Alberto Escudero

Application of 29Si and 27Al MAS NMR spectroscopy to the study of the reaction mechanism of kaolinite to illite/muscovite.

Understanding the mechanisms for illitization of clay minerals has important applications in reconstructing geologic histories and determining the origins of physical and chemical characteristics of buried sediments. While many studies have been carried out on this topic, few have focused on the mechanism of illite formation from kaolinite. The purpose of this study was to investigate more deeply the illitization of kaolinite in KOH solution at a high solid/liquid ratio (1000 mg/mL). X-ray diffraction (XRD) and infrared spectroscopy were used to follow the formation of new crystalline phases and the composition of the octahedral sheet, while the transformation of the Si and Al local environments was analyzed by 29Si and 27Al magic angle spinning nuclear magnetic resonance spectroscopy (MAS NMR). The results revealed that the first reaction stage consists of the diffusion of Al from the octahedral to the tetrahedral sheet of the kaolinite TO layers, giving rise to the precursors of the illite/muscovite nuclei. Combination of XRD with 27Al MAS NMR measurements indicated that a minimum amount of tetrahedral Al is required in the original TO layer before condensation of a second tetrahedral sheet occurs to complete the formation of the illite/muscovite TOT layers.

The hydrothermal conversion of kaolinite to kalsilite: Influence of time, temperature, and pH

Abstract Kalsilite (the low-temperature form of KAlSiO4) is used as the precursor of leucite, an important component in porcelain-fused-to-metal and ceramic-restoration systems, and it has also been proposed as a high-thermal expansion ceramic for bonding to metals. The present study reports the hydrothermal synthesis and characterization of pure kalsilite from kaolinite in subcritical conditions, as well as the characterization of the intermediate products by means of XRD, 29Si and 27Al MAS NMR, IR, SEM, and TEM. Effects of time, temperature, and pH on the reaction products are analyzed. The experimental data indicate that pure kalsilite is obtained after hydrothermal treatment of kaolinite at 300°C for 12 h in 0.5 M KOH solution. Longer reaction times increase the crystallinity of the structure, whereas lower reaction times give rise to the metastable ABW-type KAlSiO4 polymorph. Lower temperatures are not sufficient to produce kalsilite, but zeolite W is obtained instead as the unique reaction product. Finally, the pH of the aqueous solution in contact with kaolinite is an important parameter for the synthesis of kalsilite, which must be ≥13.70

Aluminum solubility in TiO2 rutile at high pressure and experimental evidence for a CaCl2-structured polymorph

Abstract Aluminum incorporation into TiO2 has been studied in the TiO2-Al2O3 system as a function of pressure at temperatures of 900 and 1300 °C using commercial Al2TiO5 nanopowder as starting material. A new orthorhombic TiO2 polymorph with the CaCl2 structure has been observed in the recovered samples synthesized from 4.5 to 7 GPa and 900 °C and from 2.5 to 7 GPa at 1300 °C. The phase transition to the α-PbO2 type TiO2 phase takes place between 7 and 10 GPa at both temperatures. Two mechanisms of Al incorporation in TiO2 rutile have been observed in the recovered samples. The substitution of Ti4+ by Al3+ on normal octahedral sites is dominant at lower pressures. High pressure induces the incorporation of Al3+ into octahedral interstices of the rutile structure, which is responsible for an orthorhombic distortion of the TiO2 rutile structure and gives rise to a (110) twinned CaCl2 type structure. This phase is probably a result of temperature quench at high pressure. Aluminum solubility in TiO2 increases with increasing pressure. TiO2 is able to accommodate up to 9.8 wt% Al2O3 at 7 GPa and 1300 °C. Temperature has a large effect on the aluminum incorporation in TiO2, especially at higher pressures. High pressure has a strong effect on both the chemistry and the microstructure of Al-doped TiO2. Enhanced aluminum concentration in TiO2 rutile as well as TiO2 grains with a microstructure consisting of twins are a clear indication of high-pressure conditions.

Mineralogical stability of phyllosilicates in hyperalkaline fluids: Influence of layer nature, octahedral occupation and presence of tetrahedral Al

Abstract Mineralogical changes in a set of phyllosilicates, differing in their layer nature, chemical composition, octahedral character, and Al content of the tetrahedral sheet, were analyzed after hydrothermal reaction in an alkaline solution. The composition of the alkaline solution was selected to simulate the first stage of cement degradation [NaOH-KOH-Ca(OH)2]. The reaction products have been analyzed by XRD, 29Si and 27Al MAS NMR spectroscopy, SEM/EDX, and TEM. The results indicate that the main factor influencing the stability of the clays is the occupation of the octahedral sheet such that all trioctahedral members withstand the alkaline attack, whereas most of the dioctahedral clays suffer a complete dissolution and crystallization of new phases. Second, clays with Al in the tetrahedral sheet of their layers are shown to be less stable than those with a pure Si tetrahedral sheet.