Raman and infrared spectroscopic study of boussingaultite and nickelboussingaultite

Spectrochimica Acta Part A 73 (2009) 420–423

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Raman and infrared spectroscopic study of boussingaultite and nickelboussingaultite Adam Culka a,∗ , Jan Jehliˇcka a,∗∗ , Ivan Nˇemec b a b

Faculty of Science, Institute of Geochemistry, Mineralogy and Mineral Resources, Charles University, Albertov 6, 128 43 Prague, Czech Republic Faculty of Science, Department of Inorganic Chemistry, Charles University, Hlavova 2030, 128 40 Prague, Czech Republic

a r t i c l e

i n f o

Article history: Received 24 June 2008 Received in revised form 29 September 2008 Accepted 23 October 2008 Keywords: Raman spectroscopy Sulphates Ammonia Boussingaultite Nickelboussingaultite

a b s t r a c t The Raman and infrared spectra of two secondary sulphate minerals, boussingaultite [(NH4 )2 Mg(SO4 )2 ·6H2 O] and nickelboussingaultite [(NH4 )2 Ni,Mg(SO4 )2 ·6H2 O] have been collected. Two bands observed at 983 and 990 cm−1 were attributed to the 1 (SO4 2− ) symmetric stretching vibration. The bands at 1133, 1096 and 1063 cm−1 in boussingaultite spectra and bands at 1149, 1093 and 1063 cm−1 in nickelboussingaultite spectra were attributed to the 3 (SO4 2− ) antisymmetric stretching vibration. The splitting of the 4 (SO4 2− ) bending vibration produced bands at 625 and 615 cm−1 in the boussingaultite spectra and 652, 624 and 602 cm−1 in the nickelboussingaultite spectra. Similarly, in the case of the 2 (SO4 ) bending vibration, the bands were observed at 454 cm−1 in the boussingaultite spectra and 482, 457 and 440 cm−1 in the nickelboussingaultite spectra. The splitting of bands is the result of lowered symmetry of sulphate ions and possibly a result of substitution of Mg ions by Ni ions in nickelboussingaultite. The bands in the NH4 + bending vibration region were observed at 1705 and 1678 cm−1 (2 ), 1460 and 1438 cm−1 (4 ) for the mineral boussingaultite. In the high wavenumber region the bands arising from the OH (bands above 3000 cm−1 ) and the NH4 + (2940, 2918 and 2845 cm−1 ) stretching vibrations were identified. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Interest in the spectroscopic study of sulphate minerals [1,4,5] has been enhanced by the recent discovery of these minerals on Mars [2,3], which is believed to be one of the proofs of pre-existing liquid water on the planetary surface. The majority of Martian sulphates consist of iron (Fe(II), Fe(III)) sulphates and jarosites. It has been suggested that on ammonia-bearing icy moons such as Titan, the ammonium sulphates mascagnite ((NH4 )2 SO4 ) and ammonium sulphate tetrahydrate could be formed [6]. On Earth, ammonium sulphates can be found in active volcanic areas such as the Larderello geothermal field or as a product of various reactions on anthropogenic former mine dumps (Talnakh Cu–Ni Deposit, Noril’sk, Russia [9]; Cameron U–Mo former mines, Coconino Co., AZ, USA) or for instance in bat guano deposits [7]. Boussingaultite, named after the French chemist Jean-Baptiste Boussingault, has

∗ Corresponding authors. Tel.: +420 22195 1517; fax: +420 22195 1496. ∗∗ . Tel.: +420 22195 1503; fax: +420 22195 1496. E-mail addresses: [email protected] (A. Culka), [email protected] (J. Jehliˇcka). 1386-1425/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2008.10.026

the chemical formula (NH4 )2 Mg(SO4 )2 ·6H2 O. It was discovered in a boric acid fumaroles environment (Travale, Montieri, Cecina Valley, Grosseto Province, Tuscany, Italy); other occurrences of boussingaultite include geyser sites and anthracite mine dump fires [9]. It belongs to the monoclinic crystal system with space group P21 /a and unit cell dimensions a = 9.324, b = 12.597, c = 6.211 Å, ˇ = 107.1◦ and Z = 2. The structure has been determined by Margulis and Templeton [8]. The magnesium atom is surrounded by an octahedron of water molecules, each forming two hydrogen bonds to oxygen atoms of the sulphate ions. Three hydrogen atoms of each ammonium ion are bonded to oxygen atoms in sulphate ions, the fourth hydrogen atom is unique in that it is equidistant from two sulphateoxygen atoms. In nickelboussingaultite, as its name suggests, Ni2+ cations are partially substituted for Mg2+ . Its chemical formula is (NH4 )2 Ni,Mg(SO4 )2 ·6H2 O. It belongs to the monoclinic crystal system with space group P21 /b unit cell dimensions a = 9.241, b = 12.544, c = 6.243 Å, ˇ = 106.9◦ and Z = 2 [10]. This work was carried out to obtain Raman spectra of boussingaultite and nickelboussingaultite which have not yet been published and also because these secondary minerals containing the NH4 + group may be important as markers in the field of exobiology.

