Nucleation and Growth Process of Sodalite and Cancrinite from Kaolinite-rich Clay under Low-temperature Hydrothermal Conditions

The synthesis of low-silica zeotypes by hydrothermal transformation of kaolinite-rich clay and the nucleation and growth processes of sodalite and cancrinite in the system Na 2 O–Al 2 O 3 –SiO 2 –H 2 O at 100 °C were investigated. The synthesis products were characterized by X-ray powder diffraction (XRPD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), Si and Al Magic Angle Spinning Nuclear Magnetic Resonance (MAS-NMR) and thermogravimetric analysis (TGA). Our data show that the sequence of the transformation of phases is: Poorly crystalline aluminosilicate → zeolite LTA → sodalite → sodalite + cancrinite → cancrinite. Synthesized materials appeared stable thermodynamically under the experimental conditions, with zeolite LTA (a metastable phase) occurring as a minor phase, compared with the presence of sodalite and cancrinite.


Introduction
The Bayer process is the principal industrial means of refining bauxite to produce alumina (Al 2 O 3 ), which must be purified before it can be refined to aluminium metal.During alumina production, bauxite is digested by washing with a hot solution of sodium hydroxide (NaOH) at elevated temperatures 1 .SiO 2 , an impurity in the bauxite in the form of kaolinite and quartz, can also be digested.After digestion, the dissolved SiO 2 subsequently reacts with the ionic species in the sodium aluminate solution to form a "desilication product" (commonly known as DSP) which primarily consists sodium aluminosilicates, such as sodalite (SOD) and cancrinite (CAN) [1][2][3][4] .Both SOD and CAN are of the common chemical formula Na 6 [ .However, their structural frameworks are different (Figure 1).SOD has a cubic framework that contains only β-cages made up of eight 6-membered rings of alternating AlO 4 and SiO 4 tetrahedra and six 4-membered rings; the free dimension of the inscribed sphere of the β-cage is 6.6 Å and access to the cage through the 6-membered ring window (channel) is ~2.2Å [3] .CAN has a hexagonal framework that contains small ε-cages consisting of five 6-membered rings and six 4-membered rings, which give rise to large continuous channels consisting of 12-membered rings with a free dimension of ~6.2 Å [5] .In general, these aluminosilicates are characterized by a three-dimensional framework, which contain cages and channels in their negatively charged frameworks because of the substitution of Si 4+ ions by Al 3+ ions 6 .Both the βand εcages need positively charged species to neutralise them 3 .Cations can enter these porous materials to balance the charge of their structural frameworks 6 .Structural data of these zeolite frameworks are published by the 'Structure Commission of the International Zeolite Association' (IZA) 7 .
The low-temperature hydrothermal conversion of kaolinite and metakaolinite to low silica zeolites (LSZs) is a very well known process, with zeolite LTA as the most common synthesis product 8 , which is a metastable phase during the formation of SOD and CAN.The synthesis of zeolites from kaolinite has been extensively investigated 5,9 .However, the improvement of the kaolinite properties by chemical methods is difficult due to its low reactivity; this clay mineral is not significantly affected by acid or alkaline treatments, even under strong conditions 10 .Therefore, kaolinite is usually used after calcination at temperatures between 550-950 °C11 to obtain a more reactive phase (metakaolinite) under chemical treatments.Several authors have reported the synthesis of metakaolinite-based zeolitic materials, including the zeolite LTA 8,[12][13][14][15] .The mechanisms and kinetics of SOD and CAN formation have been investigated by several authors [1][2][3]16 . Accrding to Barnes et al. 2 , the phase transformation from SOD to CAN was a solution-mediated process, which involved a fourstage mechanism.However, there are many inconsistencies in the published sodium aluminosilicate equilibrium solubility data particularly for the SOD and CAN phases 17 . O the other hand, whether or not the crystalline phase underwent any transformation changes (e.g.SOD to CAN) was either not determined or not taken into account 18 .
In this work, we investigate not only the low-temperature hydrothermal transformation of kaolinite and metakaolinite into SOD and CAN-type structures but also the phase transformation from SOD to CAN.On the other hand, this research is also useful in order to understand the common natural process of low to medium temperature alteration of feldespatoids in alkaline volcanic and subvolcanic rocks.

