## 3 Results and Discussion  
### Preparation and Characterization of the Materials  
Table 3 lists the chemical composition of the parent kaolins Ka and Ka-R, which have very similar composition. Ka-R has slightly higher amounts of Fe\({}_{2}\)O\({}_{3}\) and MnO than Ka. Clays usually have the reddish/brownish/yellowish color pattern on the surface of aggregates, on pore surfaces, and in the upper soil horizons as a result of oxidized transition metals such as Fe\({}^{3+}\) and Mn\({}^{4+}\). The grayish/bluish colors of clays appear within aggregates or in the deeper soil horizons and are due to Fe\({}^{2+}\) and Mn\({}^{2+}\) cations present in the layers. The Fe\({}^{3+}\) and Mn\({}^{2+}\)/Mn\({}^{4+}\) contents may justify the differences between the colors of Ka and Ka-R and should influence their reactivity depending on whether Fe\({}^{3+}\) and Mn\({}^{2+}\)/Mn\({}^{4+}\) are present as extra framework phases or if they are located in structural positions. The X-ray powder diffraction patterns of the zeolites Zeo and Zeo-R synthesized from the metakaolins M-Ka and M-Ka-R (Figure 1) reveal the presence of a single and very pure phase. Comparison with the JCPDS file 43-142 clearly shows that Zeo and Zeo-R are hydrated Linde Type Zeolite A (LTA). LTA has a three-dimensional pore structure where the pores are perpendicular to each other along the \(x\), \(y\), and \(z\) directions. The pore diameter is defined by a small ring (4.2 A) of eight oxygen atoms, which gives a large cavity with free diameter of 11.4 A. The unit cell is cubic (a: 24.64 A). The purity of zeolites synthesized from kaolin has been reported to depend on various factors such as NaOH concentration, Al/Si ratio, calcination temperature of the parent kaolin, and duration of the treatment [12]. Here, the powder X-ray diffractograms prove that the small differences in Fe and Mn contents do not interfere in the synthesis of zeolite A: there is only a small decrease in the reflection intensities of the sample Zeo-R, in agreement with literature results. This decrease indicates the presence of isomorphic substituents in the clay mineral, which may lower the crystallinity of the resulting zeolite [15]. Thermogravimetric analyses (Figure 2) of the zeolites Zeo and Zeo-R give rise to more complex curves than the curves of the parent kaolins, Ka and Ka-R (not included, but typical of this mineral). Mass loss reaches values close to 23% for both Zeo and Zeo-R, which are very close to the expected value for zeolite A. The theoretical mass loss for a 'typical' zeolite A, with formula Na\({}_{12}\)Al\({}_{12}\)Si\({}_{12}\)O\({}_{48}\)-27H\({}_{2}\)O, is 22.2%, but the mass loss strongly depends on the storage conditions. In the present case, special storage conditions were not adopted, so both zeolites Zeo and Zeo-R may have a low amount of adsorbed water. Water removal is almost complete around a temperature of 250 \({}^{\circ}\)C. The DTG curves (not shown) demonstrate that this removal occurs in three consecutive, overlapped processes centered at 65-70, 120-150, and 220-240 \({}^{\circ}\)C, which correspond to water held to the zeolite at various strengths, adsorbed water, and zeolitic water, respectively. Zeo-R undergoes a small additional mass loss at ca. 350 \({}^{\circ}\)C, which may be related to the removal of CO\({}_{2}\) fixed as carbonate during the preparation  
Figure 1: PXRD patterns of the purified parent kaolins Ka and Ka-R, the metakaolins M-Ka and M-Ka-R, and the synthesized zeolites Zeo and Zeo-R, prepared from white (**a**) and red (**b**) kaolins, respectively.