RIETVELD REFINEMENT AND VIBRATIONAL SPECTROSCOPIC STUDY OF ALUNITE FROM EL GNATER, CENTRAL TUNISIA

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Author: MOHAMED TOUMI 1, ALI TLILI 2, MOHAMED ESSGHAIER GAIED2 and MABROUK MONTACER2

RIETVELD REFINEMENT AND VIBRATIONAL SPECTROSCOPIC STUDY OF ALUNITE FROM EL GNATER, CENTRAL TUNISIA

MOHAMED TOUMI 1, ALI TLILI 2, MOHAMED ESSGHAIER GAIED2 and MABROUK MONTACER2

1Faculté des Sciences de Sfax, Département de Chimie, Laboratoire de l'Etat Solide, Route de Soukra, 3038-Sfax, Tunisie. 2Faculté des Sciences de Sfax, Département des Sciences de la Terre, Unité de Recherche GEOGLOB, Route de Soukra, 3038-Sfax, Tunisie.

Corresponding autor: Ali TLILI < alitlili@yahoo.fr,

Abstract Crystal structure of alunite (K0.72, Na0.28)Al3(SO4)2(OH)6 from El Gnater, central Tunisia, has been refined by the Rietveld method. Raman and infrared data of this mineral are also given in order to provide some further information about the mineralogy and chemistry of this alunite. The crystal system is trigonal, space group R m, with a = 6.9834(4) Å and c = 17.0899(11) Å. Final Rietveld refinement converged to Rp = 0.16, Rwp = 0.16, and RBragg = 0.07. In the alkalic site, the occupancy of potassium and sodium have been refined to 72 and 28% respectively. The Raman and infrared spectra have been investigated in order to improve previous assignments of the observed frequencies, especially for tetrahedral and octahedral vibration and OH group, which are discussed on the basis of the unit-cell group analysis and by comparison with previous observed wavenumbers of natrojarosite and synthetic alunite. Key word: alunite, El Gnater, Rietveld, Infrared, Raman

1. Introduction

The two end members alunite and natroalunite minerals are represented by the chemical formula (Kx, Na1-x)Al3(SO4)2(OH)6. Solid solution occurs between sodic and potassic end members, K>Na for alunite and K< Na for natroalunite. The crystal structure refinement of Hendricks [1] assigned alunite to the acentric space group R3m. However, has been proved subsequently that diffraction data of the alunite jarosite solid solution fit well with the centrosymetric space group R m [2]; [3]; [4]; [5]; [6]. According to these previous works, the structure of alunite is made of T-O-T layers, where T is the tetrahedral and O the octahedral layer. The octahedron is distorted being formed by two oxygen atoms, coming from the two adjacent SO4 tetrahedrons and four OH groups. Al lies at the centre of symmetry whereas the sulfur atom and the apical oxygen lie on the three-fold axis. The tetrahedron is also distorted and consists of three basal oxygen atoms (O2) and an apical one (O1). The M site (K+, Na+, H3O+, etc) is twelve-fold coordinated an is fairly regular, the surrounding anions (six oxygen (O2) and six OH groups [7]; [8]. This study is carried out on the alunite sample occurring at El Gnater, Kairouan district, located at the central Tunisia. The occurrence and geological setting of this alunite should be discussed elsewhere (Gaied, Tlili, Chaabani, Toumi and Montacer). The present work is intended to provide new data on the crystal structure of alunite and provide new insights on the attribution of the observed vibrational frequencies of this mineral, especially those related to the tetrahedral and octahedral motions.

2. Experimental Section

Potassium and sodium contents were determined by atomic absorption analysis, weight error of 2% and confirmed here by alkaline site occupancy determined from Rietveld structure refinement.

2.1 Raman spectroscopy

A Raman spectrum of a fragment of a compact block of polycrystalline alunite was recorded with a T 64000 Jobin-Yvon Multi-channel Spectrometer (in triple subtractive configuration, 1800 tr/mm grating) with a cooled CCD detector. An argon-krypton laser (coherent spectrum) with a power of about 40 mW power (on the sample) was used for the excitation (514.5 nm). Measurement was carried out at room temperature under a X50 LF objective microscope. The spectral steps typically are 0.7 cm-1.

