A. BEN HAJ AMARA. L.P.M., Facultd des Sciences de Bizerte, 7021 Zarzouna, B&erte, Tunisia

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1 Clay Minerals (1997) 32, X-ray diffraction, analysis of infrared and TGA/DTG hydrated nacrite A. BEN HAJ AMARA L.P.M., Facultd des Sciences de Bizerte, 721 Zarzouna, B&erte, Tunisia (Received 25 July 1995; revised 19 November 1996) ABSTRACT: An homogenous 8.4 A hydrate was obtained after washing intercalated KAcnacrite. X-ray diffraction analysis based on comparison of the experimental and calculated profiles enabled the amount of water (one molecule per SizAlzOs(OH)4) and the z coordinate along c* (6.5 A) to be determined. The hydration state was accompanied by a decrease in the coherent domain along c* in the order nacrite > KAc-nacrite > H2-nacrite. The IR spectrum of the hydrated nacrite showed an evolution of the structure with a shift of v(oh) from 372, 3649 and 363 cm -1 to 369, 3641 and 362 cm -1, respectively for the nacrite hydroxyls. The v(oh) stretching bands of the interlayer water appeared at 362 and 3545 cm -1 and their bending band at 1655 cm -1. The TGA/DTG analysis of the air-dry hydrated nacrite showed a loss of water at 245~ the weight loss (1 molecule per Si2A12Os(OH)4) corresponding to the interlamellar water, in agreement with that determined by XRD. Potassium acetate (KAc) complexes of kaolin minerals have been the subject of several investigations (Wada, 1965; Deeds et al., 1966; Wiewiora & Brindley, 1969). These studies showed that for each type of kaolin mineral, the removal of the intercalated KAc by washing with water gives rise to different products such as: a fully hydrated form with a spacing of 1.1 A for halloysite; a partially hydrated form for kaolinite; an anhydrous form for dickite, and a stable hydrated nacrite with a basal spacing of 8.35,~. A change of the original structure order of the kaolinite was observed by Wiewiora & Brindley (1969). Hydrated kaolinite has been reported by Costanzo et al. (198, 1984a) who investigated its preparation and properties. In their initial report, a 1 A hydrated kaolinite was obtained by the substitution of F for OH at the edge of the silicate layers of kaolinite. Three related phases were also described, an 8.4,~ hydrate, an 8.6 ]~ hydrate and a 1 ~, quasi-stable hydrate (Costanzo et al., 1982, 1984b). Costanzo et al. (1984a) and Raythatha & Lipsicas (1985) reported the preparation of the 1 A and 8.6,~ phases with no fluorination reaction. More recently, Tunney & Detellier (1994) reported the preparation of the 8.4A hydrated kaolinite obtained by washing the intercalated ethylene glycol kaolinite. In these investigations the number of water molecules per unit-cell was determined by using either differential scanning calorimetry, XRD, crystal chemistry, IR, TGA and NMR (Costanzo et al., 1984a; Lipsicas et al. 1985), absorption-desorption isotherms of water vapour (Deeds et al., 1966) or TGA (Wada, 1965; Tunney & Detellier, 1994). The aim of the present work was to determine the water position along the normal to the (a,b) plane of an air-dried hydrated nacrite using a method based on the comparison of the experimental 1 reflections obtained from an oriented powder pattern with those calculated from structural models. This method, applied to a hydrated nacrite enabled the determination of the amount of water per unit-cell and the positions of the water in the interlamellar space with an accuracy better than 7%. This study is completed The Mineralogical Society

