X-ray Diffraction Analysis

Measurements by XRD revealed that both KBE1_M2 and KGa-2 contained >90% kaolinite and small amounts of accessory minerals (Table 1). The Hinckley Index of KBE1_M2 was 1.63, characteristic of a well-ordered kaolinite (Komarneni et al. Reference Komarneni, Fyfe and Kennedy1985a; Plançon et al. Reference Plançon, Giese, Snyder, Drits and Bookin1989). The small Hinckley Index of KGa-2 of 0.5 was in accordance with published values of 0.24–0.41 (Metz and Ganor Reference Metz and Ganor2001; Du et al. Reference Du, Morris, Pushkarova and Smart2010; Ndlovu et al. Reference Ndlovu, Farrokhpay, Forbes and Bradshaw2015) and confirmed a high degree of structural disorder.

Table 1. Phase contents (wt.%) of KBE1_M2 and KGa-2

KBE1_M2 consists of 46–47 mass% ordered kaolinite and 50–51 mass% disordered kaolinite. The disordered kaolinite is characterized by 93% BB/7% BC stacking sequences. 88% of BB sequences have no additional ~b/3 stacking errors and, thus, the kaolinite is low b-axis error-ordered. KGa-2 is characterized by 86% BB stacking sequences with a strong tendency for additional ~b/3 translations (Ufer et al. Reference Ufer, Kleeberg and Monecke2015) and a low abundance of BC stacking. Thus, KGa-2 is intermediate disordered.

Both dickites are also quite pure and well ordered. The dickite from Altenberg (dickite AB) actually consisted of 100% dickite while the dickite from Kohlendorf (dickite KD) contained 4% accessory minerals.

Simultaneous Thermal Analysis

Simultaneous thermal analysis of the two natural, air-dry kaolinites (Fig. 3) showed one endothermic peak in the region between 400 and 800°C, which was associated with a maximum in the MS curve of evolved water (m/z = 18) at 562°C (KBE1_M2) and at 546°C (KGa-2) due to the dehydroxylation (DHX) of kaolinite. The normalized mass loss of 13.25 mass% and 13.24 mass% for KBE1_M2 and KGa-2, respectively, reflected perfectly the kaolinite content in both materials of ~95% determined by XRD. The DHX of KGa-2 occurred at a slightly lower temperature than did DHX of the KBE1_M2 (Figs 3 and 4).

Fig. 3. DSC curve (green colors) and MS curve (m/z =18) of evolved water (blue colors) for both kaolinites (left) and DSC curve (red colors) and MS curve (m/z =18) of evolved water (blue colors) for both dickites (right)

Fig. 4. Extent of DHX as a percentage of stoichiometric mass loss of 13.95% of kaolinites (left) and dickite (right)

At temperatures >950°C, two further peaks were visible in the DSC curve of both kaolinites. Neither peak is associated with any gas release and can be assigned to the first and second recrystallization of kaolinite into a spinel and mullite (Mackenzie et al. Reference Mackenzie, Brown, Meinhold and Bowden1985).

The DHX of both dickites occurred at between 500 and 900°C, in two steps. A two-step DHX of dickites was also observed in previous studies (Frost and Vassallo Reference Frost and Vassallo1996; Franco and Ruiz Cruz Reference Franco and Ruiz Cruz2006). The first DHX of dickite KD is very broad while dickite AB shows a sharp peak at ~650°C. The maximum of the second endothermal peak and the second peak in the MS curve of evolved water (m/z = 18) was at ~679°C (Fig. 3). The degree of DHX of dickite KD was greater, at temperatures between 400 and 650°C, before full DHX is reached for both dickites (Fig. 4). The normalized mass loss of 13.95% (dickite AB) and 13.43% (dickite KD) corresponds to the dickite content determined by XRD.

The ratio of peak areas with a peak temperature below and above 665°C was ~40:60 for both dickites (Fig. 5).