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2. Materials and methods

Table 1 Observed Raman and infrared bands of mineral boussingaultite.

2.1. Samples

Boussingaultite (NH4 )2 Mg(SO4 )2 ·6H2 O

Two samples of the minerals boussingaultite and nickelboussingaultite were investigated in this study. Boussingaultite originated from Larderello, Tuscany, Italy, and the sample investigated here comprised a coating of a micro-crystalline appearance with a yellowish colour. The nickelboussingaultite originated from Cameron, Coconino Co., AZ, USA and the sample was a white, finely granular aggregate. 2.2. Instrumental The micro-Raman analyses of minerals were carried out on a multichannel Renishaw InVia Reflex spectrometer coupled with a Peltier-cooled CCD detector. In this study excitation was provided by the 785 nm line of a diode laser. The samples were scanned from 100 to 3600 cm−1 at a nominal spectral resolution of 4 cm−1 . The chosen regions were afterwards studied at a higher resolution. The scanning parameter for each Raman spectrum was taken as 30 s, and 10 scans were accumulated using the 50× objective lens for each experimental run to provide a better signal-to-noise ratio. Spectra were calibrated using the 1332 cm−1 line of a diamond. Multiple spot analyses on different areas of the same sample provided similar spectra and confirmed the spectral reproducibility. All measurements were made at the room temperature. The Raman microspectrometric analyses were conducted on the samples without any previous preparation. For the DRIFT analyses a small amount of each sample was taken and mixed with potassium bromide in a ratio approximately 1:10. The DRIFT analyses were performed on an Equinox 55/S (Bruker) spectrometer. Totally 128 interferograms were co-added per spectrum in the spectral range between 4000 and 400 cm−1 with a spectral resolution of 4 cm−1 . All spectral manipulations such as baseline adjustment, smoothing and band component analysis were performed using the GRAMS32 software package (Galactic Industries Corporation, Salem, NH, USA). For improved demonstration of individual bands in the spectra, band component analysis was used in some cases. Best results of band fitting were achieved using a mixed Gauss–Lorentz function at a medium sensitivity. 3. Results and discussion The Raman spectra of both minerals can be divided into four regions. The situation is similar in the infrared spectra, although more overlapping of bands occurs there. The highest wavenumber region above 2600 cm−1 displays the combination of the OH and NH4 stretching vibrations. The region between 1800 and 1400 cm−1 contains the spectral signatures of the NH4 and HOH bending vibrations. The region between 1300 and 900 cm−1 contains the spectral signatures of the SO4 stretching vibrations. The signatures of the SO4 bending vibrations as well as the lattice modes occur in the region below 800 cm−1 . The Raman and infrared bands and their proposed assignment are presented in Tables 1 and 2.

Raman 3380 w 3290 w 3080 m 3040 m 2918 w 2845 w 1705 m 1678 m 1460 m, br 1436 m 1133 w 1096 vw 1063 w 983 s

626 m 616 m, br

DRIFT 3290 s 3084 s 2913 m 2848 m 1705 m 1675 m 1471 w 1433 m 1145 s 1090 s 982 m 787 w, br 724 w, br 676 w, br 625 m 615 m, sh 564 w, br

454 m 360 w 310 w <222 w

Proposed band assignment (OH) (OH) (OH) (OH) (NH4 ) (NH4 ) 2 (NH4 ) + ı(HOH) 2 (NH4 ) + ı(HOH) 4 (NH4 ) 4 (NH4 ) 3 (SO4 ) 3 (SO4 ) 3 (SO4 ) 1 (SO4 ) T (H2 O) T (H2 O) T (H2 O) 4 (SO4 ) 4 (SO4 ) T (H2 O) 2 (SO4 ) (H2 O) (H2 O) Lattice modes