Material
The raw material used in this study was a kaoliniterich clay (distributed under the name Supreme Powder and supplied by English China Clay International).Metakaolinite was obtained by thermal treatment at 600 °C of the kaolinite-rich clay.Both kaolinite and metakaolinite were used as starting materials for the synthesis of zeotypes.Other reagents used in the activation of the raw materials were: sodium hydroxide, NaOH, as pellets (99.99%,Aldrich Chemical Company, Inc.) and distilled water using standard purification methods.

Synthesis of SOD-and CAN-type structures
Zeotype synthesis was carried out under hydrothermal conditions in alkaline medium using kaolinite and metakaolinite as silica and alumina sources.A calculated amount of alkali hydroxide pellets was added to distilled water in reaction plastic beakers (150-250 mL) to prepare 1.33 M and 3.99 M NaOH solutions; the starting materials were then mixed with the alkaline solutions to produce a reaction gel with a specific molar composition.The progressive addition of reagents was carried out under stirring conditions until they dissolved to homogenize the reaction gels.Crystallization was carried out by hydrothermal synthesis under static conditions in PTFE vessels of 65 mL at 100 °C for several reaction times.Once the activation time was reached, the reactors were removed from the oven and quenched in cold water to stop the reaction.After hydrothermal treatment, the reaction mixtures were filtered and washed with distilled water to remove excess alkali until the pH of the filtrate became neutral.Then, the samples were oven dried at 80 °C overnight.The hydrogel pH was measured before and after hydrothermal treatment.The dried samples were weighted and kept in plastic bags for characterization.Table 1 presents the experimental conditions used for zeolite synthesis.

Characterization of the starting materials and as-synthesized zeotypes
X-ray diffraction (XRD) patterns of the starting materials and as-synthesized zeotypes were recorded using a Philips PW1710 diffractometer operating in Bragg-Brentano geometry with Cu-Kα radiation (40 kV and 40 mA) and secondary monochromation.Data collection was carried out in the 2θ range 3-50°, with a step size of 0.02°, a divergent slit of 1.0°, and a dwell time of 2 seconds Phase identification was performed by searching the ICDD powder diffraction file database, with the help of JCPDS files for inorganic compounds.The morphology of the solid phases were examined in a ZEISS EVO50 scanning electron microscopy (SEM) under the following analytical conditions: I probe 1 nA, EHT = 20.00kV, beam current 100 µA, Signal A = SE1, WD = 8.0 mm.Specimens were prepared by spraying dried raw material or zeolite powder onto aluminium stubs using double-sided adhesive carbon discs and sputter coated with gold to reduce static charging and then observing under SEM conditions.For EDX analysis the samples were prepared in a similar way to SEM.However, to avoid errors in the aluminium content, a carbon sample holder was used instead of aluminium stud and gold was not coated on the samples.Fourier transform infrared (FT-IR) spectroscopy was performed using a Mattson Genesis II FT-IR spectrometer in the 4000-400 cm -1 region.However, only the 1200-400 cm -1 region was investigated, taking into account that it is where the spectra showed remarkable changes.Solid-state 29 29 Si and 1 M AlCl3 aqueous solution for 27 Al.Thermogravimetric analyses (TGA) were performed on a Mettler Toledo TG 50 thermobalance between 25-700 °C at a heating rate of 20 °C/min under flowing air (200 cm 3 /min).The ground sample was directly filled into the crucible for testing.The amount was normally ~15-20 mg to minimize background noise.