Fig. 5. MS curve (m/z =18) of evolved water during DHX of dickite KD (left) and dickite AB (right) with decomposed peaks

NMR Measurements

The ²⁷Al MAS NMR spectra (Fig. 6) of the two unheated kaolinite samples showed one main resonance signal at ~0 ppm, which revealed octahedrally coordinated Al (Al^VI). Both show an additional minor resonance signal at ~75 ppm, which indicated tetrahedrally (Al^IV) coordinated Al, confirming the presence of dioctahedral mica with Al substitutions for Si in the tetrahedral sheet.

Fig. 6. ²⁷Al MAS NMR spectra of the two untreated kaolinite samples (* ssb = spinning side bands)

The ²⁷Al MAS NMR spectra of KBE1_M2 showed an additional signal between 20 and 40 ppm. Broad ²⁷Al signals at ~20 to 40 ppm were observed for poorly crystalline aluminosilicates, X-ray amorphous aluminosilicate glasses, or gels (MacKenzie Reference MacKenzie2000); or for minerals with 5-fold coordinated Al (Al^V) such as andalusite (Lippmaa et al. Reference Lippmaa, Samoson and Magi1986), grandidierite (Smith and Steuernagel Reference Smith and Steuernagel1992), or augelite (Bleam et al. Reference Bleam, Dec and Frye1989). Neither amorphous material nor the above-mentioned minerals could be detected in KBE1_M2.

With increasing temperature, the overall intensity of the ²⁷Al MAS NMR signals of both samples decreased drastically (Fig. 7). This indicates that only a small part of the remaining short-range order is reflected, as metakaolinite is characterized by a complex amorphous structure (Brindley and Nakahira Reference Brindley and Nakahira1959; Bellotto et al. Reference Bellotto, Gualtieri, Artioli and Clark1995).

Fig. 7. ²⁷Al MAS NMR spectra of KBE1_M2 and KGa-2 heated to indicated temperatures without isothermal annealing (* ssb = spinning side bands)

The recrystallization products of kaolinite are spinel and mullite. Calculations based on the structural formulaa of spinel (Verwey Reference Verwey1935; Low and McPherson Reference Low and McPherson1988; Rozita et al. Reference Rozita, Brydson and Scott2010) and mullite (Angel et al. Reference Angel, McMullan and Prewitt1991; Lee et al. Reference Lee, Cheng, Heine and Klinowski1997; Schneider et al. Reference Schneider, Schreuer and Hildmann2008) showed that the amount of Al^IV ranges between 25–37.5% and 56–58%, respectively (Fig. 8). Accordingly, the Al^VI is in the range 62.5–75% for spinel and 42–44% for mullite (Fig. 8). The measured Al^VI and Al^IV ratios match the ratio in the defect spinel/inverse spinel at ~1100°C and the ratio in the mullite at ~1400°C.

Fig. 8. Amount of Al^IV in the kaolinite–mullite reaction

Both samples showed a decreasing Al^VI signal and an increasing Al^IV signal as well as an increasing signal between 28 and 40 ppm with progressive DHX (Figs. 7 and 9). Thereby, the chemical shift of 28–40 ppm for the new signal is greater than published data of 20–35 ppm (Komarneni et al. Reference Komarneni, Roy, Roy, Fyfe, Kennedy, Bother-by, Dadok and Chesnick1985b; Rocha et al. Reference Rocha, Adams and Klinowski1990; He et al. Reference He, Guo, Zhu and Hu2003; Fabbri et al. Reference Fabbri, Gualtieri and Leonardi2013) and is closer to common chemical shifts of Al^IV (50–80 ppm) in clay minerals. With incipient recrystallization after DHX the Al^VI signal increased again and the signal between 28–40 ppm decreased (Fig. 7). The ²⁷Al MAS NMR spectra of both kaolinites at 1100°C showed one signal for Al^VI and one for Al^IV.

Fig. 9. Fractions of different Al coordinations vs. calcination temperature of KBE1_M2 and KGa-2

With increasing temperature, the quadrupole coupling constant (QCC) for all three signals in the ²⁷ Al NMR spectra increased to values >4 MHz (Fig. 10), characteristic of heavily distorted coordination shells of Al (Fyfe et al. Reference Fyfe, Bretherton and Lam2000, Reference Fyfe, Bretherton and Lam2001; van Bokhoven et al. Reference van Bokhoven, Koningsberger, Kunkeler, van Bekkum and Kentgens2000; Gore et al. Reference Gore, Abraham, Hegde, Kumar, Amoureux and Ganapathy2002; Omegna et al. Reference Omegna, van Bokhoven and Prins2003).

