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FIRST YEAR ANNUAL REPORT – DELIVERABLE N. 13 Annual Report on JRA2 Task 1- Application of NMR techniques to the study of ceramics (UNI-PG) Eu-ARTECH Access, Research and Technology for the conservation of the European Cultural Heritage Integrating Activity implemented as Integrated Infrastructure Initiative Contract number: RII3-CT-2004-506171 Project Co-ordinator: Prof. Brunetto Giovanni Brunetti Reporting period: from June 1st 2004 to May 31st 2005 Project funded by the European Community under the “Structuring the European Research Area” Specific ProgrammeResearch Infrastructures action 1 First Year Annual Report on JRA2 Task 1- Application of NMR techniques to the study of ceramics (Responsible: UNI-PG) According to Annex I of the contract, the main aim of this study is to explain the structural changes with the temperature of an iron-rich clay, as preliminary step to clarify the firing processes of the ancient ceramic technology. To this purpose, illite-rich clay samples were characterized using a multi-techniques approach, i.e. EPR, SEM-EDS, XRPD, MAS and MQ-MAS NMR. During the firing in the temperature range 6001100 °C, four main reaction processes occur: dehydration, de-hydroxylation, structural breakdown and recrystallization. Dehydration of water interlayers, located close to the aluminium octahedral sites occurs at temperatures lower than 600 °C, while the de-hydroxylation occur in the 600-700 °C temperature range. At higher temperatures (800-900 °C), the structural breakdown of the illite is observed with the full iron oxidation to Fe3+, while in the temperature range of 1000-1100 °C an iron clustering occurs, with the appearance of ferro or ferri magnetic effects, due to the presence of large iron single crystals. Furthermore, the appearance of octahedral and tetrahedral sites of aluminium is discussed as function of the investigated temperatures. Introduction Clays form a class of technologically important materials having widespread industrial applications, including catalyst support in petrochemical industries. Clays are the major components of white-ware products, bricks, roof tiles and cements. Firing involves breaking down some phases and forming new ones. Consequently clay artefacts are considered as artificial rocks formed in a kiln.1 The thermal decomposition of clays is one of the most studied ceramic reactions. Moreover clays is an important constituent of new synthetic materials such as nano-composites for advanced applications. The growth of new phases, during and after firing, is related both to the firing temperature and to the composition of the micro-sites in which the phases grow. To optimize the processing of clays, the thermal decomposition mechanisms must be fully understood. In particular, as the clay structure breaks down, details on the structure and on the distribution of atomic species in the intermediate amorphous states should be obtained. To study the modification induced by firing an illite-rich clay from Deruta, a town in the region of Umbria (Italy), was chosen. In fact, Deruta has been for many centuries one of the most important centers for the production of ceramics in Italy. The quality of these ceramics is well known, being historical wares from Deruta, especially those belonging to the Renaissance period, exhibited in the most relevant museums of the world. Several factors encouraged Deruta to become a major producer of ceramics, one of these is the availability of suitable earth from which to form clay. In fact, the hills around Deruta are particularly rich in a pure strain of clay which also washes up along the shores of the close Tiber river. Therefore the aim of this study is to clarify the structure of this clay and to explain the structural changes involved in its firing, as a preliminary step to obtain information about the firing processes of the ancient ceramic technology. To this purpose illite-rich clay samples fired at different temperatures were characterized using a multi-techniques approach, i.e. by EPR, SEM-EDS, XRPD, MAS and MQ-MAS NMR. Previous studies on illite-rich clays2 using various techniques3 showed that four main reaction processes occur during firing: dehydration, dehydroxylation, structural breakdown and recrystallization. In the 350-400 °C temperature range, the interlayer water is driven off; dehydroxylation occurs between 450 and 700 °C; the irreversible structural breakdown occurs between 800 and 900 °C, with the formation of new phases at about 900 °C. For the sake of clarity the results obtained from each technique will be discussed separately. 2 Experimental Firing Procedure. The samples were prepared using the following procedure. To easily model the clay, it was plunged in water and then dried at room temperature for one day. Samples were fired in oxidant atmosphere at different temperatures: 600, 700, 800, 900, 1000 and 1100 °C, in a kiln of a ceramist’s workshop. After an 8-hour heating cycle, the temperature was kept constant for 2 hours at the final value. Finally, the samples were slowly cooled down to room temperature inside the kiln. Analytical investigations were performed using Atomic Absorption spectroscopy, scanning electron microscopy with energy dispersion X-ray spectrometry (SEM-EDS) and X-ray fluorescence spectrometry (XRF). Compositional Analysis. The chemical composition of the samples was determined by X-ray fluorescence spectrometry using a Philips, PW 1400 with a RhK radiation. The measurements were carried out on compressed powder pellets. The loss on ignition (L.o.I) was determined by heating at 1000 °C for 12 hours. Intensities were processed according to methods previously reported by Franzini, Leoni & Saitta. A quantitative determination of the clay iron content was also carried out by a Perkin Elmer Atomic Absorption spectrophotometer mod. 1100B. Acid digestion was performed using mixtures of HClO4/HF. The composition was 5.4 0.6 % of iron for all the samples. SEM-EDS. Samples were cut to obtain thin cross-sections which were coated with carbon at high pressure to make the surface conductive. Scanning Electron Microscopy investigation was performed using a Philips XL 30 operating at 20 kV and equipped with an EDAX DX4 energy dispersive spectrometer (EDS). Elements with atomic number higher than 10 were identified. The counting rate was kept close to 1500-1800 counts per second over the whole energy