Starting materials
The XRD patterns of the starting materials are presented in Figure 2. Kaolinite (Figure 2a) in the sample is identified by its characteristic XRD peaks at 12.34° and 24.64° 2θ.However, minor impurities, such as illite, muscovite and halloysite, also occur.Metakaolinite (Figure 2b) is characterized by the disappearance of all the XRD peaks of kaolinite, accompanied by the appearance of an amorphous aluminosilicate (see the broad hump at 2θ = 13-33°, having a maximum at 2θ = ~22°).Similar results have been observed by as reported in several studies [19][20][21] .The SEM micrographs (Figure 2) reveals that kaolinite (Figure 2a) displays a platy morphology and hexagonal outlines, with small, well-formed hexagonal plates loosely packed, defining an orientation, whereas metakaolinite (Figure 2b) occurs as a mixture of large plates and stacks.
The FT-IR spectra of the starting materials (Figure 3) resemble those reported in previous studies.The vibration bands of kaolinite are described as follows.The band at 1119 cm −1 is referred to Si-O stretching vibrations, while the bands at 1034 and 1012 cm −1 are rather caused by Si-O-Si and Si-O-Al lattice vibrations 22 .The OH bending vibrations at 942 and 916 cm −1 can be referred to the 'surface OH bending' and 'inner OH bending' 23 , which are mainly caused by Al-OH groups 22 .Further, bands in low range of frequency (762, 696 and 539 cm −1 ) can be attributed to different Si-O and Al-O vibrations 24 .The conversion of kaolinite to metakaolinite is revealed by the disappearance of these characteristic bands, similar to what is reported in previous studies [19][20] .The vibration bands observed in metakaolinite were 1049, 806, 642, 571 and 428 cm -1 , with three broad bands centred at 1049, 806 and 428 cm -1 .A significant shift of the Si-O vibration bands at 1034 and 1012 cm -1 in kaolinite to a higher frequency band at 1049 cm -1 was observed, which has been assigned to amorphous SiO 2 [25]   .The stretching vibration of Al (O,OH) 6 octahedra in kaolinite 26 is observed at 539 cm −1 , but is substituted by a peak at 806 cm −1 corresponding to the vibration band of AlO 4 tetrahedron in metakaolinite.
The 29 Si MAS-NMR spectra of kaolinite and metakaolinite are given in Figure 4.The 29 Si MAS-NMR spectrum of kaolinite (Figure 4a) displays two well resolved signals at -90.9 and -91.4 ppm attributed to the existence of two different but equally populated silicon sites.Similar results have been reported in several studies 27,28 .According to Thompson and Barron1 27 , the interlayer hydrogen bonding resulting in two different silicon environments is the main reason for the 29 Si MAS-NMR signal splitting in kaolinite.The 29 Si MAS-NMR spectrum of metakaolinite (Figure 4b) displays a broad signal around -96.3 ppm, indicating its heterogeneity, with Si linked to four other Si atoms in silica polymorphs 29 and the presence of amorphous silica 30 .According to Mackenzie et al. 31 , when kaolinite is dehydroxylated, the Si atoms undergo a range of environments of different distortion and the broadness of the metakaolinite line is attributed to these variations in the Si-O-Si(Al) bond angles.The 27 Al MAS-NMR spectrum of kaolinite (Figure 4a) consists of a sharp octahedral Al-O resonance at -3.4 ppm attributed to 6-coordinated Al.The 27 Al MAS-NMR spectrum of metakaolinite (Figure 4b) contains broad overlapping resonances at -0.3, 23.4 and 45.9 ppm that can be attributed to 6-, 5-and 4-coordinated Al local environments, in agreement with data reported by Rocha 32 , although different compared with those reported by several authors 30,33 .This reveals that the dehydroxylation of kaolinite brings about major changes, especially in the octahedral Al-O layers which are most closely associated with the structural hydroxyl groups 31 .The broader and asymmetrical peak shapes of metakaolinite show its disordered structure 33 .However, it is very complicate to speculate on the metakaolinite structure, taking into account that there are a number of transitional stages, f,i, related to the temperature and duration of the kaolinite structure breakdown.
The thermal decomposition of kaolinite is illustrated in the TG and its first derivative (DTG) curves on Figure 5.At < 100 °C, low temperature release of absorbed water occurs.kaolinite undergoes a pre-dehydroxylation state 35 .Between ~450-600 °C, the kaolinite dehydroxylation occurs, with the transformation to a non-crystalline phase, metakaolinite.According to Varga 36 , the transformation of kaolinite into metakaolinite by dehydroxylation results in structural disturbances through the breaking of unstable bonds.As a result, the degree of ordering became lower than that in kaolinite as dehydroxylation progressed.The structure of metakaolinite does not collapse but, rather, retains a layered structure.A progressive decomposition of the metakaolinite occurs up to 900 °C.An exothermic peak at ~1000 °C reveals the breakdown of the metakaolinite structure and the formation of mullite or Si-Al spinel with mullite-like composition, as reported by Chakravorty and Ghosh 37 .