Fig. 10. Quadrupole coupling constants (QCC) for KBE-1_M2 (left) and KGa-2 (right)

First-Principles Calculations

The transformation of kaolinite, disordered kaolinite, and dickite into metakaolinite, metadiskaolinite, and metadickite via several steps through dehydroxylation was investigated using DFT. First, the thermodynamic grand canonical potential was calculated for the identification of stable phases for the desorption process of water. The desorption energy alone does not allow conclusions about the stability of a specific structure, however. Rather, one has to take into account the chemical potentials μ(A _i) of the surface constituents, A _i, in order to compare energetically the interfaces with different stoichiometries. The ground state of the surface is determined by the minimum of the thermodynamic grand canonical potential Ω:

$\Omega =F-{\sum }_i\mu \left(A_i\right)\bullet n_i+q\bullet \left(E_F+E_{\mathrm{VBM}}\right)$

where F = E – TS is the surface free energy. Here, F is approximated by the total surface energy, E, assuming similar entropy contributions, S, for different adsorption configurations. In fact, the differences in vibrational free energy and electronic entropy are typically several orders of magnitude smaller than adsorption energies resulting from chemical-bond formation as found in the present case. The last term on the right-hand side accounts for the energy changes due to a possible surface charge, q, which creates a dependence on the chemical potential of electrons given here by the Fermi level (E _F) measured relative to the valence-band maximum (E _VBM).

The transformation of kaolinite into the Al₁₂Si₁₂O₃₉(OH)₆ phase occurred at the third step of water removal when the chemical potential of water was –1.93 eV (Fig. 11 left). The final product was a metakaolinite at a water chemical potential of –2.67 eV. A phase which had the chemical formula of Al₁₂Si₁₂O₃₉(OH)₆ was produced. Other phases are unstable and do not exist thermodynamically. The phase transformation from disordered kaolinite into Al₁₂Si₁₂O₃₃(OH)₁₈ happens at the first step of water removal and when the chemical potential of water is –2.03 eV (Fig. 12 left). The final product was metadiskaolinite when the chemical potential of water was –2.319 eV. The intersection of the three lines of purple, red, and green (Fig. 13 left) for the phase transformation from dickite into metadickite is not very clear but was much more evident from Fig. 14. The dehydroxylation of dickite to Al₂₄Si₂₄O₇₂(OH)₂₄ took place at the second step of water removal at a water chemical potential of –2.717 eV (Fig. 14). The final product was metadickite at the chemical potential of water of –2.82 eV.

Fig. 11. Calculated phase diagram of kaolinite–metakaolinite transformation as a function of the water chemical potential (left) and p,T-phase diagram (right)

Fig. 12. Calculated phase diagram of disordered kaolinite–metadiskaolinite transformation as a function of the water chemical potential (left) and p,T-phase diagram (right)

Fig. 13. Calculated phase diagram of the dickite–metadickite transformation as a function of the water chemical potential (left) and p,T phase diagram (right)

Fig. 14. Transition of dickite (purple) to Al₂₄Si₂₄O₇₂(OH)₂₄ (red) and to final product of metadickite Al₂₄Si₂₄O₈₄ (green) at the chemical potentials of –2.717 eV and –2.82 eV, respectively

The resulting phase diagrams as a function of partial pressure of water and temperature may be compared with experimental data. Dashed lines (Figs 11, 12, and 13) indicate an excellent example for the partial pressure of H₂O equal to 10 Pa vs. temperature. The chemical potential can be related directly to experimental conditions. The pressure- and temperature-dependent deviation, Δμ(H₂O), from the zero-temperature value obtained from the DFT calculations can be estimated within the approximation of a polyatomic ideal gas by

$\Delta {\mathrm{\mu H}}_{2}O\left(p,T\right)=k_{B}T\left(ln\left(\frac{p{\lambda }^3}{k_{B}T}\right)-ln{Z}_{\mathrm{rot}}-ln{Z}_{\mathrm{vib}}\right)$

where k_B is the Boltzmann constant, p is the pressure, T is the temperature, and λ is the de Broglie thermal wavelength of the water molecule

$\lambda =\sqrt{\frac{2\pi {\hslash }^2}{m{k}_{B}T}}$

where m represents the molecular mass and ℏ is Planck’s constant divided by 2.