2.2 FT-IR spectroscopy

The FT-IR spectrum was measured with the pellet technique, mixing 1.0 mg sample for a total weight (samples + KBr) of 200 mg. The spectrum was collected on a Perkin-Elmer FT-IR system PC spectrometer in the range 4000-400 cm-1 using 30 scans with 2 cm-1 spectral resolution.

2.3 X-ray powder diffraction

The X-ray powder diffraction pattern of alunite was collected by an 'Expert High Score plus PANalytical diffractometer (at INRST-Tunis) using monochromated CuK radiation. Diffraction intensity was measured between 3° and 150°, with a 2 step of 0.017° for 7 s per step. Data were analyzed by the Rietveld method [9] using the FULLPROF2000 code [10]). Peak profiles modelled with pseudo-Voigt functions [11].

3. Structure refinement

Starting structural data were taken from Menchetti et al. [3] for ideal KAl3(SO4)2(OH)6 alunite. However, the occupancy of K and Na atoms placed in 4a were set to 74 and 26%, i.e. to the composition determined by atomic absorption spectrometry. A scale factor, six background parameters, the unit-cell parameters, the zero-point correction, the atomic fractional coordinates, the isotropic displacement parameters, three profile parameters defining the pseudo-Voigt functions, and two asymmetry parameters were refined. The occupancies of potassium and sodium on the alkaline-atom site were refined but did not deviate appreciably from the initially set values. The refinement was stopped when the R-factors stabilised and the maximum shift-to-error ratio was less than 0.001. The final reliability factors, with 13 structural parameters and 17 profile parameters refinement, are Rp = 0.16, Rwp = 0.16. Table 1 illustrates the crystallographic characteristics and the refined profile parameters together with the reliability factors for El Gnater alunite. The observed, calculated and difference X-ray powder diffraction patterns are shown in Figure 1. Atomic positions with equivalent isotropic thermal parameters and occupancy factors for the alunite are given in Table 2. Interatomic distances and angles are presented in Table 3. The chemical composition of alkaline site K2O (9.15%) and Na2O (2.16%) determined by atomic absorption (Gaied, Tlili, Chaabani, Toumi and Montacer in prep.) is close to the composition determined here by alkaline site occupancy deduced from Rietveld structure refinement. Thus the Rietveld deduced ratios of K/Na for El Gnater alunite is 0.72/0.28, close to 0.74/0.26 obtained from chemical composition determined by atomic absorption spectrometry analysis. The structure formula of El Gnater alunite can be summarized as K0.72,Na0.28)Al3(SO4)2(OH)6). A projection of the atomic arrangement of ((K0.72,Na0.28)Al3(SO4)2(OH)6), along the b axis, is given in Figure 2. The structure correspond to AlO2(OH)4 octahedra (symmetry 2/m) and SO4 tetrahedra (3m) sharing corners to form an open structure which shows the existence of tunnels running along the a axis where alkaline (symmetry m) cations are located. Aluminium is coordinated in a single slightly distorted octahedron formed by four OH groups and two oxygen atoms from two separate SO4 groups. The distance between aluminium and oxygen is 1.996(3) Å and the Al-OH bond length is 1.903(4)Å. In the pure potassium alunite KAl3(SO4)2(OH)6, the Al-OH bond length is 1.864(15) Å and Al-O is 1.963(9) Å [2]. The Aluminium octahedrally coordinated sites share only corners. They are linked together by OH groups and to the (SO42-) tetrahedron by the two oxygen ions, such that each octahedral site is surrounded by four octahedral sites and two tetrahedral sites (Fig. 2). The sulfur atom has a distorted coordination tetrahedron with average S-O distance of 1.486 Å. The S-O distances are consistent with other alunite structures [3]; [12]). The S-O1 bond is considerably shorter than the other three S-O bonds, as reported in Table 2, because apical oxygen (O1) is not coordinate to any other cation. The alkaline Na/K atoms are twelve-coordinated, formed by six oxygen O2 (from six sulfate groups) and six OH groups. The distance between Na/K and oxygen (2.818(6) Å) and Na/K-OH bond length (1.831(9) Å) are in good agreement with those of alunite [3] and natroalunite [12].