XRD analysis
As mentioned before, the structural frameworks of both SOD and CAN are similar.As a result many diffraction peaks  Between ~100-450 °C, a pre-dehydration process takes place as a result of the reorganization in the octahedral layer, first occurring at the OH of the surface 34 .After dehydration, occur at the same d-spacings for the two phases.The main identifying peaks for CAN are the 101 and 211 occurring at d ~ 4.67 and 3.24 Å, respectively, which are not found in a SOD diffraction pattern 1 .The XRD patterns of the untreated starting materials and the as-synthesized products obtained after hydrothermal treatment under alkaline conditions are presented in Figure 6.The hydrothermal treatment of kaolinite (Figure 6a) and metakaolinite (Figure 6b) under these experimental conditions resulted in the cocrystallization of SOD and CAN, along with metastable zeolite LTA.Therefore, a mix of new phases (SOD and CAN), with zeolite LTA, in the synthesis products, is observed.A higher reaction time promotes a more effective dissolution of the starting materials, accompanied by more crystalline zeotypes, as shown by an increase of the relative intensity of the XRD peaks of SOD and CAN.In general, a decrease in the kaolinite relative intensity suggests that the formation of zeolite LTA, CAN and SOD enhanced the dissolution of kaolinite by removing Al from solution, whereas the transformation of the amorphous character of the metakaolinite to crystalline phases is revealed as well.

SEM analysis
A SEM study was carried out to provide information about the particle morphology and crystal growth process.SEM images (Figures 7-8) illustrate the occurrence of the as-synthesized zeotypes obtained after activation of the starting materials, revealing a change in morphology of the original surface of the particles of the starting materials.We highlight more examples of the zeotypes obtained using kaolinite as a starting material, taking into account that the synthesis products reveal the occurrence of SOD and CAN along with traces of zeolite LTA, which as the reaction proceeds display a metastable behaviour.On the other hand, an almost pure zeolite A (with minor SOD and CAN) was obtained after transformation of metakaolinite.
Using kaolinite as a starting material, the following morphologies were observed in the as-synthesized zeotypes.Well developed crystals of zeolite LTA displaying a cubic morphology and interpenetration-twinning are observed in Figure 7a-d.Note the spherical aggregates of SOD growing out onto the surface of zeolite LTA as reported by Ríos and co-workers 6,14,15 , revealing that SOD can be the result of secondary nucleation as reported previously 38 and not the result of direct solid-state transformation.As shown in Figure 7e, f CAN occurs as lepispheres composed of intergrown thin disks that grew at the expenses of SOD (a wedge-shaped blade morphology).The phase transformation SOD → CAN has been reported by Barnes and co-workers [1][2]4 . Acording to Deng et al. 39 , CAN occurs as more tiny hexagonal needles in the lepispheres formed.We have obtained similar morphologies in the lephispheric aggregates, although at higher temperature and basicity of the medium. Th highly-symmetric hexagonal needle shape reflected CAN's hexagonal symmetry (P63) 40 and is an indication of high crystallinity.With reaction time, a small concentration of CAN is seen by XRD, although it is morphologically indistinguishable by SEM simply by grain size (Scherrer law, etc.).
Figure 8a-c show the main morphological features observed in the zeotypes synthesized from metakaolinite, typical cubic crystals of zeolite LTA (interpenetrationtwinning) and spheric aggregates of SOD. Figure 8d reveals solid spherical crystallites of SOD and residual metakaolinite.No needle-like crystals of SOD were observed.