$Z_{\mathrm{rot}}=\frac{{\left(2k_{B}T\right)}^{\frac{3}{2}}{\left(\pi I_1{I}_2{I}_3\right)}^{\frac{1}{2}}}{\sigma {\hslash }^3}$

and

$Z_{\mathrm{vib}}=\prod _{\alpha }{\left(1-exp\left(-\frac{\hslash {\omega }_{\alpha }}{k_{B}T}\right)\right)}^{-1}$

The first stable phase of Al₁₂Si₁₂O₃₉(OH)₆ was formed at the third step at 425°C as a result of releasing nine hydroxyl groups (Fig. 11, right). As expected, Al₁₂Si₁₂O₃₉(OH)₆ required heating to 655°C to release the three remaining hydroxyl groups resulting in the formation of metakaolinite as the final product. The first stable phase of Al₁₂Si₁₂O₃₃(OH)₁₈ happened at the first step of water removal with a heating temperature of 457°C as a result of releasing three hydroxyl groups (Fig. 12, right). To reach complete dehydroxylation, Al₁₂Si₁₂O₃₃(OH)₁₈ must be heated to 547°C for the final production of metadiskaolinite. The formation of the first stable phase of Al₂₄Si₂₄O₇₂(OH)₂₄ happened at the second step of water removal and at 665°C as a result of releasing six hydroxyl groups. The final product was metadickite (Fig. 13 right), which needed little more than 698°C for complete DHX.

Water molecules may form when hydroxyl groups are close enough to each other upon heating. During DHX the coordination of aluminum atoms changed. The first stable phase of Al₁₂Si₁₂O₃₉(OH)₆ (corresponding to a degree of DHX of 75%) from kaolinite is characterized by the ratio of 1:1:0 for Al^IV:Al^V:Al^VI (Fig. 15, middle). In contrast, the ratio is 0:1:1 for Al^IV:Al^V:Al^VI of the first stable dehydroxylated phase of Al₂₄Si₂₄O₇₂(OH)₂₄ (corresponding to a degree of DHX of 50%), observed for dickite (Fig. 17, middle).

Fig. 15. Stable, partially dehydroxylated kaolinite, Al₁₂Si₁₂O₃₀(Or)₉(OH)₆ (left); metakaolinite, Al₁₂Si₁₂O₃₀(Or)₁₂ (middle); and SEM images of ordered kaolinite KBE1_M2 (upper right) and metakaolinite KBE1_M2 700°C (lower right); scale bar: 2 μm

In contrast to Mackenzie et al. (Reference Mackenzie, Brown, Meinhold and Bowden1985) and White et al. (Reference White, Provis, Proffen, Riley and van Deventer2010a), no residual OH groups were necessary to maintain the layering of the metamaterials of any of the three 1:1 layer polytypes studied (Figs. 15, 16, and 17). The SEM images of the metamaterials confirmed the remaining platelet-like morphology (Figs. 15, 16, and 17 left).