range. After peak acquisition, the intensity values were used for quantification, according to the ZAF procedure and to the “standardless” approach (namely, using computed pure element intensity factors). Analytical accuracy was checked using secondary standards, i.e. samples with known composition; an agreement better than 10 % was achieved. The analytical precision was better than 0.5 % for major elements, and better than 20 % for minor elements (with a concentration ranging from 0.3 up to 3-5 wt %). To study the morphology of the samples, back-scattered images (BSE) were collected. EDS compositional studies have been carried out on bulk and on grains of the samples. For the characterization of the bulk ten measurements, taking care to avoid the iron-rich grains, were performed scanning different areas with dimensions about 800x800 m2. Then the results were obtained using the arithmetic average. Instead, the areas used for the compositional studies of the grains were related to the dimensions of the grains. XRPD. X-ray powder diffraction patterns were collected in the 3-100° 2 range according to the step scanning procedure with CuK radiation on a Philips X’PERT APD diffractometer; a PW3020 goniometer equipped with a bent graphite monochromator on the diffracted beam, 0.5° for divergence and scatter slits and a 0.1 mm for receiving slit were used. The LFF tube operated at 40 kV and 30 mA. To minimize preferred orientations, the sample was carefully side loaded into an aluminium sample holder with an oriented quartz mono-crystal underneath. The diffraction patterns for quantitative analysis were collected in the 3–100° 2θ range with a 13 s/step data collection time. The diffraction patterns for qualitative analysis were collected in the 3-60° 2θ range with a 3 s/step data collection time. Microcrystalline silicon powder (200 mesh) was used as a reference standard and it was added to the samples in a Si / sample = 1/10 weight fraction ratio. EPR Spectroscopy. Electron paramagnetic resonance spectra were recorded at the X-band frequency, 9.1 GHz, at 297 K using a Bruker EMX EPR spectrometer, equipped with the EMX High Sensitivity Probehead. Samples were finely powdered, weighted, 124 mg, and inserted into 4 mm EPR quartz tubes. In order to perform the Fe3+ quantitative analysis, six samples were prepared containing alum (KAl(SO4)2*12H2O)/ferric-alum (Fe(NH4)(SO4)2*12H2O) mixtures with known amounts of Fe3+, namely : 9.65%, 7.72%, 5.79%, 3.86%, 1.93% and 0.37%. NMR spectroscopy. Samples, 130 mg, were inserted in 4 mm zirconia rotors sealed with Kel-F caps. All spectra were recorded on a Bruker ASX200 NMR spectrometer. The spinning rate was always 12 KHz. 27 Al NMR. 27Al MAS spectra were recorded at 52.15 MHz. The /2 pulse width was 3 s; number of scans (ns) = 800, time domain (td)=512 data points, recycle delay 60 sec.. The choice of this long recycle delay is by no means casual, but due to the presence of a long relaxing component. Before the relaxation measurement a careful match of the magic angle was performed. T 1 spin-lattice relaxation time was 3 measured using the Saturation Recovery sequence. These measurements were repeated twice at two different spin-rates, 5 and 12 KHz. The longest T1 component is 12 sec., while the other component is 0.05 sec.. 27 Al 1H CP MAS NMR spectra. The cross-polarization was performed applying the variable spinlock sequence “RAMP” 4. The ramp can be applied on the X or on the 1H channel. The ramp causes one of the channels to be spin-locked slightly off the Hartmann-Hahn condition, except in the middle of the ramp where the spin-lock is exactly matched at the Hartmann-Hahn condition. This method allows to overcome the motional modulation of the aluminium and proton coupling caused by spinning the sample at high rate. The ramp was applied on the 1H channel and, during the contact time , the amplitude of the spin-lock increased from 50% up to 100% of its maximum value. All spectra were recorded using the TPPM15 1H decoupling sequence, with a decoupling field strength of 75 KHz 5. CP-MAS spectra were recorded at different contact times , with ranging from 0.2 up to 20 ms. The signal in the cross-polarized spectra showed a maximum at = 0.5 ms, therefore this contact time was used in all spectra. 27 Al 3Q MAS NMR spectra6 - 9 were obtained with a two pulses sequence: p1 – t1 – p2 – aq; the phase cycling was composed of six phases for the selection of triple-quantum coherences. The pulse durations p1 and p2 were optimised to 3 s and 7 s respectively. The delay t1 between the two pulses was incremented regularly, t1= 6 s, in the 3Q dimension (f1), with a total of 100 increments, td1 = 100, and according to a TPPI acquisition scheme; 256 data points were used in the f2 dimension; 732 transients were recorded for each t1 increment. The ppm scale was referenced to 0 frequency in the f2 dimension and to 30 in the f1 dimension, using Al(H2O)63+ as an external reference. The 3Q MAS spectra were zero filled and Fourier transformed using 512 x 512 data points. The centres of gravity G1 and G2 of the 2D spectral ridges in the f1 and f2 dimension respectively can be used for estimating the isotropic chemical shift iso and the second order quadrupolar effect Pq while is the slope PQ ν 0 of the quadrupolar axis QIS: δ iso ξδ G2 p δ CG1 ξ p being δ CG1 δ G1 p and 2 9 4I2I 1 δ CG1 δ G2 ; 10 p 2 1 10 6 in the case reported here: p=3, I=5/2 and 0 =52.15 MHz and =3/4 19 3 ν 2 ; the QIS axis is defined by ν 1 ν 2 12 4 In the 3Q MAS spectra, the A axis is defined by ν 1 and the CS axis is defined by ν1 3ν 2 10. Spectral deconvolution. The deconvolution of 27Al MAS and 3Q MAS NMR spectra was performed using the dm2004 program 11 . Deconvolution of 27Al 1D MAS spectra. In the dm2004 program the Q-MAS ½ model was selected to model the central transition under infinitely fast MAS spinning. Each resonance was characterized by the following parameters: the isotropic chemical shift iso, the amplitude, the quadrupolar coupling constant Cq, the asymmetry parameter q and EM, a parameter used to apodize the theoretical lineshape. The starting values of iso and Cq were extracted from the 3Q-MAS spectra; then the best fit procedure was applied, obtaining the best fit values for all parameters and the integral of the resonances. The test version of dm2004 allowed us to fit the resonance due to the octahedral site. In fact, using the test version of dm2004 it was