2 464 A. Ben Haj Amara QUANTITATIVE DETERMINATION OF THE NUMBER AND LEVEL OF THE INTERCALATED WATER BY XRD The diffracted intensity for a unit-cell along the rod of reciprocal space is given by the relation (Reynolds, 198; Drits & Tchoubar, 199) c b J a, i i I JO > U I 3O lool(o) = k pl Fo2ot() Got() (1) where is the Bragg angle, k is a constant and pl is the Lorentz-polarization factor. pl = 4(1 + cos 2 2)/sin2 where ~ is the powder ring distribution factor (Reynolds, 1986) and takes the form: O= [~} [[-~]erf(q)-[~](1-e-c~)} FIG. 1. X-ray diffraction patterns for nacrite (a), the intercalated AcK-nacrite (b) and the hydrated nacrite(c); Cu-K~t radiation. by infrared (IR) spectroscopy and TGA/DTG analysis. METHODS The hydrate was prepared from Tunisian nacrite (Jbel Slata, Kef) (Ben Haj Amara et al., 1995); 2 g of clay were ground with 3.5 g of KAc and kept in 2 ml of a saturated KAc solution for several days until the intercalation was completed (Fig. 1). The particle size >5 gm was used for XRD and <2 ~tm for the other methods. The resulting KAc complex was washed with water and air dried (Fig. 1). X-ray diffractograms of oriented powder were obtained with a Philips PW171 diffractometer equipped with a horizontal goniometer and a vertical objectplane, using Ni-filtered Cu-Kt radiation. The intensities were measured every.2 ~ (2) as the number of counts registered in a period of 1 s. Slits of incident and diffracted beam were used with an angular aperture of.25 ~ The number of l reflections obtained by this method was nine. Infrared spectra were recorded with a Perkin- Elmer spectrophotometer model 983G; compounds were examined in KBr pressed disks under air-dry conditions. The TGA/DTG thermograms were obtained at ambient temperature using a SETARAM 24 instrument. where Q F and F = ~12 + F x/2~* sin F1, F2 are the divergence of incident and diffracted beam respectively; o* is the standard deviation of the orientation function, which was determined by using the technique established by Reynolds (1986). Figure 2 shows a plot of Ln [I())] vs. )2 for the hydrated nacrite. The excellent fitting of the data into a straight line shows that the orientation function was indeed Gaussian. These results are in agreement with those of Taylor & Norrish (1966), Lippman (197) and Hall et al. (1983) who used different experimental arrangements and concluded that a Gaussian function explains the orientation ~7 r" J 6 ~ 4 > (.2 FIG. 2 Plot intensity as a function of tilt angle [Ln(l(to)) vs. )2; (3"* is the standard deviation of the orientation function.

Analysis of hydrated nacrite 465 state of well-oriented clays. The value of o*, obtained from the slope of the straight line is 5 ~. This value indicates a well-oriented sample.

3 Analysis of hydrated nacrite 465 state of well-oriented clays. The value of o*, obtained from the slope of the straight line is 5 ~. This value indicates a well-oriented sample. The structure factor of the unit-cell is Foo() and takes into account the structural formula of the mineral, the number of water molecules per unitcell and their z coordinate (A) in the perpendicular direction to the layer. The interference function is Goo() which depends on the stacking mode along the normal to the (a,b) plane and the average of the coherent domain (Ben Brahim et al., 1983; Drits & Tchoubar, 199). G() takes the form: Gool( ) = 1 + 2Re (offm')e ~-~ (1-2x282s2)nexp(2xnSdo2)) where co(m) is the statistical distribution of the crystallite thickness along c*, S is the modulus of the scattering vector: S = 2sin/L, and ~52 is the mean square deviation of the basal period fluctuations. The presence of this type of defect increases the diffuse background and decreases the intensity of the individual reflections. This implies that the distance doo2 is an average period about which small fluctuations occur (Drits & Tchoubar, 199). Thus the true distance is dt = doo In this work the ~52 value is equal to.4 A, then the yield reduction of the,1 reflection is <1% and 9% of the,18 reflection. Then the interference function takes the elementary form: G() (M) sin 2 (MxSdoo2) Z.., M sin2 (xsdoo2) M The analysis of the basal reflections allows determination of the structural characteristics of the hydrated nacrite. In practice, comparison of the experimental diffractograms with synthetic powder patterns calculated from expression (1) permits the elaboration of a structural model with explicit reference to the following parameters. (1) The basal period, the mean number of layers per stacking and their statistical law of distribution as shown by expression of Goo(). Indeed, the maximum of this function and the FWHM are related to the basal spacing and the thickness of the coherence domain respectively (Reynolds, 198; Drits & Tchoubar; 199). (2) The number and the z coordinate of water molecules per unit-cell which are deduced from the ratios 1oo~(l = 4, 6, )llooz. Indeed, the examination of expression (1) shows that the ratio loo/loo 2 represents the structure factor ratio IFooll2/IFoo212 which is sensitive to the chemical composition of the unit-cell and the z coordinates of atoms in it. We note that the l reflections of K~,1 and K~,2 are not resolved even for the high values and an average of K~, (1.5418,~) was chosen in order to resolve the theoretical data. Analysis of the profile of the OOl reflections The data given in Table 1 indicate that there were no anomalies concerning the width at half maximum intensities (FWHM) for all 1 reflections. The examination of l reflection profiles shows a single basal spacing of 8.42,~ which is in agreement with the positions of all reflections but not with the apparent experimental dot periods. This is due to the slope of the square of the structure factor (Reynolds, 1968; Ross, 1968; Tettenhorst & Robertson, 1973; Ben Brahim et al., 1983). The ratio values loo2/loot (1 = 4, 6, 8 and 1) are different from those obtained by Wada (1965) and Deeds et at. (1966). In the first step, five reflections (2, 4, 6, 8 and,1) were used to determine the number TABLE 1. Parameters of apparent coherent domain, basal spacing for l reflections and experimental Ioo2/Iooi (l = 4, 6, 8 and 1) ratios of the air dried hydrated nacrite ,1 loo2/ioot (ldoo/)app Mapp Mapp is the apparent number of layers calculated from Sherrer formula and (ldoot)app is the apparent basal period.