4. Spectroscopic analysis

4.1 Group theory analysis

Factor group analysis by the correlation method developed by Fateley et al. [13], gives the distribution of irreducible representation, excluding acoustic modes. Γvib = 8A1g(R) + 3A2g(-) + 11Eg(R) + 4A1u(-) + 10A2u(IR) +14Eu(IR). The internal modes of the SO42- derive from the mode of the free tetrahedron with symmetries A1 (referred to as ν1), E(ν2), and F2(ν3 and ν4) of the corresponding Td point group. Table 4 shows the correlation diagram between the free SO42- group vibrations in Td symmetry and the lattice SO42- internal vibrations in D3d factor group symmetry through the C3 symmetry of one SO42- in the crystal. The correlation shows that six Raman and six infrared lines are predicted: Raman  3 stretching modes: 1ν1(A1g) ; 2ν3 (A1g, Eg).  3 deformation modes: 1ν2(Eg) ; 2ν4 (A1g, Eg). Infrared  3 stretching modes: 1ν1(A2u) ; 2ν3 (A2u, Eu).  3 deformation modes: 1ν2(Eu) ; 2ν4 (A2u, Eu). The internal modes of AlO2(OH)4 octahedra derive from the modes of the free Al(OH)6. For an Al(OH)6 ion having Oh symmetry, there are six normal modes of vibration, symmetric stretching modes ν1(A1g), asymmetric stretching modes ν2(Eg) and ν3(F1u), asymmetric bending mode ν4(F1u), symmetric bending mode ν5(F2g), and inactive mode ν6(F2u). If two of the OH group of Al(OH)6 are replaced by an oxygen atom, the symmetry is lowered from Oh to D4h ( trans - isomer). As a result, the selection rules are changed as show in Table 5. The AlO2(OH)4 trans-isomer (D4h) is expected to give three ν(AlOH) stretching (A1g + B1g +Eu) and two ν(AlO) stretching (A1g + A2u) modes. The correlation diagram between the free AlO2(OH)4 vibration in D4h symmetry and the lattice AlO2(OH)4 internal vibration in D3d factor group symmetry through the C2h symmetry of one AlO2(OH)4 in the crystal (Tab. 5), shows that 5 Raman (2A1g + 3Eg) and 5 IR (2A2u + 3Eu) active ν(AlO6) stretching vibration ν(AlO6) of the AlO2(OH)4 octahedra. The result is in agreement with that proposed by Breintinger et al. [14].