possible to take into account the presence of a chemical shift distribution using the Czjzeck model 12. According to this model the line to be fit was characterized by the following parameters: iso, Cq, q, EM, the amplitude, the width of the isotropic chemical shift distribution CS, and the parameter d; this parameter d, ranging between 1 and 5, for a Gaussian isotropic distribution of EFG is an exponent equal to 5 13. Deconvolution of 27Al 3Q- MAS spectra. The deconvolution of 3Q-MAS spectra was applied for deconvoluting the spectral ridges due to Al in tetrahedral sites. Before performing the deconvolution of the spectra, a shearing transformation was applied to align the anisotropic lines parallel to the f2 axis14. In modelling multi-dimensional experiments some problem arises from the large number of variable parameters to be taken into account. Therefore we started fitting the corresponding 1D MAS spectrum and 4 then we used the obtained parameters as starting parameters in the deconvolution of the 3Q MAS spectrum. In addition to the parameters which characterize a resonance in a 27Al 1D MAS spectrum, a parameter was added to compute the two dimensional spectrum, that is the line-width in the f1 dimension. 29 Si NMR. MAS NMR spectra were recorded at 39.76 MHz. The /2 pulse width was 4 s, number of scans (ns) =4000, recycle delay = 60s, time domain (td)=512 data points. CP-MAS spectra were obtained using the following parameters: /2 pulse width 3.5s, contact time 1 ms, recycle delay 3s. The ppm scale was referenced to tetramethylsilane (TMS). To model each resonance, the deconvolution of 29Si MAS spectra was performed using the dm2004 program11. The Gaussian/Lorentzian model was selected. Each resonance was characterized by the following parameters: amplitude, position, the width at half height and a function (xG/(1-x)L) of the Gaussian/Lorentzian ratio (1 for a Gaussian lineshape and 0 for a Lorentzian lineshape); we used only Lorentzian lineshapes, fixing this ratio to 0. 23 Na NMR. MAS NMR spectra were recorded at 52.94 MHz. The /2 pulse width was 4.5 s, number of scans (ns) = 12000, recycle delay = 1 s, time domain (td) = 512 data points. Results XRF. The chemical composition of the samples was obtained by X-ray fluorescence analyses. The samples composition of the main components ranges as follow: Na2O = 2.8 - 3.1%, MgO = 3.3 – 3.5%, Al2O3 = 15.9 – 16.7%, SiO2 = 56.7 – 58.4%, P2O5 = 0.1 - 0.2%, K2O = 2.2 – 2.4%, CaO = 5.2 – 5.4%, TiO2 = 0.6 – 0.7%, MnO = 0.1 – 0.2%, Fe2O3 = 5.5 – 5.7% L.o.I = 3.8 –6.8%. The composition of all samples is practically constant apart, as expected, the loss on ignition. SEM-EDS. The SEM-EDS was used to determine micro-textural and micro-chemical features of the samples. All samples show the same micro-texture which consists of grains (from 10 to several hundreds micrometers) within a fine-grained porous matrix. The pore size is variable and can reach up to 100 m. EDS analysis of the grains shows different compositions. Many grains are quartz-grains15, others are aluminum-silicate of alkaline metals (Na, K, Ca, and Mg); these minerals belong to the unfired clay or are formed after the structural breakdown of the phyllosilicates. Other grains are rich of Ba and S, or of Mn, or P, or Fe (Table 1) 16. A more detailed treatment is required for the description of the iron content observed both in the grains and in the matrix of the samples. The dimension of the iron rich grains ranges from 50 m2 up to several hundreds of m2. In the samples fired at T < 900 °C the iron content is lower than 60 wt % (Table 2), whereas in the samples fired at T 900 °C it can be as high as 90 wt % (Table 3). In the sample fired at 1100 °C a particular grain made up both octahedral and tetrahedral crystals with dimension of about 150 x 200 m2 and an iron content of about 90 wt % is observed (figure 1a and 1b). The iron content of the matrix of the samples was studied considering the average composition of ten areas of about (800 x 800 m2); the chosen areas did not contain iron grains. The iron content in the samples matrix decreases as the firing temperature increases (see figure 2). This observation suggests that the iron migrates from the matrix toward preferential sites of crystallization. In fact the amount of iron in grains increases from 60% up to 90%. XRPD: Qualitative phases analysis. The mineralogical phases in all samples were identified with a search/match on the PDF database on the basisof the characteristic reflections. The unfired clay contained Quartz, (Qz, SiO2, PDF n. 46-1045), K-feldspar (Kfs, (K,Na)(Si,Al)4O8, PDF n. 84-0710), Na-rich plagioclase (Napg, NaAlSi3O8, PDF n. 18-1202), Illite 2M (Ill, (K)(Al,Mg,Fe)2(Si,Al)4O10(OH)2 n(H2O), PDF n. 26-0911), Calcite (Cc, CaCO3, PDF n. 13-0198) and ordered Anorthite (An, Ca2Al(AlSi)O7, PDF n. 71-0788); the basal reflections of Montmorillonite 15A (Mm, (Na,Ca)0,3(Al,Mg)2Si4O10(OH)2·n(H2O), PDF n. 29-1498) and chlorites (Chl, Na0,5(Al,Mg)6(Si,Al)8O18(OH)12·5(H2O), PDF n. 40–0744) were also identified. The reflections of the last two phases are beyond doubt less intense than the others. Therefore Montmorillonite and Chlorites can be neglected in the further considerations. The pattern of the sample fired at 800°C showed the disappearance of the calcite peak, due to the breakdown of the carbonates, while the illite basal reflections were still present. The Hematite (Hem, Fe2O3, PDF n. 33-0664), Gehlenite (Geh, Ca2Al(AlSi)O7, PDF n. 35-0755) and Diopside (Di, CaMgSi2O6, PDF n. 24-0203) peaks were identified as new-formation phases, and the intensity of their reflections is more enhanced in the samples fired at 1000°C and 1100°C. 5 The illite peaks disappear above 900 °C, whereas the peaks of Na-Ca plagioclase (27-29° 2θ region) increase and they are well defined in the samples heated at 1000 and 1100 °C (see figure 3a). XRPD:Quantitative phases analysis. The quantitative mineralogical phase composition of the samples fired at 800, 900, 1000 and 1100 °C, was estimated using the Rietveld method implemented in the program GSAS 17. The weight fraction of the phases present in the bulk was calculated according to the Bish & Howard formula 18. Its generalized form can be written as m Wm = amSm/ a S k 1 k k , where Wm is the weight fraction of the mth component in the sample and ak is its calculated density, expressed as ak = ZkMkUk (Zk is the number of chemical formula units in a unit cell, Mk is the