FT-IR analysis
The FT-IR spectra of the starting materials and assynthesized products obtained after its hydrothermal treatment with alkaline solutions are shown in Figure 9.The characteristic vibrations bands of the starting materials disappeared with reaction time.In general, the characteristic bands of the starting materials shift of transmittance to lower wavenumbers indicates that there are more Al substitution in tetrahedral sites of the silica framework with NaOH acting as a structure modifier as reported in other studies 41,42 .
As shown in Figure 9a, the absorption bands of the synthesis products at low temperature show the asymmetric and symmetric vibration bands characteristic of a mixture of LTA zeolite, SOD, and CAN in the region 1200-400 cm -1 .Barnes et al. 4 summarized the reported assignments of vibrations for SOD and CAN in this range.Coincident with the disappearance of kaolinite, characteristic zeolite bands appeared on the spectra, including the asymmetric Al-O stretch located in the region of 1250-950 cm -1 , and their symmetric Al-O stretch located in the region of 770-660 cm -1 .The region 1200-850 cm -1 shows a single vibration band centred at 960-965 cm -1 , associated with the presence of CAN.The transmittance of the asymmetric stretch of the Si-O-Al in the SOD framework consists of a single band at 980 cm -1 , which split into four bands at 1110, 1050, 1020, and 960 cm -1 assigned to CAN 4 .However, this study reports only the bands at 975 and 960-965 cm -1 , corresponding to SOD and CAN, respectively.The region 850-750 cm -1 reveals a weak vibration band at 766-768 cm -1 , characteristic of CAN.Several bands in the region 750-650 cm -1 indicate the presence of a mixture of SOD and CAN in the as-synthesized products.The bands in the region 650-500 cm -1 are related to the presence of the double rings (D4R and D6R) in the framework structures of the zeolitic materials 9 .The bands in the region 500-420 cm -1 are related to internal tetrahedron vibrations of Si-O and Al-O of the zeotypes.The region 800-400 cm -1 can be considered as the fingerprint region for zeolite LTA, SOD, and CAN as suggested in previous studies 4,41 .As shown in Figure 9b, The characteristic vibration bands of metakaolinite disappeared, accompanied by the appearance of new peaks that reveal   the occurrence of zeolite LTA.The band at 1047 cm -1 was shifted to lower frequency bands (at 973-980 cm -1 ); the band at 800 cm -1 disappeared in the zeolitic products; the low intensity bands at 635 and 565 cm -1 were shifted to higher (at 673-690 cm -1 ) and lower (at 530-538 cm -1 ) frequency bands, respectively.The band at 424 cm -1 was shifted to lower frequency bands (at 411-417 cm -1 ).The typical bands of zeolite LTA representing the asymmetric Al-O (973-980 cm -1 ) and symmetric Al-O (673-690 cm -1 ) stretches, double rings (530-538 cm -1 ) in the framework structures of the zeolitic materials were observed and they show a constant intensity.The FT-IR peak width is an indication of the level of the crystallinity of the synthesis products.
3.2.4. 29Si and 27 Al MAS-NMR Figure 10 shows the 29 Si and 27 Al MAS-NMR spectra of the starting kaolinite and representative as-synthesized zeotypes.The 29 Si MAS-NMR spectrum of the synthesis products show three resonances at -83.6, -86.3 and -91.2 ppm.The chemical shifts at -83.6 and -86.3 ppm can be attributed to Q 4 (4Al) environments in SOD and CAN presence of the two sites indicates imperfect Al/ Si ordering in tetrahedral sites.CAN was reported to have a chemical shift of -87.3 ppm 4 .Therefore, in this study the signal at -86.3 ppm can be presumably attributed to CAN.In some studies, 29 Si MAS-NMR has been used to differentiate between SOD and CAN 44,45 .However, according to Barnes et al. 4 , no difference in chemical shift between these zeotypes can be recognized in terms of 29 Si MAS-NMR.The -91.2 ppm chemical shift can be assigned to the presence of residual kaolinite.The 27 Al MAS-NMR spectrum of the synthesis products shows a single resonance at 59.5 ppm, which can be assigned to tetrahedral Al in the framework of the zeotypes, but with some octahedral Al corresponding to the presence of residual kaolinite.The reaction of the kaolinite with NaOH as mineralizer resulted in a shift of Al from octahedral to tetrahedral coordination, which is likely due to the dissolution of this clay mineral and the subsequent precipitation of the zeotypes.The amount of Al( 6) is greatest in kaolinite (Figure 10a) where the initial Al concentration was larger compared with that in the synthesis products (Figure 10b).The decrease in the Al(6)/Al(4) ratio can be explained by the dissolution of kaolinite, with Al coordination changing from octahedral to tetrahedral, which is consistent with the formation of zeolite-type materials.
Figure 11 shows the 29 Si and 27 Al MAS-NMR spectra of the starting metakaolinite and representative as-synthesized zeotypes.The 29 Si MAS-NMR spectrum of the synthesis products shows a sharp signal at -89.5 ppm, which is characteristic of Q 4 (4Al) sites in zeolite LTA, similar to that observed by Akolekar et al. 20 .A weak signal at -85.9 ppm can be attributed to the contribution of Q 4 (4Al) sites of SOD and CAN. 29Si MAS-NMR results point out the change of the silica morphology from amorphous SiO 2 in the metakaolinite to crystalline SiO 2 in the zeotypes, with the total dissolution of the starting metakaolinite to form zeolite-type structures and the incorporation of aluminium.The 27 Al MAS-NMR spectrum of the synthesis products shows a single resonance at 58.2 ppm, which can be assigned to tetrahedral Al in the zeotypes.
Figure 10. 29Si and 27 Al MAS-NMR spectra of the (a) starting kaolinite and (b) representative as-synthesized zeotypes obtained after hydrothermal transformation of kaolinite in 1.33 M NaOH solutions at 100 °C after 120 hours of reaction.