4 466 A. Ben Haj Amara and the z coordinates of the intercalated water molecules. The z coordinates of the nacrite used in this work were taken from the Zheng & Bailey (1994) refinement. The origin is placed on the sheet of surface oxygen atoms of the unit-cell. Expression (1) was used to calculate the intensities for the 1 reflections. The calculated values of the ratio (loo2/loo4, loo2/loo6, loo2/loo8, Ioo2/loo.lO) in terms of the number of water molecules (,.5 or 1 oper Si2AlzoOs(OH)4) and their coordinate z (,~) (5 A < z < 7 A) are given in Fig. 3. These results show that all ratios are sensitive to the number of water molecules per unit-cell. A similar study of these ratios for a fixed number of water molecules per unit-cell revealed the influence of their coordinates, e.g. Fig. 3 shows that the ratios 12]I6 and loo2/loo8 are very sensitive to the water molecule positions. It therefore appears that the use of the relative intensities of the five recordable l reflections of the hydrated nacrite is sufficient to determine its hydration parameters (number of water molecules and their z coordinates per unitcell) without any ambiguity. The number of layers per stacking M and their distribution law ~(M) are determined from the G() function. To confirm these parameters, nine reflections were used. The best agreement between experimental and theoretical patterns for the nine 1 reflections is given in Fig. 4. The resulting structural parameters are as follows: (a) M = (16 1) layers and corresponds to an average diffracting domain of L = ( A). The ~(M) used is symmetric around 16 layers and extends from 1 to 32 layers. It was 7 1oo2/I4 OH 2 ~ 6- I~H~O 5H2 4 Ioo2/Ioo6 H2 ~' 2 IH loo2/loo8 H2 5-4 IH2 3 loo2 ~loom H2 ~~1 H 2 H2 5 6! 7 5 O >z(a), 6 FIG. 3. Evolution of the relative intensities of the 1 reflections as a function of the quantity of the intercalated water and their z ordinates (in.~) along c*.