molecular weight and Uk is the unit-cell volume). The amorphous content was estimated with the internal standard method 19. The amount of the amorphous phase Wa was calculated directly from the weight of the internal standard according to the following equation: Wa = 1 Ws where Ws is the weight fraction of the 1 1 Ws Ws, c internal standard added to the mixture and Ws, c is its calculated weight fraction after the Rietveld refinement. The structural data (cell edges and atomic parameters) of these phases were downloaded from the American Mineralogist Crystal Structures Database 20 and they were used for the Rietveld refinement of the diffraction patterns. The structure of illite was derived from that of muscovite, decreasing the K/Si ratio to a value of 0.25. The Rietveld procedure was performed refining the profile shape, the background and the scale factor for each phase. The profile was modelled using a pseudo-Voigt profile function for each phase, in which the Gaussian, the Lorentzian and the sample displacement term were refined. The cell parameters for the plagioclase structures (albite and anorthite solid solutions) were also refined; at the end of the refinement the shifts of all parameters were minor than their standard deviations. Figure 3b shows the Rietveld plot of the sample fired at 800 °C. Table 4 shows the relative amount of the phases present in the samples fired at 800, 900, 1000 and 1100 °C; the amorphous content is also shown in the same Table. The statistical uncertainty of the weight fractions is strongly dependent on some factors, these are the crystallinity degree, the presence of strong scatterers, the variability in stoichometry and mainly the overlapping degree of the most intense reflections of each phase 21. In our case the estimated standard deviations of the weight fractions of Quartz, Calcite and Hematite fell in the range 0.001 ÷ 0.01 because their reflections had a low overlapping degree. The estimated standard deviation of the weight fraction of plagioclases/feldspars phases ranges between 0.01 ÷ 0.1 because of the higher overlapping degree of the reflections of these phases and their high chemical variability. Figure 3c shows the percentage of the weight fraction of each phase after the Rietveld refinements for the samples fired at 800, 900, 1000 and 1100 °C. In all samples quartz is the predominant phase and its weight fraction shows a maximum at 1000 °C (37%) and then a decrease. In the sample fired at 900 °C the amorphous content markedly increases, at the same time, a decrease of the illite weight fraction and the breakdown of the calcite structure are observed. Then, in the samples fired at 1000 and 1100 °C (22%), the amorphous content progressively decreases. The feldspars/plagioclases phase increases with the firing temperature; in the sample fired at 1100 °C, up to 10-15% weight fraction has been found. These values must be taken with caution because they are only indicative of a general trend. The high chemical variability and the strong overlapping degree of the characteristic reflections of these phases, do not allow a good repartition of the weight fraction of each phase. The simultaneous increase of the weight fractions of these phases with the temperature is indicative of the reaction of the alumino-silicatic amorphous matrix, with K+ and Na+ ions. This reaction gives rise to the neo-formation of Na-rich K-feldspatic phases. The formation of new phases such as Gehlenite and Diopside is due to the reaction of Ca 2+ with the phyllosilicate matrix, while Anhortite formation probably occurs at the Quartz crystals grain boundaries 22. The general increase of the amorphous matrix up to 900 °C is mainly due to the clay minerals decomposition, whereas, above this temperature, a general re-crystallization of new phases is observed, as shown by the decrease of the amorphous content observed in the samples fired at 1000 and 1100 °C. 6 EPR spectra. In clays Fe can be present as Fe2+ and as Fe3+. Traces of Mn2+ may be also present and eventually other paramagnetic compounds in trace. Therefore, to perform a characterization of paramagnetic centers present in the clay, a quantitative EPR study was performed. The EPR spectra of the fired clays are shown in figure 4. In all samples EPR spectra show the presence of Fe 3+ and, in the sample fired at low temperature also of Mn2+. EPR spectra allow the distinction of samples in two sets, i.e. samples fired at 600, 700 and 800 °C , named set S1, and the ones fired at 900, 1000 and 1100 °C, named set S2. The EPR linewidth of Fe3+ is about 100 mT for the first group of samples whereas it is about 50 mT for the second group. In the set S1, where iron is present mostly as Fe2+, the quantity of Fe3+ is smaller; the spectra show an intense resonance absorption at g = 2.4 and a shoulder in the region of g = 4.2 (rhombic site). Spectral line widths are dominated by dipolar broadening 23. In the set S2, the Fe2+ in the clay is fully oxidized to Fe3+. An intense resonance at g = 2.1 (octahedral site) is observable 24, 25. The spectral line-widths are dominated by exchange narrowing 23. The intense resonance at g = 2.1 begins to appear in the sample fired at 800 °C. The spectrum of the clay fired at 900 °C is intermediate between the two sets; the shape of the Fe 3+ resonance is similar to that one of the S2 set, its intensity however is intermediate. The presence of Mn 2+ is observable only in samples fired at T 800 °C, whereas at higher temperature, Mn2+ is oxidized to Mn3+, therefore no more observable by EPR. Note that, in the unfired sample, the Mn2+ resonance is the stronger one, data not shown. Quantitative analyses were performed using ferric-alum dispersed in alum. In figure 5a the straight line represents the result of the titration; the dashed horizontal line represents the intensity of an hypothetical clay sample containing un-clustered 5.4 % Fe3+ i.e. the value corresponding to the total amount of Fe3+ as given by elemental analysis. It can be easily seen that in clays fired at the higher temperatures, the EPR spectrum shows an anomalously high intensity, see figure 5b. Therefore the presence of ferromagnetic or ferrimagnetic domains of Fe3+ contained in clusters is strongly suggested. A decrease of the EPR intensity in the sample fired at 1100 °C on respect to the one fired at 1000 °C might be due to coupled Fe3+ ions in clusters. 27 Al NMR MAS spectra. 