4.2. Data acquisition of infrared and Raman spectra

Infrared and Raman spectra of El Gnater alunite (K0.72,Na0.28)Al3(SO4)2(OH)6 (Figs. 3-4), are similar to those of K-Na alunite and K-Na jarosite reported in the literature [14], [15]; [16]. The tentative assignment of observed infrared and Raman vibration bands are listed in Table 6. In the high wavenumber region above 3000 cm-1, the infrared spectrum of alunite from El Gnater (Fig. 3) show very strong peak of OH stretching vibration laying at 3483 cm-1, in the intermediate position between the OH stretching position of alunite (3495 cm-1) [14] and that of natroalunite (3456 cm-1) [16]. Frost et al. [17] reported also that infrared OH stretching bands for K-alunite (3479 cm-1) is higher than Na-alunite (3454 cm-1), such that the substitution of K by Na decreases the wavenumber of OH stretching vibrations in Na-alunite compared to their K equivalent. This wavenumber shift has been reported for Raman spectra of silicates rock forming minerals especially for the solid solutions of trioctahedral and dioctahedral (K, Na)-mica [18]. The assignment of infrared band observed at 1637 cm-1 (Fig. 2) is not clear. Drouet et al. [16] suggested that this band is due to the HOH deformation. This band is also observed, but not assigned by Frost et al. [17], at 1628.9 for K-alunite and at 1643.9 cm-1 for Na-alunite. The band observed at 1629 cm-1 for jarosite was attributed to the bending motion of the surface absorbed water [19]. This band is may be related to the water absorbed by KBr. Group theory analysis predicts three stretching modes ν(SO4) (1 ν1 , 2 ν3) in each infrared and Raman spectra (Tab. 4). However, both Raman and infrared spectra show four bands respectively at (1025, 1080, 1150, 1188 cm-1) and at (1027, 1093, 1160, 1213 cm-1) (Figs. 3-4). Breintinger et al. [14] concluded that these group of bands are due to the ν1(SO4), ν3(SO4) (doublet) and O-H deformation. The strong band in the Raman spectrum (Fig. 4) observed at 1025 cm-1 is attributed to the symmetric stretching mode ν1(SO4). This band is observed at 1027 cm-1 in the infrared spectrum of alunite (Fig. 3) and it is reported for Raman spectra of K-alunite, Na-alunite, and ammonium alunite at 1026.4, 1027.0 and 1026.3 cm-1 respectively [17]. Infrared band of K-alunite was listed at 1026 cm-1 [17]. The single band for the SO42- symmetric stretching mode was observed at 1030 cm-1 in both infrared and Raman spectra of alunite [20]. Therefore, the fact that there is only one signal corresponding to the nondegenerate ν1(SO4) mode, observed in all studies, is consistent with the high symmetry R m space group model. The infrared and Raman spectra of K-alunite occurring at 1165 and 1158 cm-1 respectively and also the Raman band at 1152 cm-1 for NH4-alunite have been attributed to one of the ν3(SO4) antisymmetric stretching modes [17]. These authors assigned the whole Raman bands for K-alunite observed at 1228.4, 1194.2, 1158.0 and 1082.4 cm-1 to the ν3(SO4) antisymmetric stretching modes. Although factor group analysis (Tab. 4) predicts only two bands for the antisymmetric stretching ν3(SO4). Moreover Breintinger et al. [14] suggest that the band laying 1160 cm-1 can be attributed to OH deformation mode, because it was shifted to ~ 850 cm-1 for deutrated K-alunite. This shifting can not be suitable with the finding of Frost et al. [17]. Likewise the Raman bands observed, for El Gnater alunite, at 1080 and 1188 cm-1 and their infrared equivalents observed at 1093 and 1213 cm-1 can be attributed to the ν3(SO4). The Raman and infrared bands at 1150 and 1160 cm-1 respectively, are assigned to O-H deformation motions as well. This proposition is also consistent with the assignment of OH deformation reported by Serna et al. [20], which observed only one Raman band for of K-alunite at 1081 cm-1 and two infrared band at 1085 and 1225 cm-1. Several absorptions are also observed in the 400 to 1000 cm-1 region, where factor group analysis predicts:  Two Raman bands (A1g + Eg) and two infrared bands (A2u + Eu) for the asymmetric bending vibration (ν4)SO4.  One band in Raman (Eg) and one in IR (Eu) for the symmetric bending ν2(SO4).  Five active stretching ν(AlO6) vibrations of the AlO2(OH)4 octahedra for each Raman and infrared spectra.  The OH deformation modes for each spectrum. According to Drouet et al. [16], it is possible to assign the absorption bands observed in the infrared spectrum of natroalunite by comparison with the natrojarosite sample. The differences observed between the two spectra are apparently due to the change in octahedra vibrations between FeO6 and AlO6. The absorption bands assigned to FeO6 and ν4(SO4) vibrations are reported respectively at (477, 513, and 575 cm-1) and (668 and 628 cm-1) for jarosite [21]. Drouet et al. [16] concluded that the bands, observed at : 486, 515, and 592 cm-1 for natroalunite can be assigned to AlO6 octahedral vibrations, but they warn that the latter is surprisingly intense. The band observed at 631 cm-1 is attributed to asymmetric bending mode of sulfate group ν4 [16]. According