5 Analysis qf hydrated nacrite o 4 2O 5 2O, OO6 o ~- 3 J 2,4 i I i,4 8 ~5 el I 5O ~ 12 >~ FIG. 4. Agreement between experimental (... ) and calculated (full lines) of the 1 reflections. The calculated intensities are normalized to the experimental intensities; Cu-K~ radiation. observed that the coherence domains decrease from untreated to hydrated nacrite (515,~ for nacrite, 39 A for KAc-nacrite and 135 A for the hydrated nacrite). The decrease of the coherence domain as observed by Wiewiora & Brindley (t969) in kaolinite is due to the KAc and water treatments. (b) The water content is one molecule per Si2AI2Os(OH)4 and situated at 6.5 _+.5 A (z = corresponds to the base of the tetrahedral sheet). This number of water molecules is identical to that determined by Deeds et at. (1966) and Costanzo et al. (1984a) but is different from that determined by Wada (1965) (.78 molecules per half unit-cell) in the hydrated nacrite and Tunney & Detellier (1994) (.6 molecules per half unit-cell) in the hydrated kaolinite. The reliability factor given by the formula R = Z(lo-lc)ls is equal to 7% (lo and 1~ are the integrated intensities of the observed and calculated 1 reflections, respectively). DIFFERENTIAL GRAVIMETRIC THERMAL ANALYSIS The TGA/DTG analysis of hydrated nacrite wet to the touch (Fig. 5) showed a two-stage loss of water at 18 and at 245~ The low-temperature (18~ endotherm is due to water out of the interlamellar space (when the hydrate is heated at 6~ the first loss is not observed). The second loss (245~ corresponds to the interlamellar water. The weight losses, determined with a precision better than 7%, correspond to 13 and 1 water molecules per Si2A125(OH)4, respectively. The third weight loss at 625~ is due to dehydroxylation. The number of interlamellar water molecules is in agreement with that determined by XRD. The measured bulk density of nacrite and of the air-dried hydrate are 2.54 and.61, respectively. This result indicates an increase in the porosity and confirms that the amount of water obtained by TGA is situated in the interlamellar space. INFRARED SPECTROSCOPY The IR spectrum of the nacrite (Fig. 6a) shows three bands attributed to v(oh) at 372, 3649 and 363 cm -1 similar to those previously reported by Farmer (1974). In the present investigation the bands at 372 and 3649 cm -1 were replaced by a single band at 369 em -1 in the IR spectrum of the air-dry hydrated nacrite (Fig. 6b). The band due to the inner surface hydroxyls, which typically appears at 363 cm -1 was displaced to the 3618 cm -~ indicating that change took place in the orientation and environment of the inner surface hydroxyls as the result of the presence of the water keyed in the ditrigonal hole (hole water). The same shift from 362 to 3612 cm -1 was observed by Costanzo & Giese (199) in the hydrated kaolinite. After drying at 6~ for 24 h, the IR spectrum of the hydrate (Fig. 6c) shows four bands at 3695, 364, 362 and 362 cm -1. Previous IR studies of halloysite (Tarasevich & Gribina, 1972) and hydrated

6 468 A. Ben Haj Amara o -2 ATG --~ 2 I I 4 I 6! 8~ 3 H2 ) T(~ 8OO ) ~ -4-6 /~4x 5~ (1H2) 627oc DTG.-.11 FIG. 5. The TGA/DTG thermal analysis of the air-dried hydrate. kaolinite (Costanzo et al., 1984a) indicated that hole water has its v stretching band between 3572 and 3586 cm ~ and its v stretching band between 3522 and 3555 cm -~ (the exact position of the band seems to depend on the state of hydration). Associated water (Costanzo et al., 1984a) has only a broad poorly resolved stretching band that can extend between 32 and 34 cm -~. Both types of water have their v bending bands near 165 cm -~. The IR spectrum of the homogeneous hydrate showed a vl band at 3545 cm -j and v2 band at 1657 cm -l. The v3 band appeared at 362 cm-j when the hydrate was heated at 6~ Heated at 1~ the water bands were shown at 359, 354 and 165 cm -l. This result is in agreement with one type of hole water which has its C2 axis normal to the layer (Costanzo et al., 1984a). In this orientation the water hydrogens point toward the basal oxygens and are able to bond to the basal oxygens. The lone pair electrons of water oxygen can accept hydrogen bonds from the hydroxyls of the opposite surface (Costanzo et al., 1984a). The IR spectrum of hydrated nacrite appears very different from the IR spectrum of the 8.35 A hydrated nacrite (Wada, 1965). Wada (1965) found that the OH-stretching band at 363 cm -~ due to the inner surface of nacrite was not perturbed by the hydration process, and he observed a moderately intense band at 358 to 359 cm -l assigned by the author to OH-O(H2) interaction in nacrite. Costanzo & Giese (199) found that the OH-stretching band at 362 cm -~ due to the inner O-H kaolinite was shifted to 3612 cm -J for their hydrated kaolinite (kao(f)-h2 8.4,~). They also observed a moderately intense band at 3655 cm -~ and a sharp peak due to the deformation vibration of water at 158 cm -1. The local environment of the intercalated water in nacrite seems to be similar to that of the kaolinite (kao-h2 8.4 A(EG)) reported by Tunney & Detellier, 1994 who observed the stretching band of water at 355 cm -~ and the deformation vibration at 1655 cm -1. In some cases, a sharp absorption band at 3599 cm -1 is also observed: this absorption band is related by Tunney & Detellier (1994) to the inhomogeneity in the bulk sample not detected by XRD analysis. The structural OH-deformation band was displaced from 935 to 97 cm -1. This would seem to indicate a strong interaction between intercalated water and the hydroxyl surface of nacrite. In contrast, the AI-OH deformation due to the inner OH group of nacrite observed at 912 cm -1 shifted to 95 cm -1 in the air-dry hydrated nacrite (99 cm -1 for the heated hydrate) indicates an interaction between the hydroxyls of the sheet and the oxygen of the intercalated water. The same change was observed by Wada (1965). The Si-O-AI stretching band shifted from 535 cm -1 to 555 cm -~. It was observed that the Si-O-Si stretching band at 47 cm -1 and the Si-O stretching bands at 112, 135 and 1 cm -j were slightly shifted. Any theory of H bonding suggests a dependence between frequency shifts of OH-stretching vibrations and the length R of H-bridge O-H...O. Some authors