27Al MAS NMR characterization of many different clay minerals allows a useful comparison of the Al MAS spectral characters to the structural and compositional parameters of the clay 26. Many naturally occurring clays contain substantial amount of iron which affects the NMR spectra. The extent of the influence of paramagnetic iron on the NMR spectra provides information on the distribution of the iron in the clay. As shown by elemental analysis and X-rays fluorescence techniques, our samples contain a considerable amount of iron, about 5.4 %. As the firing temperature increases, Fe2+ oxidizes to paramagnetic Fe3+. As previously observed 27 a considerable amount of paramagnetic iron does not impair an NMR study. In fact, it has been previously shown that the observation of a 27Al NMR signal in an iron containing clay strongly depends upon the distance between Fe3+ and Al. As reported in the literature 28 the extent of Fe3+ segregation can be assessed by the effect of 27Al NMR signal loss caused by the presence of Fe3+. It is possible to select a “wipeout” radius (R 6 Å) around Fe nuclei, within which the 27Al NMR signal is lost. Accordingly, in the NMR spectrum of an iron containing clay, resonances due to some Al atoms are not observable. It is also known that any Fe atom absorbed in the clay matrix is highly favoured to be incorporated into the Oh (octahedral) layer, with Fe3+ replacing Al3+ into the Oh layer 29. As a consequence in iron rich clays, the Al(Td)/Al(Oh) signals obtained by NMR may appear larger than expected, affecting a quantitative evaluation. In figure 6, the 27Al MAS NMR spectra of the starting clay sample (a) and of samples fired at different temperatures, T=600 (b), 700 (c), 800 (d), 900 (e), 1000 (f), 1100 (g) °C, are shown; in the figure the deconvoluted spectra have been overlapped to the experimental ones, along with the components obtained from the deconvolution. 27Al spectra allow to separate samples in two sets referred as S1 and S2 respectively, i.e samples fired at temperatures lower than 900 °C and samples fired at temperature equal or higher than 900 °C; S1 also includes the unfired clay sample. All samples belonging to S1 show at least two main, broad resonances: the low frequency resonance apparently centred at about 50 ppm is characteristic of Al in a tetrahedral environment (Td), whereas the resonance apparently centred at about 0 ppm is characteristic of Al in an octahedral environment (Oh). This finding is in agreement with the large amount of illite found using XRPD. In fact illite has a 2:1 layer structure with a plane of octahedral coordinated cations sandwiched between two inward pointing sheets of tetrahedral 30. In the illite structure one out of four Si4+ ions are replaced by Al3+ in the tetrahedral sheet; besides some octahedral Al3+ may be replaced by Fe2+ and Mg2+. 7 These replacements give raise to a negative charge which is neutralized by large cations in the interlayers spaces. Therefore the resonances of Al both in Td and in Oh environments are observed. As the firing temperature increases, the signal of Al in Oh environment decreases, fully disappearing at temperature higher than 800 °C. This observation may be rationalized by two effects. In fact, as the firing temperature increases, Fe2+, which preferentially replaces Al in Oh environment, oxidizes to paramagnetic Fe3+ which affects the intensity of the signal of Al in Oh environment. Moreover, as the firing temperature increases, the illite structure breaks down and the signal due to Al in Oh environment does not exist any more. According to XRPD data, as the illite structure breaks down, new phases are observed; all these new phases contain Al only in the Td environment 31, 32. As previously shown, in the 27Al MAS NMR spectra significant differences exist among different clays, including dehydroxylated fired clays 32. For instance in the case of Kaolinite and Hallosyte, besides the resonances due to Al in Td and Oh environment, a resonance at about 26 ppm has been reported. This resonance has been ascribed to Al in a penta-coordinated environment. AlO5 has been also found in the 2:1 system Pyrophyllite 33 whereas, in other clay minerals, no signal due to AlO5 has been reported 34. In the 27Al spectra, see figure 6, no evidence of resonances due to Al in a penta-coordinated environment (20 – 30 ppm) is observed. However the spectra are too broad for definitely ruling out the presence of Al in a penta-coordinated environment, which however may be present in a small amount. Therefore for obtaining more information, the spectral resolution must be improved. 27 Al 3Q MAS NMR spectra. To obtain 27Al NMR spectra with improved resolution, we performed triple-quantum MAS NMR experiments. This 2D method 6, 9 involving multiple-quantum excitation in combination with the magic angle spinning, is able of refocusing the second order quadrupolar effects which broaden the 27Al signals. The 3Q-MAS spectra are shown in figure 7: unfired clay (a), and clays fired at T = 600 (b), T = 700 (c), T = 800 (d), T = 900 (e), T = 1000 (f) and T = 1100 °C (g). In the 2D map, in the multi-quantum dimension, the lines are three times more separated by their isotropic chemical shift than in MAS single quantum dimension, moreover they are free of second order quadrupolar effects. In the 2D spectra, figure 7, the dotted lines represent the different orientation of signals in an 3Q-MAS spectrum: “A” denotes the anisotropic axis, “QIS” is the axis giving the direction of the induced quadrupolar shift and “CIS” axis gives the direction of the isotropic chemical shifts. 