Many environmental applications in the inorganic remediation field are based on the swelling and ion-exchange capacities of smectites, even though these can be affected by hydrothermal treatment in water and acidic media. Here a systematic study of the properties of layered silicates that could affect their hydrothermal stability at different pH is described: type of layers, octahedral occupancy, layer charge, and origin of the layer charge. The silicates studied were selected on the basis of their different characteristics associated with these properties. Kanemite (1:0 phyllosilicate), kaolinite (1:1 phyllosilicate), and pyrophyllite and talc (2:1 phyllosilicates with no-layer charge) were examined in order to determine the effect of layer structure, whereas the hydrothermal reactivity of silicates with different layer charge was analyzed by comparing the talc-hectorite-Laponite® and talc-saponite-trioctahedral vermiculite series. Samples were treated hydrothermally at 300ºC for 48 h in pure water and in a 0.01 M HNO3 solution and the final products were analyzed by X-ray diffraction, scanning electronic microscopy, and solid-state nuclear magnetic resonance spectroscopy. All layered silicates, except for kanemite, were found to remain intact after hydrothermal treatment in water and acidic media, with only minimal short-range structural changes observed. The extent of the structural changes depended on the octahedral sheet occupancy (greater extent) and the number of isomorphic substitutions (lesser extent), both of which weaken the structure.

Micaceous layer silicate clay minerals are attractive materials for applications involving non-linear optics because of their low cost and ability to form well ordered, platy aggregates, but such applications require precise knowledge of the dielectric behavior of the clay. The purpose of the present study was to use Terahertz time-domain spectroscopy (THz-TDS) to determine the dielectric properties of certain cleavable layered clay minerals, including muscovite, vermiculite, phlogopite, and biotite. The samples were characterized by X-ray diffraction and Fourier transform infrared spectroscopy as well as chemical analysis by Energy dispersive X-ray spectroscopy. The THz frequency window investigated was the far-infrared region of 3.3 to ∼40.0 cm−1 corresponding to 0.1 and 1.2 THz, respectively. The samples were selected so that the hydrated form of the interlayer cation, e.g. Mg2+ present in the interlayer gallery of vermiculite, could be compared to species such as phlogopite, biotite, and muscovite with the dehydrated form of interlayer cations such as K+ or Na+. The frequency-dependent complex index of refraction of these natural materials was determined to vary between 2.50 and 2.80. The presence of water in the interlayer space of vermiculite was reflected in the detection of increased values of the absorption index in comparison with the muscovite, phlogopite, and biotite.

The surface properties of various synthetic layered silicates, Na-kanemite, Nakenyaite and magadiite, exchanged with H, K and Ca, were examined using high-resolution nitrogen and argon adsorption and the data were treated using the Derivative Isotherm Summation method. Using argon as an adsorbate, the aspect ratio of platelets can be determined. In the case of magadiite exchanged with various cations, the stacking of particles is influenced by the nature of the exchangeable cations, thicker platelets being observed for ions with low polarizability. Highresolution argon adsorption data also confirm some structural information previously deduced from Raman spectroscopy experiments concerning the existence of rather open six-membered rings at the surfaces of both magadiite and kenyaite. Furthermore, in the low-energy domain of the isotherms, argon forms a very organized film on basal planes, suggesting a commensurate relationship between silica framework and argon atoms for both magadiite and kenyaite, contrary to what is observed for kanemite. Nitrogen adsorption results reveal the presence of polar sites on the surface of all the investigated minerals but does not allow us to propose an unequivocal assignment for such sites.

A synthetic protonic magadiite was used as a silica source to prepare zeolitic material (PLS-1) in the presence of tetramethylammonium hydroxide and water. The conversion of the protonic magadiite to the PLS-1 phase was achieved at 150°C after 5 days, or at 170°C after 3 days for SiO2: TMAOH:H2O molar ratios of 2.54:1:4.4. The synthesis of the pure PLS-1 phase depended also on the amounts of tetramethylammonium hydroxide and water used. Analysis by 29Si magic angle spinning nuclear magnetic resonance spectroscopy confirmed the layered character of the PLS-1 phase with a resonance at −93 ppm, and its dehydroxylation-condensation process. The chemical formula of (TMA)2Si18O33(OH)6 for PLS-1 was refined with the Rietveld method and the tetrahedron-splitting model. The later model has been proposed to describe the presence of silanol defects in the layered structure of PLS-1. Upon calcinations of the PLS-1 phase at temperatures >400°C, the removal of TMA cations and dehydroxlyation of PLS-1 layers resulted in a three-dimensional structure phase identified as the CDS-1 phase, with a chemical formula of Si18O36. The CDS-1 phase exhibited a large specific surface area of 288 m2/g and microporous character, as indicated by the nitrogen adsorption isotherms. The temperature-programmed desorption profile of ammonia indicated that CDS-1 exhibited one weak type of acid sites, confirmed, by pyridine desorption studies, as weak Lewis acid sites.