TG analysis
DTG curves of the representative as-synthesized zeotypes are shown in Figure 12.The synthesis products show up to four dehydration steps.The position of these DTG peaks and the number of dehydration steps can be attributed to the different compensating cation-water binding energies as well as to the different energy associated with the diffusion of the desorbed water through the porous structure of the as-synthesized products; their weight loss percentages reflects the water loss from the zeolite structure, and the amount of desorbed water is related with the number of compensation cations in the framework of the zeolite 46 .The peaks observed between 39-52 °C correspond to surface water in zeolitic materials; the peaks observed between 100-162 °C indicate zeolitic water, although in some cases in this to two peaks occur, which can be explained by the heterogeneous nature of the assynthesized products.

Phase transformation from SOD to CAN
The SOD and CAN zeotypes present a ratio of Si/Al = 1, being one of the few structures that have compensation  anions 47 .Therefore, it is not surprising that they can be synthesized under similar conditions.The formation of a specific structure (SOD-type or CAN-type), depends on, among other factors, the symmetry, the charge of the anion and the basicity of the medium 47 .Walton et al. 48studied on the influence of the reaction condition of the hydrothermal crystallization and the transformation of zeolite LTA into SOD by time resolved in situ energy dispersive X ray diffraction.
According to Barnes et al. 2 , the phase transformation from SOD to CAN was a solution-mediated process, which involved a four-stage mechanism: (1) dissolution of SOD, (2) relative CAN supersaturation as a result of SOD dissolution, (3) nucleation of CAN, and (4) simultaneous growth of CAN nuclei and dissolution of SOD.
Results from this study are in agreement with regard to SOD forms first and then transforms slowly to CAN at a rate that increases with increasing temperature 4,17 .Consequently, the formation of these aluminosilicate phases may be considered as being dominated by kinetics rather than thermodynamics 2 .
A mixture of LTA zeolite, SOD, and CAN was obtained at 100 °C, as demonstrated by XRD patterns shown in Figure 2. The formation of these zeolitic materials is in agreement with the previous study by Ríos et al. 14 , which suggest that zeolite LTA is the main zeolitic phase at shorter reaction times, whereas SOD-and CAN-type structures were formed from hydrogels at longer reaction times.