3Q-MAS spectra of samples belonging to S1 show two main well separated spectral ridges, respectively due to Al in Td and in Oh environment. It must be noted that the spectral ridges of Al in T d environment are rather well aligned along the CS axis, whereas the spectral ridge of Al in Oh environment appears out of the CS alignment, thus showing the presence of a chemical shift distribution. Note also that the cross-peak due to the Td site lies along the A axis, whereas the cross-peak due to the Oh site appears not well aligned along this axis. According to this observation we infer that the quadrupolar coupling constant of Al in T d site is larger than that one of Al in Oh site. It is worth noticing that the QIS axis is the axis along which the mass centres of the ridges from different species with the same isotropic chemical shift but different quadrupolar constants are located. In the 3Q-MAS spectrum of the unfired clay, at least two spectral ridges due to Al in tetrahedral sites, Td1 and Td2 aligned along the QIS axis, are clearly observable, see figure 7a, furthermore also the isotropic projection along the F1 dimension shows a clear multiplicity. The 3Q-MAS spectra of samples fired at 600, 700, and 800 °C show again the presence of spectral ridges due to Al into two different tetrahedral environments, however as the firing temperature increases the chemical shifts of these two tetrahedral sites, Td1 and Td2, become very close to each other, making the observation of their separation increasingly difficult, see figures 7 b,c,d. The 3Q-MAS spectra of samples belonging to S2 show only the spectral ridges due to Al in T d environment, see figures 7e,f,g. Two spectral ridges, T d1 and T2d, due to Al in two slightly different Td environments, both aligned along the QIS axis, are observed: T d1 and Td2 have chemical shifts close to each other, whereas their quadrupolar coupling constants are rather different. According to the literature 35, 36 using the centres of gravity G1 and G2 (in the f1 and f2 dimension respectively) of the 3Q-MAS spectra, it is possible to estimate the isotropic chemical shift iso and the second order quadrupolar effect Pq of each spectral ridge. The obtained parameters were used as starting parameters in a best fit procedure applied for deconvoluting the 1D MAS spectra. The isotropic chemical shifts isoTd1 and isoTd2, the quadrupolar coupling constants CqTd1 and CqTd2, the asymmetry parameters qTd1 and qTd2 of Al in Td sites, and isoOh and CqOh of Al in the Oh site are reported in Table 5. The Czjzeck model 12 used for the simulation of the resonance of Al in Oh site accounts for a chemical shift distribution 13, the width of the chemical shift distribution CS is also reported in Table 5. The used program, dm2004, gives also the area of each 27Al resonance. It is then possible to obtain the percentage 8 of Al in Oh site and the percentage of Al in Td site, data not shown. However, it must be pointed out, that, due to presence of Fe3+, this percentage is not quantitative. In fact, the resonances due to 27Al atoms within the wipe out sphere of Fe3+, are fully lost; therefore the obtained values refer only to those sites outside the Fe3+ wipe out sphere 27. In the unfired clay Al is present in an almost equal amount in Oh and Td sites. As the firing temperature increases, the amount of Al in Oh progressively decreases; in samples fired at temperature higher than 800 °C, the resonance at about 4.5 ppm fully disappears, reflecting the structural breakdown of the illite structure; this observation fully agrees with the XRPD data previously shown. The expanded tetrahedral region of all spectra, after applying the shearing procedure 14 is shown in figure 8, with the experimental spectra shown on the left side, and the simulated spectra shown on the right side. It is worth to note that the set of parameters obtained for the Td sites from the simulation of 1D-MAS spectra is very similar to the corresponding set obtained from the best fit of the 3Q-MAS spectra. On samples fired at a temperature lower than 900 °C, two tetrahedral sites have been always found. As previously observed, these two sites give rise to NMR signals with very similar chemical shift values whereas the quadrupolar coupling constant values are different 37. A possible interpretation of the structural difference of these two environments is that slightly different distortions may change the Electric Field Gradient (EFG) distribution, and, as a consequence, the value of the quadrupolar coupling constant. In fact the quadrupolar coupling constant Cq is proportional to the largest of the three principal axes of the EFG tensor at the nucleus 34. It has been previously shown that, in some cases, either the distortion of the local environment or long range lattice effects may affect the magnitude of the Electric Field Gradient (EFG) and, therefore, the magnitude of Cq 38. Therefore two Td sites may show resonances with isotropic shifts very similar to each other, but with different Cq values. In samples fired at temperature higher than 800 °C, Al spectra show only two tetrahedral environments. The parameters obtained for the two Al resonances are very close in chemical shifts whereas again they show different Cq values . Dehydration and dehydroxylation of fired clays. Many factors may affect the efficiency of the crosspolarization process of the resonances of quadrupolar nuclei under MAS conditions. Thus resonances of two distinct sites may display markedly different cross-polarization properties, exhibiting matching conditions at very different field strength. Carefully taking into account these points, we may asses that the efficiency of the transfer of the magnetization from proton nuclei to Al nuclei is closely related to the total proton concentration in the sample. According to the literature39, in hydrous aluminates such as Gibbsite, the interlayer water is associated to Al in Oh environment. Moreover, earlier studies on Illite-rich clays 31,40 showed that, in the 350 – 400 °C range of firing, the interlayer water is driven off, therefore only the protons of the hydroxyl groups participate to the CP-process. The dehydroxylation takes place between 450 – 700 °C. In figure 9, the 27Al MAS (a) and CP MAS (b) spectra of the unfired clay are shown. According to the literature 39 the CP-MAS NMR spectra show that the available protons are closely associated with Al in Oh site, as this site provides a considerable CP-MAS signal intensity whereas no CP-MAS intensity is observed for the resonance of Al in Td sites. The intensity of the CP-MAS signal decreases in the sample fired at 600 °C and, due to the dehydroxylation, the signal fully disappears in the sample fired at 700 °C (data not shown). 