Fig. 16. Stable, partially dehydroxylated disordered kaolinite, Al₁₂Si₁₂O₃₀(Or)₃(OH)₁₈ (left); metadiskaolinite, Al₁₂Si₁₂O₃₀(Or)₁₂ (middle); and SEM images of disordered kaolinite KGa-2 (upper right) and metadiskaolinite KGa-2 615°C (lower right); scale bar: 2 μm

Fig. 17. Stable, partially dehydroxylated dickite, Al₂₄Si₂₄O₆₀(Or)₁₂(OH)₂₄ (left); metadickite, Al₂₄Si₂₄O₆₀(Or)₂₄ (middle); and SEM images of dickite AB (upper right) and dickite AB 700°C (lower right); scale bar: 5 μm

The DHX of kaolinite into metakaolinite, disordered kaolinite into metadiskaolinite, and dickite into metadickite must be implemented only at the stationary point by use of the Birch-Murnaghan diagram (Bleam et al. Reference Bleam, Dec and Frye1989). Hence, expansion or contraction of the simulation super cells in order to compute the optimized structure at the stationary point is required. The unit cells of metakaolinite and metadiskaolinite expanded after complete dehydroxylation at the stationary points (Fig. 18). The most striking finding was that the super cells of metakaolinite and metadiskaolinite at the stationary points expanded by roughly 4% and 8% in comparison with kaolinite and disordered kaolinite, respectively. In contrast, no significant changing in the super-cell volume after complete dehydroxylation of dickite was observed.

Fig. 18. Birch-Murnaghan diagram of kaolinite-to-metakaolinite, disordered kaolinite-to-metadiskaolinite, and dickite-to-metadickite for calculation of the optimized structure at the stationary point

The volume of kaolinite decreased after DHX by ~0.8 to 2%, determined by dilatometer measurements of kaolins, and the volume of dickite increased after DHX by ~6.5 to 7% (Schomburg and Störr Reference Schomburg and Störr1984). While the calculated values from the Birch-Murnaghan equation of state predict the changes of the material at the atomic level, a direct correlation with measurements performed by optical dilatometer might by difficult.

One of the most important differences in polytype structures concerning the material in the present study is the differing orientation of OH groups. From a thermodynamic point of view, the hydrogen bonding of dickite is more stable than the zigzag hydrogen bonding of kaolinite between layers. This finally affects the dehydroxylation temperatures and volume changes of the super cells. T _DHX,disKao < T _DHX,Kao < T _DHX,Dic was found experimentally and confirmed results from previous studies (e.g. Stoch and Wacławska Reference Stoch and Wacławska1981a, Reference Stoch and Wacławska1981b).

H-bonding plays a considerable role in binding layers together. To discover the strongest clay mineral, the total H-bonding energy between two immediate layers was summed and divided by the area influenced. The bond length of OH determines the calculation of the H-bonding energy (Jones et al. Reference Jones, Jenkins and King2006). From DFT calculations, dickite is considered to be the strongest material with an average energy of –0.032 eV/Ų, which is in accordance with experiments. Kaolinite and disordered kaolinite were specified as the second and third strongest materials after kaolinite with average energies of –0.031 eV/Ų and –0.02 eV/Ų, respectively.

Furthermore, two different energies (–0.03 eV/Ų and –0.032 eV/Ų) for dickite were found, correlating to the B-C-B-C stacking sequence of layers. In contrast to that, kaolinite and metakaolinite contain only a simple B-B-B sequence of layers, which leads in the end to only one sticking energy for each material. The number of sticking energies correlated with the number of peaks that were found in the STA experiment instead of with stepwise release of inner and outer hydroxyl groups of the octahedral sheet (Frost and Vassallo Reference Frost and Vassallo1996).

The T _DHX of disordered KGa-2 is less than that of well-ordered KBE1_M2. The disorder in KGa-2 is caused by b/3 translation. For dickites, no structural models are available to describe disorder by stacking faults. In any case, the two different vacancies in the dioctahedral 1:1 layers cause a two-step dehydroxylation and the a/3 translation of the 1:1 layers results in a higher dehydroxylation temperature.

The deviation for the results between experimental and simulation studies is negligible. Two main reasons are elucidated briefly here. First, results from the computational study were limited to the thermodynamic point of view. Some of the processes might be kinetically driven or kinetically hindered. Both would mean that a thermodynamic sequence of structures is not representative. Second, as White et al. (Reference White, Provis, Proffen, Riley and van Deventer2010b) reported, more than 3×10⁸⁴ possible transformation paths exist to obtain metakaolinite from kaolinite. Thus, the results here demonstrated a possible DHX path correctly, as calculation of all possible paths is impossible.