7 Analysis of hydrated nacrite 469 b JJ+~Nacrite ~ ~ ~ ~ ~ ~+ I I I I i,~i+i i. * water +7/1\..+.H+O(air d+'> D'l~" ~ J~l n o... X<+: d Y t) 4 ) cm'l FIG. 6. Infrared spectra of the nacrite (a), air-dried hydrate (b) 6~ heated hydrate (c) and IO~ heated hydrate (d). assume that the repulsion between OH and the oxygen atom originates largely in the lone pair/lone pair repulsion of the oxygen atoms. On this basis, Bellamy & Owen (1969) suggested an empirical function in which the decrease of stretching frequency vibration of free OH (Av) vs. R is calculated from the Lennard-Jones 12-6 potential function. The formula was used for the calculation of hydrogen bond length in hydrated nacrite and kaolinite, e.g. the hydrogen bond lengths for 3586, 3545, 353 and 344 cm -1 are 2.87, 2.86, 2.85 and 2.79 A, respectively. If it is supposed that the bond length variation is related to the z coordinate variation, Az is still <.5,~ if 3586 < v(oh) < 3535 cm -1 and.15 ~, if 3545 < v(oh) < 344 cm-k These differences which exist between 8.4 hydrated nacrite and 8.4 A hydrated kaolinite can be correlated with the water position in the interlamellar space but to resolve this problem it is necessary to determine the exact position (xyz) of water and the stacking mode for all hydrate in the (a,b) plane. CONCLUSIONS The methods used indicate that the hydration process was accompanied by a decrease of the coherence domain along c* and an increase in the porosity (the densities before and after the hydration process are 2.54 and.61, respectively). Comparison of the experimental and simulated X- ray powder diffraction patterns elucidate the number of the intercalated water molecules (1 molecule per Si2A12OsO(H)4 ) and their z coordinate (z = 6.5,~). Extra stability of the intercalated water has been observed by TGA/ DTG. The IR spectroscopy of the hydrated nacrite indicates that one type of water molecule was intercalated having its hydroxyls oriented to basal oxygen surface of the tetrahedral sheet. The perturbation of all IR bands attributed to the hydroxyls were observed showing that the octahedral sheet was perturbed by the hydration process. No noticeable change was found for the tetrahedral sheet. REFERENCES Bellamy L.J. & Owen A. (1969) A simple relationship between the infrared stretching frequencies and the hydrogen bond distances in crystals. Spectrochim. Acta, 25, Ben Brahim J., Armagan N., Besson G. & Tchoubar C. (1983) X-ray diffraction studies on the arrangement of water molecules in a smectite. I- Homogeneous two-water-layer Na beidellite. J. AppI. Cryst. 16, Ben Haj Amara A., Ben Brahim J., Ben Ayed N. & Ben Rhaiem H. (1995) Prtsence de la nacrite dans des anciens gisements de Pb-Zn. Clay Miner. 31, Costanzo P.M. & Giese R.F. (199) Ordered and disordered organic intercalates of 8.4-A synthetically hydrated kaolinite. Clays Clay Miner. 38, Costanzo P.M., Clemency C+V. & Giese R.F. (198)

47 A. Ben Haj Amara Low-temperature synthesis of a 1-,~ hydrate of kaolinite using dimethylsulfoxide and ammonium fluoride. Clays Clay Miner. 28, 155-156. Costanzo P.M., Giese R.F. & Clemency C.V.