29 Si MAS spectra. In figure 10, the 29Si MAS NMR spectra of the unfired and fired clays are shown. The corresponding deconvoluted spectra have been superimposed to the experimental ones, along with the single components obtained from the deconvolution procedure. In all spectra a resonance centred at –107 -108 ppm due to quartz is observed; this resonance is much sharper in the spectra of clays fired at T= 600, 700 and 800 °C, thus showing a certain amount of short range disorder in the clays fired at the higher temperatures. The resonance centred at about –93 ppm can be easily ascribed to the Q3 units of Illite; this resonance is observed in the spectra of samples fired at temperatures lower than 900 °C; at higher temperatures, when the structural breakdown of illite occurs, this resonance is no more observable. In agreement with XRPD data, the resonances centred at about –96 and –101 may be safely attributed to the presence of K-Feldspars41. The peak broadening and the extra intensity in the –86 -100 ppm range may be tentatively attributed to the resonances of a range of silicon environments in strained regions between Albite and Anorthite-rich domains 41, 42. In some sample a shoulder centred at –113 -115 ppm is also observable, see Table 6; this shoulder might be possibly due to silica polymorphs or to silica glass with a broad distribution of the SiOSi bond 9 angles. In fact, according to the literature41, the resonances of silica polymorphs appear at the highest field of the 29Si chemical shift range of silicates, from -107 (quartz) to to –121 ppm, whereas silica glass may show a broad resonance centred at about –110 -111 ppm Note that no evidence of Gehlenite , observed with XRPD, was found neither in the 29Si NMR spectra. It is important to point out that, even if a considerable amount of paramagnetic iron is found in the sample, in order to avoid saturation of the signal from the crystalline quartz, a recycle delay of at least 60 s must be used. Therefore, in the crystalline quartz domain, no paramagnetic iron is present and the observed line broadening is possibly due to disorder. The 29Si MAS (bottom) and CP-MAS (top) spectra of the unfired clay are shown in figure 11. As expected, the quartz spectral resonance fully disappears in the CP spectrum of the unfired clay, as compared to the simple MAS spectrum of the same clay, see figure 11. 23 Na MAS NMR spectra. 23Na MAS NMR spectra of all clays are shown in figure 12. In clays fired at the higher temperatures the spectrum shows a single broad line, rather symmetric and unstructured, centred at about –27 ppm from NaCl. This line well corresponds to literature data and it is due to Albite glasses 43 and/or Na-rich Plagioclase 34. Spectra of clays fired at lower temperatures appear complex and poorly resolved. A comparison with literature data does not allow a significant assignment. A further study at higher field and higher spin rate will be performed. Conclusion The use of a large number of chemical physical techniques allowed a clear description of the effect of firing on a complex material such as an Fe rich clay. Taken all together these characterization methods have shown the following items: -Interlayer water is driven off at temperatures lower than 600 ºC. As shown by 27Al CP-MAS NMR spectra, interlayer H2O is located close to octahedral sites. -Dehydroxilation occurs in the temperature range 600-700 ºC. -In the 800-900 ºC temperature range, due to a full breakdown of the Illite structure, Al in octahedral sites disappears; in the same temperature range all Fe is oxidized to Fe 3+ . -In samples fired at 1000 and 1100 ºC a clustering of the iron has been observed accompanied by the presence of large iron single crystals with the appearance of ferro or ferri magnetic effects. -The Al in octahedral site present at temperatures lower than 900 ºC presents a continuous chemical shift distribution pointing to the presence of slightly distorted sites. -In the full temperature range the presence of at least two tetrahedral Al sites has been revealed, characterized by different values of the quadrupolar coupling. -In some samples silica polymorphs or silica glasses with a broad distribution of the SiOSi bond angles have been also found. Few other observations can be obtained comparing XRPD and NMR data. In fact both set of data show a maximum quartz content in the sample fired at 1000ºC. Both techniques confirm the presence of K-Feldspars 34 , observed by XRPD and by NMR. Albite which, according to XRPD data, increases in the samples fired at the higher temperature was also observed in the 23Na NMR. However Gehlenite , observed with XRPD, was not found neither in the Al and in the Si NMR spectra. The same for anhortite not observed neither in the Al and in the Si NMR spectra. However, some extra intensity observed in the Si spectra, may be tentatively attributed to a range of silicon sites in strained regions between Albite and Anorthite-rich domains 41, 42. Even if the set of parameters obtained from 3Q MAS experiments and from the deconvolution procedure makes sense, the accuracy in determining the isotropic chemical shift and quadrupolar coupling constant may be improved by the combined evaluation of MAS and 3Q MAS spectra measured over a range of different Larmor frequencies. Measurement at different H0 fields will allow the assessment of the relative importance of second order quadrupole parameters vs. line broadening mechanisms in determining the shape and the line width characteristic of an observed NMR resonance composed of overlapping resonances from different sites44. Moreover, the resolution of the observed Td and Oh Al sites, might be improved at higher field and higher spin rate. Moreover Si spectra obtained at higher field will surely be more informative. Acknowledgements Thanks are due to Prof. D. Massiot for the version of the program dm2004.Thanks are due to Dr. M. Liberi and Dr. R. Melzi , Bruker Biospin Milano for the use of the Bruker EMX EPR spectrometer. We thank Dr. F. 10 Ziarelli for the stimulating and useful discussions.Thanks are due to Dr. A. De Stefanis and to Mrs P. Cafarelli for elemental analysys. This work was performed as part of the Eu-ARTECH project within the VI Framework Program. 11 Table 1: Composition W(%) of different grains typically found in the samples. Aluminium Silicate Other crystals W(%) W(%) W(%) W(%) W(%) W(%) W(%) W(%) Na2O 0.6 13.6 1.6 0.6 2.2 1.7 0.9 1.4 MgO 1.3 1.7 0.9 3.3 6.8 1.9 0.6 4.4 Al2O3 4.4 19.9 20.9 1.8 20.0 4.0 3.0 3.4 SiO2 89.7 63.8 59.1. 24.8 35.5 40.0 7.0 7.7 P2O5 n.d. n.d. n.d. 0.9 n.d. 46.5 n.d. n.d. SO3 0.2 0.5 0.5 12.7 0.5 2.6 26.2 0.8 K2O 0.6 0.1 15.8 0.2 2.7 0.5 0.3 0.4 CaO 0.9 0.1 1.0 53.8 5.0 1.4 1.5 23.3 BaO n.d. n.d. n.d. n.d. n.d. n.d.