to Breintinger et al. [14]the bands observed in infrared spectrum of alunite at : 681, 631, 602, 529, and 432 cm-1 are obviously valence vibrations ν(AlO6) of the AlO2(OH)4 actahedra. These authors concluded that, this infrared range is dominated by the strong valence vibrations of highly polar Al-O bands, which hidden the deformation modes δ(OH)// and ν4(SO4) expected in this domain. This assignment is supported by the shifting about 15 cm-1 of all bands on deutration, which is closely identical to the ratio ~ (m(OD)/m(OH))1/2. This ratio is right only where the central atom is not moving. The result proposed by Suresh et al. [22] agree with the preceding ratio, where Al-O valence vibrations of the Al(OH2)6 octahedra observed at 565 cm-1 in the RbAl(SO4)2.12H2O was shifted to 520 cm-1 with D-H substitution. This shifting agree as well with the calculated ratio (m(OD2)/m(OH2))1/2. The argument of Breintinger et al. [14], is not consistent for all stretching frequencies of XY6 octahedral (X = central atom and Y = ligand). In ν3, both X and Y atoms are moving, and subsequently the mass effect of the X atom cannot be ignored completely. But, for ν1 mode, the central atom is not moving and the mass effect of the X atom can be ignored. The infrared band of alunite observed at 631 cm-1, have been attributed to ν(AlO6) of the AlO2(OH)4 octahedra [14]. This attribution is not accurate because ν1 is infrared inactive and all stretching frequencies of AlO6 of the AlO2(OH)4 octahedra, derive from ν3(AlO6) (Table 5). The ration 1.029 proposed by Breintinger et al. [14] is true only were the central atom is not moving at all. Taking into account that the central atom is moving (ν3 is infrared active) the ration of the reduced mass should be equal to 1,08. Therefore the attribution of the band at 628 cm-1 for jarosite and the band at 631 cm-1 for natroalunite to ν4(SO4) by Drouet et al. [16] is convincing, since they appear roughly at the same position. In the present study the infrared absorption bands of alunite in the 400 - 1000 cm-1 region are observed at : 702, 691, 673, 668, 629, 600, 550, 525, 515, 488, 471, 461, 435 cm-1. According the discussion above, the strong bands observed at 668 and 629 cm-1 can be assigned to the ν4(SO4) and the band observed at 673, 525, 515, 488, 471 and 461 cm-1 are due to the ν(AlO6) of the AlO2(OH)4 octahedra corresponding apparently to five stretching and one bending motion of ν(AlO6). The band observed at 600 cm-1 must be assigned to the OH deformation modes according to the attribution proposed by Ross [23] for K-alunite (602 cm-1). Serna et al. [20], indicate that this deformation band occurs at 602 cm-1, 600 cm-1 and at 600 cm-1 for (K-alunite), (Na-alunite) and (H3O+- alunite) respectively. The 435 cm-1 band may be related to ν2(SO4) vibrations, but according to the most literature data, some uncertainties concerning the attribution of this band are not yet solved. Raman spectrum of El Gnater alunite (Fig. 4) in the 400 - 1000 cm-1 spectral region, shows five bands at : 651, 640, 561, 509 and 486 cm-1. These positions are close to those for alunite reported by Breintinger et al. [14] at : 655, 610, 561, 510 and 486 cm-1. By comparison of the Raman spectra of alunite, deutrated alunite and their hydroxylated equivalent (KAl3(SO4)2(OH)6), it's possible to assign the 651 and 640 cm-1 bands to the triply-degenerate asymmetric bending mode (ν4) of sulfate group and the band observed at 486 cm-1 to the symmetric bending motions ν2(SO4). Thus the number of deformations modes ν4 and ν2 of SO4 is consistent with the prediction of the factor group analysis (Tab. 5). The two bands observed at 509 and 561cm-1 have been attributed to the two δ(OH)// modes. Frost et al. [17] attributed the bands observed at 588.2, 505, 481.7 cm-1 (K-alunite) and at 517, 486.2, 485.5 (Na-alunite), to the bending vibration of ν2(SO4). Factor group analyses (Tab. 4) predicts only one Raman band for the ν2(SO4). Then, the attribution of the bands laying around 500 cm-1 [17], to ν2(SO4) bending may be not accurate. In low wavenumber region 3 - 400 cm-1, the Raman band at 389, 400 and 381 cm-1 for K-alunite, Na-alunite and NH4-alunite respectively, has been attributed to Al-O stretching vibrations [17]. Likewise the weak bands observed at 383, 392 cm-1 for Raman spectrum of El Gnater alunite (Fig. 4) can be attributed to Al-O stretching vibrations. According to Frost et al. [17], the strong band at 235, 238 and 235 cm-1 for K-alunite, Na-alunite and NH4-alunite respectively, has been attributed to OH---H hydrogen bands. This band is observed in our Raman spectrum at 234 cm-1. But this band can be also overlapped the Al-O vibrations. Since, the bands at 241, 245, and 243 cm-1 for K-alunite, Na-alunite and H3O+-alunite respectively have been assigned by Serna et al. [20] to Al-O stretching vibrations. The attribution of the observed band at 160, 154, 28, 10 and 5 cm-1 is confused. Since the vibration bands from translational and librational mode of SO42- associated with the translational cations are expected below 250 cm-1 [20].