2913 and 2848 cm−1 in the infrared spectrum are attributed to the NH4 stretching vibrations (Fig. 1). Two bands at 1705 and 1678 cm−1 in the Raman spectrum and the less well resolved bands at 1705 and 1675 cm−1 in the infrared spectrum were assigned to the combination of the (2 ) NH4 and HOH deformation vibration of water molecules (Fig. 2). The (4 ) NH4 stretching vibration is demonstrated by two bands both in the Raman (1460 and 1436 cm−1 ) and in the infrared spectrum (1471 and 1433 cm−1 ). The antisymmetric stretching vibration 3 of the SO4 group is given by two strong bands in the infrared spectrum (1145 and 1090 cm−1 ). In the Raman spectrum the very weak bands located at 1133, 1096 and 1063 cm−1 (Fig. 3) arise from the same vibra-

Table 2 Observed Raman and infrared bands of mineral nickelboussingaultite. Nickelboussingaultite (NH4 )2 Ni,Mg(SO4 )2 ·6H2 O Raman

DRIFT

Proposed band assignment

3453 m

(OH) (OH) (NH4 ) + (OH) (NH4 ) 2 (NH4 ) + ı(HOH) 4 (NH4 ) 4 (NH4 ) 3 (SO4 ) 3 (SO4 ) 3 (SO4 ) 1 (SO4 ) 1 (SO4 ) T (H2 O) T (H2 O) 4 (SO4 ) 4 (SO4 ) 4 (SO4 ) T (H2 O) 2 (SO4 ) 2 (SO4 ) 2 (SO4 ) (H2 O) (H2 O) Lattice modes

∼3280 w, br ∼3200 m, br ∼2940 w, br ∼1660 w, br ∼1460 w, br 1149 w 1093 w 1063 w 1027 w 990 s

3.1. Boussingaultite

652 w 624 w 602 w

Starting from highest wavenumbers, the weak intensity, broad Raman bands at 3380, 3290, 3080 and 3040 cm−1 are attributed to the to the OH stretching vibrations of the water molecules. The same vibrations are observed in the strong broad bands at 3290 and 3080 cm−1 in the infrared spectrum. Two bands located at wavenumbers 2918 and 2845 cm−1 in the Raman spectrum and

482 w 457 w 440 w 341 w 312 w <240 m

1658 m, br 1473 w 1462 w 1143 s, sh 1098 s

992 w 760 m, br, sh 675 ms 620 ms 604 ms 520 m 438 w 423 w, sh

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tion. The strongest band in the Raman spectrum at 983 cm−1 is attributed to the 1 symmetric stretching vibration of the SO4 group, the intensity of this band in the infrared spectrum is quite large, which would suggest that the symmetry of the sulphate groups in the mineral is reduced from tetrahedral. Medium intensity bands at wavenumbers 625 and 615 cm−1 (Raman) and 626 and 616 cm−1 (infrared) are attributed to the 4 bending vibration of the SO4 group. A medium intensity band at 454 cm−1 in the Raman spectrum is attributed to the 2 bending vibration of the SO4 group. The broad bands at wavenumbers 787, 724, 676 and 564 cm−1 can be attributed to the torsion vibrations of water molecules. These bands are absent in the Raman spectrum. The Raman bands at 360 and 310 cm−1 could also be attributed to the vibrations of water molecules, while bands appearing at 222 cm−1 and below are attributed to the lattice modes. Fig. 1. Raman spectra of the NH4 and OH stretching vibration region (100× magnified): boussingaultite (A) nickelboussingaultite (B).

Fig. 2. Raman spectra of the NH4 bending vibration region (50× magnified): boussingaultite (A) nickelboussingaultite (B).

Fig. 3. Raman spectra of the SO4 stretching vibration region: boussingaultite (A) nickelboussingaultite (B).

