341 and 294 cm−1 . Much information can be obtained from the study of the hydroxyl stretching region of the tsumcorites. The minerals may contain hydroxyl units or water molecules in the structure or both. The Raman spectrum of ferrilotharmeyerite, natrochalcite, thometzekite and tsumcorite in the ∼2000–4000 cm−1 region are shown in Fig. 3a, b, c and d, respectively. The Raman spectrum of ferrilotharmeyerite shows a low intensity band at 3440 cm−1 and two more intense bands at 2973 and 2636 cm−1 . The first band is assigned to the hydroxyl stretching vibration of the OH unit and the latter two bands to the OH stretching of the water units. The FGA (factor group analysis) suggests that there should be two Raman active modes; one for the water and one for the OH unit. The position of the bands suggests that the OH units are strongly hydrogen bonded and the OH–OAs distances are short. A comparison can be made with the Raman spectrum of gartrellite. The Raman spectrum of gartrellite shows three bands at 3404, 3229 and 2999 cm−1 . Gartrellite is said to have a triclinic structure which means the FGA would predict two Raman bands in the OH stretching region. The mineral gartrellite Pb[(Cu,Zn)(Fe3+ , Zn, Cu)] (AsO4 )(OH,H2 O)2 has both OH and H2 O units. It is probable that the high wavenumber band which is comparatively sharp may be assigned to the symmetric stretching vibration of the OH unit. Here we are comparing the position of the band at 3440 with a band at 3404 cm−1 . The intensity of the band at 3404 cm−1 is much stronger than the band at 3440 cm−1 for ferrilotharmeyerite. The band at 3440 is
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broad and of low intensity as a consequence the defining of its position is more difficult. This implies that the degree of substitution of hydroxyls is much greater in gartrellite as compared with ferrilotharmeyerite. The Raman spectrum of natrochalcite appears simpler. Two bands are observed in the spectrum at 3153 and 3189 cm−1 . The bands appear to be band separated in the 77 K spectrum and three bands are observed at 3199, 3138 and 3049 cm−1 . Natrochalcite (NaCu2 (SO4 )2 (OH,H2 O)2 ) contains both hydroxyl and water units. The positions of the hydroxyl stretching vibration of these two units appear to be similar. In the infrared spectrum bands for natrochalcite are observed at 3349, 3158 and 2906 cm−1 . The Raman spectrum of thometzekite more closely resembles that of ferrilotharmeyerite. The spectra in the hydroxyl stretching region of this mineral are more complex. Raman bands are observed at 2332, 3808, 3268 and 3511 cm−1 . Infrared bands for ferrilotharmeyerite are observed at 3538, 3469, 3286, 2929 and 2549 cm−1 . The mineral thometzekite does not have any hydroxyl units in the formula. Thus all of the bands are due to OH stretching vibrations of water units. 4. Conclusions The existence of free arsenic compounds in the environment is a soil contaminant and is a health hazard. The removal of arsenate from the environment is of great importance. One method of removing arsenate is the formation of minerals in the natural environment. Among the many arsenate minerals are the tsumcorite mineral group. Such minerals are commonly found as secondary minerals in the mineral deposits of Australia. It is apparent that the formation of the arsenates of the tsumcorite mineral group offers a mechanism of arsenate removal. The analysis of minerals containing arsenate units is readily characterised by Raman spectroscopy. Among the many arsenate containing minerals is the tsumcorite mineral group. The tsumcorite arsenate minerals are characterised by typical spectra of the AsO4 3− units. The symmetric stretching modes are observed in the 840–880 cm−1 region; the antisymmetric stretching modes are observed in the 812–840 cm−1 region. Some bands are observed around 765 cm−1 region and are attributed to water librational modes. The 4 bending modes are observed around 499 cm−1 and the 2 bending modes in the 300–360 cm−1 region. Multiple bands are observed in these regions indicating a loss of symmetry of the AsO4 unit. Extensive isomorphic substitution of sulphate for arsenate was observed. The fundamentals of the spectra are related to the structure of the minerals. A comparison is made with the Raman spectrum of the sulphate based mineral, natrochalcite. This comparison proves the presence of sulphate in the two minerals gartrellite and thometzekite. Gartrellite shows a much greater sulphate isomorphic substitution than thometzekite. The range of OH stretching wavenumbers shows a range of hydrogen bond strengths based upon the range of calculated hydrogen bond distances. High wavenumber bands around 3400–3500 cm−1 indicate the presence of OH units in the tsumcorite minerals. Acknowledgements The financial and infra-structure support of the Queensland University of Technology, Chemistry discipline is gratefully acknowledged. The Australian Research Council (ARC) is thanked for funding the instrumentation. Mr. Dermot Henry of Museum Victoria is especially thanked for the loan of the tsumcorite minerals for Raman and infrared spectroscopic analysis.