Nucleation and growth process of SOD and CAN
During recent years, several research groups have made great efforts to successfully bridge the gap between the general formation mechanisms of crystalline phases and the crystallization of complex structures, such as those of zeolites 49 .However, specific information concerned with crystallization mechanism and kinetics is limited 9 .The chemistry of evolution of relatively simple zeolite systems, can be affected by several factors, such as the formation of intermediate metastable phases (e.g.zeolite LTA), the occurrence of simultaneous reactions such as precipitation and dissolution of a gel phase, nucleation and growth of zeotypes, dissolution of the early metastable phases, nucleation and growth of more stable phases such as SOD and CAN, with a dissolution of the initial crystals, and nucleation and growth of the crystalline phases that reached chemical equilibrium.However, the reaction history of the zeolite synthesis developed in this study can be summarized as follows.
The process of synthesis can be described by two main stages: (1) the dissolution of aluminosilicate compounds releasing Si and Al and (2) the zeolite crystallization.Zeolites are typically crystallized from amorphous aluminosilicate precursors in aqueous in the presence of alkali metals.A simple scheme of the crystallization of an amorphous aluminosilicate hydrogel to SOD and CAN is given in Figure 13.
At the beginning of the synthesis process, a dissolution of the starting materials occurred, with the production of an amorphous gel characterized by the presence of small olygomers (Figure 13a).A dissolution process promotes the formation of the nutrients (ionic species) which then are transported to the nucleation sites, indicating that the ionic species are not static, because they necessarily need to move (transportation) to the nucleation sites.A nucleation process (Figure 13b) produced an equilibrated gel.During nucleation, hydrogel composition and structure are significantly affected by thermodynamic and kinetic parameters.A polymerization of SiO 4 tetrahedra proceed (Figure 13c), which is represented by TO 4 primary tetrahedral building units have been joined, revealing how they link together to form larger structures.A polymerization is the process that forms the zeolite precursors, which contains tetrahedral of Si and Al randomly distributed along the polymeric chains that are cross-linked so as to provide cavities large enough to accommodate the charge balancing alkali ions.During crystal growth (Figure 13d) the TO 4 units were linked, with the formation of 4-ring and 6-ring, composed by 4 and 6 tetrahedral atoms, respectively, to create a large structure (secondary building unit) like the SOD β-cage.The crystallization of zeolite LTA occurred by linking the same secondary building units together (Figure 13e).A phase transformation occurred as represented by the sequence of reaction: zeolite LTA → SOD → CAN (Figure 13f).
According to Breck 9 , the hydrogel composition and structure is controlled by the size and structure of the polymerization species and gelation controls the nucleation of the zeolite crystallites, which is supported by the crystal size and morphology of zeolites grown from hydrogels.It is clear from data reported by Barnes et al. 2 that at lower temperature and NaOH concentration than those used in this study zeolite LTA transforms to zeolite P. We consider that SOD should appear after zeolite LTA without the formation of zeolite P.However, Ríos 15 reported the co-crystallization of SOD + CAN, which was followed by the crystallization of the JBW-type structure at higher temperature and NaOH concentration.Ogura et al. 50reported that cyclic tetramer and hexamer species could be found in the liquid phase to generate SOD cages.SOD nucleated homogeneously in the liquid phase and grown without intervention of hydrogel 51 .On the other hand, results reported by Lee et al. 51 clearly explain that SOD crystallization takes place by the solid− solid transformation without involving any solution or amorphous-mediated process.However, the transformation of SOD to CAN involves a solution-mediated mechanism with SOD dissolution and subsequent CAN precipitation as suggested by Barnes et al. 2 .They also demonstrated that CAN is formed at expense of two dimorphs of cubic SOD, which show a notable change in unit cell size as reported by Zheng et al. 17  The mineral transformations and the stability of the end products were controlled by experimental parameters like NaOH concentration, temperature and Si/Al ratio.However, they were likely affected by several factors, such as the formation of intermediate metastable phases and the occurrence of different chemical processes, including precipitation and dissolution of a gel phase, nucleation and growth of zeotypes, dissolution of the early metastable phases, nucleation and growth of more stable phases (SOD and CAN), with a dissolution of the initial crystals and nucleation and growth of the crystalline phases that reached chemical equilibrium.
Ríos et al. 6,14,15 have demonstrated that the occurrence of a sequence of chemical reactions from amorphous aluminosilicate gel to crystalline zeotypes, which can be summarized as follows: Poorly crystalline aluminosilicate → zeolite LTA → SOD → SOD + CAN → CAN.
The hydrothermal transformation of the starting kaolinite and metakaolinite at low temperature presumably reveals a similar sequence of reactions, starting with an amorphous material after dissolution of the aluminosilicate phases, followed by zeolite LTA as the first and metastable phase formed, SOD-type structure, a mixture of SOD-type and CAN-type structures and finally CAN (apparently more stable than SOD) as the stable and final synthesis product.However, according to Barnes et al. 2 , CAN appeared more stable than SOD, concluding that CAN would be the final product, whereas amorphous material, zeolite LTA and SOD would be transition phases.Fechtelkord et al. 52 argued that SOD is the thermodynamically more stable phase and CAN formation is kinetically favored at lower NaOH concentrations.Similar results can be assumed in this study.We are in agreement with the fact that these phase transformations did not occur in the solid state and, therefore, represent a solution-mediated mechanism as suggested by previous studies 2,39 .According to XRD data a co-crystallization of zeolite LTA, SOD and CAN occurs, being very difficult to evaluate phase transformations due to the very constant behaviour of the XRD peaks of these zeotypes and to the overlapping of peaks of SOD-type and CAN-type structures, which makes difficult to quantifying SOD and CAN mixtures.However, SEM data reveal phase relationships in which (1) lepispheric aggregates of SOD and CAN grow onto the surface of cubic crystals of zeolite LTA or (2) CAN grows at the expense of SOD.Under the experimental conditions considered by Deng et al. 39 there is enough evidence about phase transformations that occur in this chemical system.However, in this study a slight higher temperature (100 °C) was used, which can explain why zeolite LTA (a metastable phase) occurs as a minor phase, compared with the presence of SOD and CAN.