8 47 A. Ben Haj Amara Low-temperature synthesis of a 1-,~ hydrate of kaolinite using dimethylsulfoxide and ammonium fluoride. Clays Clay Miner. 28, Costanzo P.M., Giese R.F. & Clemency C.V. (1984a) Synthesis of a 1-A hydrated kaolinite. Clays Clay Miner. 32, Costanzo P.M., Giese R.F. & Lipsicas M. (1984b) Static and dynamic structure water in hydrated kaolinite. I- The static structure. Clays Clay Miner. 32, Costanzo P.M., Giese R.F., Lipsicas M. & Straley C. (1982) Synthesis of a quasi-stable kaolinite and heat capacity of interlayer water. Nature 296, Deeds C.T., van Olphen H. & Bradley W.F. (1966) Intersalation and interlayer hydration of minerals of the kaolinite group. Proc. Int. Clay Conf. Jerusalem, 2, Drits V.A. & Tchoubar C. (199) The modelization method in the determination of the structural characteristics of some layer silicates: internal structure of the layers, nature and distribution of stacking faults. Pp in: X-ray Diffraction by Disordered Lamellar Structures, Springer-Verlag, Berlin. Farmer V.C. (1974) The layer silicates. Pp in: The Infrared Spectra ~f Minerals (V.C. Farmer, editor). Mineralogical Society, London. Hall P.L., Harisson R., Hayes M.H.B. & Tuck J.J. (1983) Particle orientation distributions and stacking arrangements in size-fractionated montmorillonite measured by neutron and X-ray diffraction. J. Chem. Soc. Faraday Trans. 79, Lippman F. (197) Functions describing preferred orientation in flat aggregates of flake-like clay minerals and in other axially symmetric fabrics. Contrib. Mineral. Pet. 25, Lipsicas M., Straley C., Costanzo P.M. & Giese R.F. (1985) Static and dynamic structure of water in hydrated kaolinite - Part. II. Dynamic structure. J. Colloid Interf Sci. 17, Raythatha R. & Lipsicas M. (1985) Mechanism of synthesis of 1-A hydrated kaolinite. Clays Clay Miner. 33, Reynolds R.C. (1968) The effect of particle size on apparent lattice spacings. Acta Cryst. 24, Reynolds R.C. (1986) The Lorentz-polarization factor and preferred orientation in oriented clay aggregates. Clays Clay Miner. 34, Ross M. (1968) X-ray diffraction effects by non ideal crystals of biotite, muscovite, montmorillonite. Z. Kristallogr. Kristallogem. 126, Tarasevich Y.I. & Gribina I.A. (1972) Infrared spectroscopic study of the state of water in halloysite. Kolloidnyi Zh. 34, (in Russian). Taylor R.M. & Norrish K. (1966) The measurement of orientation distribution and its application to quantitative X-ray diffraction analysis. Clay Miner. 6, Tettenhorst R. & Robertson H.F. (1973) X-ray diffraction aspects of montmorillonite. Am. Miner. 58, Tunney J. & Detellier C. (1994) Preparation and characterization of an 8.4 A hydrate of kaolinite. Clays Clay Miner. 42, Wada K. (1965) Intercalation of water in kaolin minerals. Am. Miner. 5, Wiewiora A. & Brindley G.W. (1969) Potassium acetate intercalation in kaolinite and its removal: effect of material characteristics. Proc. Int. Clay Conf Tokyo, 1, Zheng H. & Bailey S.W. (1994) Refinement of the nacrite structure. Clays Clay Miner. 42,

RAMAN MICROPROBE SPECTROSCOPY OF HALLOYSITE

Clays and Clay Minerals, Vol. 45, No. 1, 68-72, 1997. RAMAN MICROPROBE SPECTROSCOPY OF HALLOYSITE R.L. FROST AND H.E SHURVELLt Centre for Instrumental and Developmental Chemistry, Queensland University