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References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
F. Cesbron, J. Fritsche, Bull. Soc. Fr. Mineral. Crystallogr. 92 (1969) 196. G. Cocco, Periodico Mineral. (Rome) 21 (1952) 103. C. Palache, C.H. Warren, Camb. Am. J. Sci. 26 (1908) 342. C. Palache, Am. J. Sci. 237 (1939) 447. S. Skramovsky, J. Stepan, Cas. Cesk. Lek. 19 (1939) 4. B.H. Geier, K. Kautz, G. Mueller, Neues Jahrb. Mineral. Monatsh. (1971) 305. P. Keller, H. Hess, P.J. Dunn, Neues Jahrb. Mineral. Monatsh. (1979) 389. P. Keller, H. Hess, P.J. Dunn, Chem. Erde 40 (1981) 105. P. Keller, J. Innes, P.J. Dunn, Neues Jahrb. Mineral. Monatsh. (1986) 523. Kharisun, M.R. Taylor, D.J.M. Bevan, A.D. Rae, A. Pring, Mineral. Mag. 61 (1997) 685. W. Krause, K. Belendorff, H.J. Bernhardt, K. Petitjean, Neues Jahrb. Mineral. Monatsh. (1998) 111. K. Schmetzer, B. Nuber, O. Medenbach, Neues Jahrb. Mineral. Monatsh. (1985) 446. W. Krause, K. Belendorff, H.J. Bernhardt, C. McCammon, H. Effenberger, W. Mikenda, Eur. J. Mineral. 10 (1998) 179. I.M. Rumanova, G.F. Volodina, Doklady Akad. Nauk S. S. S. R. 123 (1958) 78. E. Tillmanns, W. Gebert, Acta Crystallogr. B 29 (1973) 2789. P.A. Williams, Oxide Zone Geochemistry, Ellis Horwood Ltd., Chichester, West Sussex, England, 1990.
[17] R.L. Frost, J. Cejka, J. Sejkora, J. Plasil, B.J. Reddy, E.C. Keeffe, Spectrochim. Acta A 78 (2011) 494. [18] R.L. Frost, S.J. Palmer, Spectrochim. Acta A 78 (2011) 248. [19] R.L. Frost, S.J. Palmer, Spectrochim. Acta A 78 (2011) 1255. [20] R.L. Frost, S.J. Palmer, Spectrochim. Acta A 78 (2011) 1250. [21] R.L. Frost, S.J. Palmer, Spectrochim. Acta A 79 (2011) 1794. [22] R.L. Frost, S.J. Palmer, Spectrochim. Acta A 79 (2011) 1215. [23] R.L. Frost, S.J. Palmer, Spectrochim. Acta A 79 (2011) 1210. [24] R.L. Frost, S.J. Palmer, S. Bahfenne, Spectrochim. Acta A 78 (2011) 1302. [25] R.L. Frost, S.J. Palmer, R.E. Pogson, Spectrochim. Acta A 79 (2011) 1149. [26] R.L. Frost, B.J. Reddy, S.J. Palmer, E.C. Keeffe, Spectrochim. Acta A 78 (2011) 996. [27] J.W. Anthony, R.A. Bideaux, K.W. Bladh, M.C. Nichols, Handbook of mineralogy, in: Arsenates, Phosphates, Vanadates, vol. IV, Mineral Data Publishing, Tucson, Arizona, 2000. [28] S.C.B. Myneni, S.J. Traina, G.A. Waychunas, T.J. Logan, Geochim. Cosmochim. Acta 62 (1998) 3285. [29] S.C.B. Myneni, S.J. Traina, G.A. Waychunas, T.J. Logan, Geochim. Cosmochim. Acta 62 (1998) 3499. [30] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, Wiley, New York, 1986. [31] R.L. Frost, P.A. Williams, W. Martens, P. Leverett, J.T. Kloprogge, Am. Mineral. 89 (2004) 1130. [32] P. Keller, Neues Jahrb. Mineral. Monatsh. (1971) 491. [33] F.K. Vansant, B.J.V.D. Veken, J. Mol. Struct. 15 (1973) 439. [34] F.K. Vansant, B.J.V.D. Veken, H.O. Desseyn, J. Mol. Struct. (1973) 425.