5. Conclusion

The determination of the structure of natural alunite from El Gnater, central Tunisia, (K0.72,Na0.28)Al3(SO4)2(OH)6 using Rietveld structure refinement in the space group R m, allows a final observed factor RBragg = 0.07, for 214 reflections. An accurate determination of the occupancy of alkaline site and the unit cell parameters, a = 6.9834(4) Å and c = 17.0899(11) Å, have been proposed as well. In addition the spectroscopic study of the same specimens, based on the factor group analysis and on the previous published data of natural and synthetic alunite, let as to clarify the attribution of the observed bands near 1000cm-1 to ν(SO4) (1ν1 , 2ν3) and δ(OH). In general, the infrared bands of these vibrations have a position rather close to that of Raman vibrations. In the medium wavenumber region 400-1000 cm-1 the assignment of peaks is complicated by the occurrences of several types of vibration bands: three bending motions of SO4 (2ν4 + 1ν2), five stretching vibrations of νAlO6) and some OH deformation vibrations. The two expected bands of bending motions of ν4(SO4), corresponding to 651-640 cm-1 for Raman and 668-629 cm-1 for infrared, can be recognised as well as Raman and infrared ν2(SO4) vibrations laying at 486 and 435 cm-1 respectively. In this infrared wavenumber range, the band of alunite 673, 525, 515, 488, 471 and 461 cm-1 can be assigned to octahedral vibration ν(AlO6).The infrared peaks at 600, 550 cm-1 and the Raman peaks 561, 509 cm-1 can be attributed to the OH deformation.