- 59.5 n.d. TiO2 0.4 n.d. n.d. 0.5 n.d. 0.4 n.d. n.d. MnO n.d. n.d. n.d. n.d. 24.0 0.2 n.d. 53.6 FeO 1.8 n.d., not detected 0.3 0.4 1.6 3.4 1.0 1.0 4.9 Table 2: Composition of some iron-rich grains of the sample fired at temperature lower than 900°C. 600°C 700°C 800°C 50x10m 20x20m 20x10m 60x100m 70x130m 40x60m 30x20m 20x15m 20x10m W(%) W(%) W(%) W(%) W(%) W(%) W(%) W(%) W(%) Na2O 2.4 2.5 0.9 2.1 1.6 1.5 2.4 1.8 1.6 MgO 5.5 12.8 2.9 6.5 2.6 2.9 5.0 2.4 4.9 Al2O3 9.7 20.2 5.8 16.3 7.1 8.3 10.9 17.5 12.1 SiO2 18.7 36.1 12.5 31.9 14.0 19.4 23.5 24.6 17.0 P2O5 0.8 0.8 1.7 n.d. n.d. n.d. n.d n.d. n.d. SO3 0.2 1.0 12.0 0.7 1.0 1.2 1.3 0.8 0.8 K2O 0.9 1.1 1.2 1.9 0.5 0.9 0.7 1.0 1.4 CaO 3.0 2.0 11.0 2.0 14.1 10.4 12.0 1.4 2.1 TiO2 n.d. n.d. n.d. 0.7 0.4 0.4 0.1 0.7 0.9 MnO 1.6 n.d. 0.6 n.d. n.d. 10.2 n.d. n.d. n.d. FeO 57.3 23.6 51.3 38.0 58.7 44.8 44.1 49.9 59.3 n.d., not detected 12 Table 3: Composition of some iron-rich grains of the sample fired at temperature higher than 800°C. 900°C 1000°C 1100°C 20x50m 80x120m60x120m 30x30m 100x30m 100x60m 150x200m 60x40m 80x80m Elements W(%) W(%) W(%) W(%) W(%) W(%) W(%) W(%) W(%) Na2O 3.4 2.8 2.2 n.d. 2.2 2.2 0.8 1.1 1.9 MgO 4.9 4.2 4.1 4.9 3.8 2.6 1.1 1.2 3.9 Al2O3 7.4 3.8 7.2 7.5 5.5 4.9 2.1 3.6 6.0 SiO2 12.2 5.2 17.2 9.4 13.2 14.4 3.1 9.7 18.0 P2O5 n.d. 2.7 n.d. 3.7 2.0 0.5 0.4 n.d. 0.7 SO3 1.7 2.1 2.0 0.4 1.2 0.5 0.7 0.7 n.d. K2O 0.7 0.1 0.4 0.5 0.2 0.4 0.4 0.4 0.2 CaO 3.3 6.6 5.4 2.9 5.4 2.5 0.4 1.0 8.5 TiO2 0.7 0.5 0.2 0.4 0.5 0.3 0.8 0.5 0.4 MnO n.d. 0.7 2.4 n.d. 1.2 0.7 n.d. n.d. n.d. FeO 65.8 71.4 58.9 70.4 64.8 71.2 90.3 81.8 60.4 Table 4: Percentage of different phases in samples fired at 800, 900, 1000 and 1100°C as obtained by Rietveld method. Phases (%) 800°C 900°C 1000°C 1100°C Qz (%) 26.9 26.9 37.6 27.5 Hem (%) 1.0 1.3 1.0 1.5 NaPg (%) 5.7 5.8 7.2 10.5 Geh (%) n.d. 1.7 2.0 4.2 Di/Augite (%) n.d. 1.4 7.0 6.5 Kfs (%) 5.6 5.4 5.1 13.9 An (%) 10.3 4.7 9.1 13.9 Ill (%) 21.1 8.3 n.d. n.d. Mont (%) <1 n.d. n.d. n.d. Cc (%) 1.3 n.d. n.d. n.d. Amorphous (%) 27.1 44.5 31.0 22.0 Agreement Rwp= Rwp=0.120 Rwp=0.120 Rwp=0.142 indices 0.124 Rp=0.09 Rp=0.09 Rp=0.107 Rp= 0.09 GOF=2.9 GOF=4.9 GOF=3.9 GOF=3.5 Rwp = ( w(Io-Ic)2 / wIo2)1/2 Rp = |Io-Ic| / Io GOF = ( w(Io-Ic)2 / (No - Nvar))1/2 n.d., not detected 13 Table 5. 27Al NMR parameters as obtained from the deconvolution procedure applied to 1D MAS NMR spectra as well as 3Q MAS NMR spectra. iso = isotropic chemical shift; Cq = quadrupolar coupling constant; q = asymmetry parameter; CS= width of the chemical shift distribution. T(K) isoT1(ppm) CqT1(KHz) unfired 72 2065 600 71 2683 700 66 2373 800 66 2373 900 63 2160 1000 64 3266 1100 64 3298 qOh was fixed at 0.61 qT1 isoT2(ppm) CqT2(KHz) 0.1 62 3099 0.9 64 3371 0.8 61 3227 0.8 61 3227 0.0 60 3096 0.0 61 2431 0.0 57 1992 qT2 isoOh(ppm) CqOh(KHz) CS(ppm) 0.5 4.7 2451 37 0.6 4.6 1711 18 0.6 4.6 1841 19 0.6 4.5 1708 17 0.0 0.0 0.0 Table 6. 29Si Chemical shifts and relative integrals as obtained applying the deconvolution procedure to 1D MAS spectra. Sample (ppm) unfired 600C 700C 800C 900C 1000C 1100C -110.1 -107.6 -107.3 -108.1 -108.0 -108.4 -108.2 % A 49 16 15 21 33 46 42 (ppm) -100.8 -100.5 -101.0 -101.8 -100.8 -100.7 % A (ppm) % A (ppm) 14 2 5 12 22 28 -97.4 -96.6 -96.9 -97.3 -96.8 -95.0 -95.0 27 21 13 28 21 15 13 -87.3 -87.2 -86.7 -89.1 -91.0 -89.7 -88.9 % A 13 17 35 9 33 8 8 (ppm) % A (ppm) % A -92.4 -92.7 -92.6 -93.2 11 32 35 16 -112.9 20 -115.6 -113.3 10 10 The peak broadening and the extra intensity in the -90-100 ppm range is tentatively attributed to a range of silicon site environments in strained regions between albite and anhortite-rich domains in the modulated structure (Ripmeester,J.A.; Davidson,D.W. J.Mol.Structure 1981, 75, 67 ). Glass of aluminosilicate G.Engelhardt and T.Michel, J.Wiley and Sons Norwich, G.B. 1987 page 97. Si O SiOSiOSi O Si 14 Figure 1 a) Grain formed by octahedral and tetrahedral crystals with dimension of about 150 x 200 m and a content of iron of about 90 wt %. It was found in sample fired at 1100°C. b) Magnification of the same grain, nice octahedral crystal is observable. 15 Figure 2 Iron content in the bulk of the samples fired at different temperatures. 16 Figure 3 a) XRD patterns collected at different temperatures along with labelling of the main phases. b) Rietveld plot of the sample fired at 800°C: experimental, calculated and difference spectra are shown, the sticks represent the calculated Bragg positions for all the phases. c) weight fractions of the main mineralogical phases in samples fired at 800, 900, 1000 and 1100°C. 17 Figure 4. EPR spectra of fired clays. 18 Figure 5 a)Titration line used for the quantitative analysis of Fe 3+, the dashed vertical line corresponds to the amount of Fe 3+ present in fired clays as obtained by elemental analysis, the horizontal line represents the corresponding EPR integral ; b) amount of Fe 3+ in clays fired at the temperatures shown in the abscissa axis. The dashed horizontal line represents the theoretical amount of Fe 3+ . 19 Figure 6. 27Al MAS NMR spectra of clays and their deconvolution; a) unfired; b) fired at 600 ºC; c) fired at 700 ºC; d) fired at 800 ºC; e) fired at 900 ºC; f) fired at 1000 ºC; g) fired at 1100ºC. 20 Figure 7 Un-sheared 27Al 3Q MAS NMR spectra of clays; a) unfired; b) fired at 600 ºC; c) fired at 700 ºC; d) fired at 800 ºC; e) fired at 900 ºC; f) fired at 1000 ºC; g) fired at 1100 ºC. In f2 the MAS spectrum is shown, whereas in f1 the isotropic projection is shown The CS, QIS and A axes have been defined in the experimental section. 21 Figure 8 (continue) 22 Figure 8 Sheared 27Al 3Q MAS NMR spectra of clays, only the Td region is shown. On the left experimental spectra, on the right simulations. a) unfired; b) fired at 600 ºC; c) fired at 700 ºC; d) fired at 800 ºC; e) fired at 900 ºC; f) fired at 1000 ºC; g) fired at 1100ºC. 23 Figure 9. 27 Al NMR spectra of the unfired clay: a) CP-MAS, b) MAS 24 Figure 10 29Si MAS NMR spectra and their deconvolution of clays; a) unfired; b) fired at 600 ºC; c) fired at 700 ºC; d) fired at 800 ºC; e) fired at 900 ºC; f) fired at 1000 ºC; g) fired at 1100 ºC. 25 Figure 11 29Si NMR spectra of the unfired clay: a) CP-MAS, b) MAS 26 Figure 12 23Na MAS NMR spectra of clay: a) unfired clay; b) fired at 600ºC; c) fired at 700ºC; d) fired at 800 ºC; e) fired at 900ºC; f) fired at 1000ºC; g) fired at 1100ºC. 27 References (1) Duminuco, P.; Messiga, B.; Riccardi, M. P. Thermochimica Acta 1998, 321, 185. (2) Altaner, S.P.; Weiss, JrC.A.; Kirkparick, R.J. Nature 1988, 331, 699. (3) de Araújo, J.H.; da Silva, N.F.; Acchar, W.; Gomes, U.U. Materials Research 2004, 7, 359. 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