Celia Marcos

Study of the dehydration process of vermiculites by applying a vacuum pressure: formation of interstratified phases

abstract Structural transformations between the different hydration states of three vermiculite samples from Sta. Olalla (Huelva, Spain), Paulistana (Piaui, Brasil) and West China, have been observed by X-ray diffraction at atmospheric pressure, P = 1.4 × 10-2 mbar and P = 2.4 × 10-4 mbar. The samples were studied in flake and powder forms. The effect of vacuum has been proven to be the same as that of temperature, i.e. it causes dehydration of vermiculite, but with a different evolution through the different hydration states. In fact, under vacuum, the process seems to be inhibited at a one-water layer hydration state (1-WLHS), without a further dehydration of samples to a zero-water layer hydration state (0-WLHS). Furthermore, the dehydration process has been shown to occur through different interstratified states in each vermiculite. This result has been related to the interlayer Mg-cation content, due to its affinity to water molecules. The interstratified states have been analysed by the direct Fourier-transform method. The vermiculite from Sta. Olalla exhibits the most complex process, with formation of three different interstratified phases: two phases characterized by an interstratification of interplanar distances, d = 11.5 - 13.8 Å and d = 9.6 - 11.5 Å , respectively, and a practically segregated phase characterized by d = 13.8 Å. For the vermiculite from China, an interstratified phase not previously reported has been found, with an interplanar distance of 12.10 Å. The inhibition of dehydration at 1-WLHS, as observed, could be used in applications such as adsorption and separation technology of gases and liquids, or in heterogeneous catalysis processes.

Semi-ordered crystalline structure of the Santa Olalla vermiculite inferred from X-ray powder diffraction

Abstract A sample of Mg-vermiculite from Santa Olalla (Spain) was studied by X-ray powder diffraction, electron microprobe, and thermo-gravimetry. The 3D structure is described as a disordered stack of two types of 2D building blocks, which are made up of one talc-type layer and one interlayer space containing hydrated Mg2+ cations. We have succeeded in the refinement of both the atomic positions and occupancies of exchangeable cations and water molecules in the interlayer space of this vermiculite using the program package DIFFaX+. The position of the Mg2+ cations is the only difference between the two layers. Besides the water molecules associated to the octahedrally coordinated Mg2+, we also located water molecules in the interlayer space. The structural analysis confirms that vermiculite is a semi-ordered crystalline material characterized by the existence of a large density of defects due to random ~±b/3 translations along the crystalline [010] direction. In this way, this structure can no longer be described by means of a unit cell repeated in 3D space. Instead, long-range order is only recognized in the a-b plane. The 3D structure is described by means of a recursive method.

Vermiculite surface structure as imaged by contact mode AFM

The {001} surfaces of three vermiculite samples from different geological origins have been examined with an atomic force microscope (AFM) in the contact mode of operation. Images of the basal O plane of the sheet of tetrahedra and the plane of OH groups of the octahedral sheet of the Mg-interlayer were recorded. Differences in the appearence of the interlayer hydroxide surface of the three vermiculites are attributed to their variable Mg-interlayer contents and the presence of biotite-vermiculite mixed-layer mineral associated with vermiculite. The mean unit cell dimensions for both cleavage halves in each sample are consistent with those of the bulk structure within experimental error, indicating that the surface structure of neither the tetrahedral sheet, nor the OH plane of the sheet of interlayer show noticeable reconstruction.

Theoretical and applied implications of the structural order of irradiated vermiculite

Abstract Vermiculite is a layered silicate with a complex crystalline structure, as it is characterized by the existence of a large density of defects- even in the case of the most pure vermiculite [e.g., Santa Olalla, Huelva (Spain)]. As a result of their lamellar structure, vermiculite structures present a broad diversity of behavior and are interesting from both the applied and scientific point of view. Vermiculite is used to examine interesting physical properties such as mixed-cation effects and two dimensional magnetism. The existence of frustration and disorder is a key feature for understanding the mechanisms of spin-glass, for example. The dimensionality of magnetic interactions, which plays a central role in controlling the critical dynamics of SG systems, is still not resolved. Probably, magnetic studies on structurally ordered vermiculites will elucidate the true nature of spin-glass-like phases. One way to provide structurally ordered vermiculites might be by irradiation with ultraviolet or γ rays. These types of radiation induce structural order in vermiculites leading to materials with enhanced opto-electrical properties, which improve its utility as an electronic insulator and a thermoluminescence dosimeter for innovative dosimetry applications in a radiation-rich environment (Kaur et al. 2014, this issue). Other layered minerals irradiated with γ rays exhibit enhanced radiation shielding capacities and electronic insulating properties.