References [1] S.B. Hendricks, Am. Mineral., 1937, 22, 773. [2] R.Wang, W. F. Bradley, H. Steinfink, Acta Crystallogr., 1965, 18, 249. [3] S., Menchetti, and C. Sabelli, N. Jb. Miner. Mh., 1976, 9, 406. [4] T. Kato, & Y. Miúra, Mineral. J., 1977, 8, 419. [5] J.T. Szymański, Can. Mineral., 1985, 23, 659. [6] C.L. Lengauer, G. Giester, and E. Irran, Powder Diffr., 1994, 9, 265. [7] H. Schukow, D.K. Breitinger, T. Zeiske, F. Kubanek, J. Mohr, R.G. Schwab, and Z. Anorg, Allg. Chem., 1999, 625, 1047. [8] G.A.Lager, G.R. Swayze, C.K. Loong, F.J. Rotella, J.W. Richardson, and R. Stoffregen, Can. Mineral., 2001, 39, 1131. [9] H.M. Rietveld, Acta Crystallogr., 1967, 22, 151-152. [10] J. Rodrigaez-Carvajal, 1994, "Program FULLPROF", version 2.6.1. [11] L.W. Finger, D.E. Cox, and A.P. Jephcoat, J. Appl. Crystallogr., 1994, 27, 892. [12] K.Okada, J.-I Hirabayashi, and J.Ossaka, N. Jb. Miner. Mh., 1982, 12, 534. [13] W.G. Fateley, F.R. Dollish, N.T. McDevitt, and F.F. Bentley, 1972, New York, [14] D. K. Breitinger, R. Krieglstein, A. Bogner, R. G. Schwab, Th. H. Pimpl, J. Mohr, and H. Schukow, J. Mol. Struc., 1997, 408/409, 287. [15] C. Drouet, and A. Navrotsky, Geochim. Cosmochim. Acta, 2003, 67, 2063. [16] C. Drouet, K. L. Pass, D. Baron, S. Draucker, and A. Navrotsky, Geochim. Cosmochim. Acta, 2004, 68, 2197. [17] R.L. Frost, R.A.Wills, M.L. Weier, W.Martens, J.T. Kloprogge, Journal of Molecular Structure, 2006, 785, 123. [18] A. Tlili, D.C. Smith, J.M. Beny, and H.A. Boyer, Mineral. Mag., 1989, 53, 165. [19] P. Makreski, G. Jovanovski, and S. Dimitrovska, Vibrational Spectroscopy, 2005, 39, 229. [20] C.J. Serna, C. Parada Cortina, J.V.G. Garcia Ramos. Spectrochim. Acta, 1986, 42A, 729. [21] D. Baron, and C. D. Palmer, Geochimi. Cosmochim. Acta, 1996, 60, 185. [22] G. Suresh, R. Ratheesh, R. S. Jayasree, V.U. Nayar, G. Keresztury, Journal of solid state chemistry, 1996, 122, 333. [23] S.D. Ross, Sulfates and other Oxy-anions of Group VI., In Farmer, V.C., The infrared spectra of minerals, Ed. V.C. Farmer, Min. Soc. London, 1974, 539pp.

Caption of figures and tables

Fig. 1 : Rietveld plots of alunite (K0.72,Na0.28)Al3(SO4)2(OH)6 from El Gnater. Observed (point), calculated (line) and difference (lower) profiles are shown. Vertical tick marks refers to the position of calculated Bragg reflections.

Fig. 2 : Projection of crystal structure of alunite from El Gnater, onto the (b, c) plane, showing the SO4 tetrahedron and the AlO2(OH)4 octahedron, while alkaline atom (K/Na) are shown in ball format.

Fig. 3 : Infrared spectrum of El Gnater alunite.

Fig. 4 : Raman spectrum of alunite from El Gnater.

Table 1 : Details of Rietveld Full-Profile refinement for El Gnater alunite (K0.72,Na0.28)Al3(SO4)2(OH)6.

Table 2 : Fractional atomic coordinates, occupancy factors, and temperature factors for El Gnater alunite. Standard deviations are given in parentheses.

Table 3 : Bond lengths (Å) and selected angle (°) for alunite ((K0.72,Na0.28)Al3(SO4)2(OH)6) from El Gnater. Standard deviations are given in parentheses.

Table 4 : Correlation scheme for the internal vibrations of SO42- group.

Table 5 : Correlation diagram for the AlO2(OH)4 octahedron.

Table 6 : The wavenumber, intensity and assignment of the majority of the bands in the infrared and Raman spectra of alunite (K0.72,Na0.28)Al3(SO4)2(OH)6.

Fig. 1 : Rietveld plots of alunite (K0.72,Na0.28)Al3(SO4)2(OH)6 from El Gnater. Observed (point), calculated (line) and difference (lower) profiles are shown. Vertical tick marks refers to the position of calculated Bragg reflections.

Fig. 2 : Projection of crystal structure of alunite from El Gnater, onto the (b, c) plane, showing the SO4 tetrahedron and the AlO2(OH)4 octahedron, while alkaline atom (K/Na) are shown in ball format.