Conclusions
Zeotypes such a s SOD and CAN were successfully synthesized via hydrothermal treatment using kaolinite and metakaolinite as starting materials and NaOH as activating agent.However, zeotype formation was assessed to be fairly unsuccessful via zeolite LTA, taking into account that the synthesis products area characterized by the occurrence of SOD and CAN as metastable phases.The structural similarity between zeolites and feldspathoids explain why they can coexist in the as-synthesized products and the dominant crystalline phase depends on formation conditions.The main crystalline phases obtained after synthesis process were zeolite LTA, SOD and CAN.The synthesis products are strongly controlled by the starting material, taking into account that metakaolinite is a more reactive phase than kaolinite.There were two major chemical processes involved in the reaction between the starting kaolinite and metakaolinite and alkaline solutions: dissolution of the starting materials followed by the formation of zeolitic materials.Future research, should consider to develop further experiments under well-optimized conditions for obtaining individual phases instead of mixtures, and evidently in order to clarify the sequence of phase crystallization.

Figure 1 .
Figure 1.Framework structures of SOD and CAN, showing their characteristic cages and channels, adopted and modified from the Database of Zeolite Structures 7 .

Figure 6 .
Figure 6.X-ray diffraction patterns of the as-synthesized zeotypes after reaction of the starting (a) kaolinite and (b) metakaolinite in 1.33 M NaOH solutions at 100 °C for several times.

Figure 7 .
Figure 7. SEM images showing several synthesis products obtained by hydrothermal treatment of the starting kaolinite in (a) (b) 1.33 M NaOH solutions, after 6 hours of reaction at 100 °C, (c) (e) 1.33 M NaOH solutions, after 120 hours of reaction at 100 °C, and (f) 3.99 M NaOH solutions, after 120 hours of reaction at 100 °C.

Figure 8 .
Figure 8. SEM images showing several synthesis products obtained by hydrothermal treatment of the starting metakaolinite in (a) (d) 1.33 M NaOH solutions, after 52 hours of reaction at 100 °C.

Figure 9 .
Figure 9. FTIR spectra of unreacted raw materials and the as-synthesized zeotypes obtained after hydrothermal transformation of metakaolinite in 1.33 M NaOH solutions at 100 °C after several reaction times.

Figure 11 .
Figure 11. 29Si and 27 Al MAS-NMR spectra of the (a) starting metakaolinite and (b) representative as-synthesized zeotypes obtained after hydrothermal transformation of metakaolinite in 1.33 M NaOH solutions at 100 °C after 52 hours of reaction.

Figure 12 .
Figure 12.TG/DTG curves from 25 to 700 °C of the representative as-synthesized zeotypes obtained after hydrothermal transformation of (a) kaolinite and (b) metakaolinite in 1.33 M NaOH solutions at 100 °C after 120 hours and 52 hours of reaction, respectively. .

Figure 13 .
Figure 13.Illustration of the proposed sequence for the formation of zeotypes.

Table 1 .
Synthesis conditions for transformation of kaolinite and metakaolinite into SOD and CAN zeotypes.