ЖУРНАЛ НЕОРГАНИЧЕСКОМ ХИМИИ, 2008, том 53, № 12, с. 1975-1983
СИНТЕЗ И СВОЙСТВА ^^^^^^^^^^
RIETVELD REFINEMENT AND VIBRATIONAL SPECTROSCOPIC STUDY OF ALUNITE FROM EL GNATER, CENTRAL TUNISIA
© 2008 r. Mohamed Toumi*, Ali Tlili**
*Faculté des Sciences de Sfax, Département de Chimie, Laboratoire de l'Etat Solide, Route de Soukra, 3038-Sfax, Tunisie **Faculté des Sciences de Sfax, Département des Sciences de la Terre, Route de Soukra, 3038-Sfax, Tunisie Поступила в редакцию 23.10.2007 г.
Crystal structure of alunite (K072, Na0.28)Al3(SO4)2(QH)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 3 m, with a = = 6.9834(4) À and c = 17.0899(11) À. 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 words: alunite, El Gnater, Rietveld, Infrared, Raman.
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  assigned alunite to the acentric space group R3m. However, it has been proved subsequently that diffraction data of the alunite jarosite solid solution fit well with the centrosy-
metric space group R 3m [2-6]. 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, Chaa-bani, Toumi and Montacer). The present work is intended
1 Corresponding author: Ali Tlili firstname.lastname@example.org.
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
Table 1. Experimental details and miscellaneous data of the Rietveld refinement of (K^^^^A^SO^ ■ 2(OH)6
Diffractometer PANalytical 'Expert High Score plus
Radiation CuKa, 45kV, 40 mA
Receiving slit [°] 0.01
Angular range [20°] 3-150
Step scan increment [20°] 0.017
Count time [sec/step] 7
Miscellaneous Room temperature, no sample rotation
Space group R 3 m (N° 166)
Cell parameters [A] a = 6.9834(4) A, c = 17.0899(11) A
Volume [A3], Z 721.78(8), 3
Number of reflections 214
Number of structural parameters 13
Number of profile parameters 17
Half width parameters U = 0.043(3), V = -0.01(1), W = 0.013(1)
Peak shape, n Pseudo-Voigt, 0.58(1)
Zero point [20°] 0.121(2)
Asymmetry parameters P1 = 0.076(7), P2 = 0.048(1)
Reliability factors Rp = 0.16, Rwp = 0.16, Reragg = 0.07, Rf = 0.08, Rexp = 0.04, & = 0.05
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 dif-fractometer (at iNRST-Tunis) using monochromated CuAa radiation. Diffraction intensity was measured between 3° and 150°, with a 29 step of 0.017° for 7 s per step. Data were analyzed by the Rietveld method  using the FULLPR0F2000 code . Peak profiles modelled with pseudo-Voigt functions .
3. STRUCTURE REFINEMENT
Starting structural data were taken from Menchetti et al.  for ideal KAl3(S04)2(0H)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 frac-
tional 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 K20 (9.15%) and Na20 (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 re-
RIETVELD REFINEMENT AND VIBRATIONAL SPECTROSCOPIC STUDY OF ALUNITE 1977 36000 r
20 40 60 80 100
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.
finement. 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(S04)2(0H)6.
A projection of the atomic arrangement of (K0.72, Na028)Al3(S04)2(0H)6, onto the (b, c) plane, is given in Figure 2. The structure correspond to Al02(0H)4 octahe-dra (symmetry 2/m) and S04 tetrahedra (3m) sharing corners to form an open structure which shows the existence
of tunnels running along the a axis where alkaline (sym-
metry3 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) A and the Al-OH bond length is 1.903(4) A. In the pure potassium alunite KAl3(SO4)2(OH)6, the Al-OH bond length is 1.864(15) A and Al-O is 1.963(9) A . The Aluminium octahedrally coordinated sites share only corners. They are linked together by OH groups and to the
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 в (A2)
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)
0(1) 6c 1 0 0 0.3917(2) 0.3(1)
0(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 (A) 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) х 6 2.818(6) S-O(1) 1.468(3)
(K,Na)-OH х 6 2.831(9) S-O(2) х 3 1.492(4)
Al-OH х 4 1.903(4) O(1)-S-O(2) х 3 108.8(4)
Al-O(2) х 2 1.996(3) O(2)-S-O(2) х 3 110.1(3)
OH-Al-O(2) х 4 93.3(4) OH-Al-OH х 2 92.4(3)
OH-Al-OH х 2 88.6(9) OH-Al-O(2) х 4 87.7(5)
(SO4 ) 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 A. 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 3, because apical
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