Fig. 3 : Infrared spectrum of El Gnater alunite.

Fig. 4 : Raman spectrum of alunite from El Gnater.

Table 1 : Experimental details and miscellaneous data of the Rietveld refinement of (K0.72,Na0.28)Al3(SO4)2(OH)6. Diffractometer Radiation Receiving slit [°] Angular range [2] Step scan increment [2] Count time [sec/step] Miscellaneous Space group Cell parameters [Å] Volume (Å3), Z Number of reflections Number of structural parameters Number of profile parameters Half width parameters Peak shape,  Zero point [2] Asymmetry parameters Reliability factors PANalytical 'Expert High Score plus CuK, 45kV, 40 mA 0.01 3-150 0.017 7 Room temperature, no sample rotation R m (N° 166) a = 6.9834(4) Å, c = 17.0899(11) Å 721.78(8), 3 214 13 17 U = 0.043(3), V = -0.01(1), W = 0.013(1) Pseudo-Voigt, 0.58(1) 0.121(2) P1 = 0.076(7), P2 = 0.048(1) Rp = 0.16, Rwp = 0.16, RBragg = 0.07, Rf = 0.08 Rexp = 0.04, 2 = 0.05

Table 2 : Fractional atomic coordinates, occupancy factors, and temperature factors for El Gnater alunite. Standard deviations are given in parentheses. Atom Wyck Occ x/a y/b z/c (Å)2 Na 3a 0.28(3) 0 0 0 1.9(1) K 3a 0.72(3) 0 0 0 1.9(1) Al 9d 1 0.5 0 0.5 1.3(2) S 6c 1 0 0 0.3056(2) 1.8(1) O(1) 6c 1 0 0 0.3917(2) 0.3(1) O(2) 18h 1 0.7814(3) 0.2186(3) 0.0560(2) 0.8(1) OH 18h 1 0.1300(3) 0.8700(3) 0.1375(5) 0.4(1)

Table 3 : Bond lengths (Å) and selected angle (°) for alunite ((K0.72,Na0.28)Al3(SO4)2(OH)6) from El Gnater. Standard deviations are given in parentheses. _________________________________________________________________________ (K,Na) - O(2)  6 2.818(6) S - O(1) 1.468(3) (K,Na) - OH  6 2.831(9) S - O(2)  3 1.492(4)

Al - OH  4 1.903(4) O(1) - S - O(2)  3 108.8(4) Al - O(2)  2 1.996(3) O(2) - S - O(2)  3 110.1(3)

OH - Al - O(2)  4 93.3(4) OH - Al - OH  2 92.4(3) OH - Al - OH  2 88.6(9) OH - Al - O(2)  4 87.7(5) _________________________________________________________________________

Table 4 : Correlation scheme for the internal vibrations of SO42- group.

Table 5 : Correlation diagram for the AlO2(OH)4 octahedron.

Table 6 : The wavenumber, intensity and assignment of the majority of the bands in the infrared and Raman spectra of El Gnater alunite (K0.72,Na0.28)Al3(SO4)2(OH)6.

Raman IR Assignments Raman IR Assignments 3483 vs ν(OH) 509 m δ(OH) 1637 w δ(HOH) 488 w ν(AlO6) 1188 w 1213 vs ν3(SO4) 486 m-s ν2(SO4) 1150 e 1160 e δ(OH) 471 w ν(AlO6) 1080 w 1093 vs ν3(SO4) 461 vw ν(AlO6) 1025 vs 1027 s ν1(SO4) 435 s ν2(SO4) 702 vw 392 m-w ν(AlO6) 691 vw 383 m-w ν(AlO6) 673 e ν(AlO6) 234 s δ(OH---H) 651 s 668 vs ν4(SO4) 160 vs 640 w 629 vs ν4(SO4) 154 m-s 600 vs δ(OH) 28 w 561 e 550 m δ(OH) 10 vs 525 m 5 s 515 m

s : strong; m: medium; w : weak ; e : shoulder ; v : very; m-s : medium to strong; m-w : medium to weak.