3.2. Nickelboussingaultite A broad weak band at around 3280 cm−1 in the Raman spectrum is due to the stretching vibration of the OH group. The broad band with a stronger intensity located at 2940 cm−1 is a manifestation of a combination of OH and NH4 stretching vibrations (Fig. 1). In the infrared spectrum these modes are given by one band at 3453 cm−1 and a very broad shoulder of this band centred at 3200 cm−1 . The band at 1658 cm−1 in the infrared spectrum is assigned to the combination of the (4 ) NH4 deformation vibration and a HOH deformation vibration of the water molecule. The corresponding Raman band is weak and located at around 1660 cm−1 (Fig. 2). The bands located at 1473 and 1462 cm−1 in the infrared spectrum (Fig. 2) are due to the NH4 bending vibration (2 ). In the Raman spectra the same vibration is shown by a broad weak band at 1460 cm−1 . The antisymmetric stretching vibration 3 of the SO4 group is given by two strong bands in the infrared spectrum (1143 and 1098 cm−1 ). Corresponding bands in the Raman spectrum (Fig. 3) are of a weak intensity and are located at 1149, 1093 and 1063 cm−1 . One weak band in the infrared spectrum (992 cm−1 ) and one strong band in Raman spectrum (990 cm−1 ) were assigned to the 1 symmetric stretching vibration of the SO4 group. Spectral signatures of the 4 bending vibrations of the SO4 group are stronger in the infrared spectrum (wavenumbers 675, 620 and 604 cm−1 ) (Fig. 4) than in the Raman spectrum (652, 624 and 602 cm−1 ) (Fig. 5). The SO4 bending 2 vibration is given by the bands at 438 and 423 cm−1

Fig. 4. DRIFT spectrum of mineral boussingaultite (A) and nickelboussingaultite (B).

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4. Conclusions Two naturally occurring sulphate minerals containing NH4 + have been characterised by Raman and infrared spectroscopy. In both minerals, spectral signatures corresponding to their structural and chemical characteristics were observed, with some significant differences between the minerals. Observation of multiple bands in the antisymmetric stretching and in the bending regions suggests a lowering in symmetry of the sulphate ions. Of the two minerals, nickelboussingaultite exhibited a greater reduction of symmetry, a possible account for the nickel atoms substitution. It was possible to distinguish between the spectral manifestations of the OH and the NH4 groups in the stretching and bending regions of the Raman spectra of both minerals, which may be of importance, since life processes and decay of organic matter may be involved in the formation of these minerals. Fig. 5. Raman spectra of the SO4 bending vibration region (10× magnified): boussingaultite (A) nickelboussingaultite (B).

in the infrared spectrum and bands at 482, 457 and 440 cm−1 (Fig. 5) in the Raman spectrum. The multiplicity of these bands clearly indicates a reduction of symmetry of the sulphate group which is apparently greater than in case of boussingaultite: six bands in SO4 bending regions comparing to three bands in the Raman spectrum of boussingaultite. These changes as well as the shift in wavelengths of major bands suggest the influence of the nickel atoms that substitute magnesium atoms in the structure of mineral. In addition, the broad bands at wavenumbers ∼760, 675 and 520 cm−1 can be attributed to the torsion vibrations of water molecules. They can be observed solely in the infrared spectrum. The weak bands at 341 and 312 cm−1 can be also attributed to water vibrations and bands at 240 cm−1 and bands at lower wavenumbers in Raman spectrum are attributed to the lattice modes.

Acknowledgement This work was supported by the Grant Agency of Charles University (grant no. 133107) and by the grant from the Ministry of Education of the Czech Republic (MSM0021620855). This support is gratefully acknowledged. References [1] R.L. Frost, R.A. Wills, W. Martens, M. Weier, Spectrochim. Acta A 62 (2005) 869. [2] D.T. Vaniman, D.L. Bish, S.J. Chipera, C.I. Fialips, J.W. Carey, W.C. Feldman, Nature 431 (2004) 663. [3] M.E.E. Madden, R.J. Bodnar, J.D. Rimstidt, Nature 431 (2004) 821. [4] A. Wang, J.J. Freeman, B.L. Jolliff, I.M. Chou, Geochim. Cosmochim. Acta 70 (2006) 6118. [5] R.L. Frost, R.A. Wills, M.L. Weier, W. Martens, S. Mills, Spectrochim. Acta A 63 (2006) 1. [6] A.D. Fortes, P.M. Grindrod, S.K. Trickett, L. Vocadlo, Icarus 188 (2007) 139. [7] H. Winchell, R.J. Benoit, Am. Miner. 36 (1951) 590. [8] T.N. Margulis, D.H. Templeton, Zeitschrift für Kristallographie 117 (1962) 344. [9] F.C. Hawthorne, Am. Miner. 71 (1986) 1545. [10] H. Montgomery, E.C. Lingafelter, Acta Cryst. 17 (1964) 1478.