Clay Minerals
Ray L. Frost

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Clay science is alive and well -- but perhaps not in the traditional departments of geology and natural resources. In the new millennium, clay science is thriving in schools of chemistry, departments of engineering, and research programs of environmental science. The fashionable research of the present is based on developing new materials for specific purposes.

Many of these new materials are crafted on the nano scale and are designed to have the chemistry controlled into the clay. These new and innovative materials are designed for specific purposes. The chemistry of the new material is controlled such that outcomes are specific and targeted. Some examples include: the use of pillared clays with large pore sizes for the production of molecular sieves. A pillared clay is formed by the insertion of large cations such as the Al13 keggin structure in the clay interlayer through cation exchange. Low temperature calcination at around degrees Celsius results in the chemical bonding of the aluminum-13 to the adjacent siloxane layers. The application of titania pillared clays for the photocatalytic and wet oxidation of low concentrations of recalcitrant organics in drinking water; the development of hydrotalcite structures for use as catalysts; and the integration of clays into polymers as nano-composites.

The most cost efficient way to produce these new materials is to use naturally occurring minerals such as montmorillonite, kaolinite, saponite and many others. The reason is that clays are much less expensive to mine than to synthesise.

The research in the nano-chemistry of clays is still at the beginning. This is not to say that clays don't have their traditional uses as adsorbents, such as in kitty litter; as filter materials, such as in bleaching earths for the decolorization of oils; and, of course, in traditional ceramic materials such as cement production, brick manufacturing and visual art ceramics. But clay science research has moved on and is now progressing in different directions.

These new uses for clays bring with them the need for innovative techniques to study the changes the molecular structures of clays experience when clays are chemically modified.

These techniques include modern surface analysis techniques such imaging techniques that produce high-resolution spectral images, showing the nature of the chemical bonding. This technique can be used to study the chemical bonding of the aluminum pillars in aluminum-pillared clays. Another technique is modern electron microscopy, including combining transmission electron microscope and environmental scanning electron microscope images with electron probe analysis. This technique can be applied to study the molecular assembly of cations in hydrotalcites. Raman microscopy and other vibrational spectroscopic techniques can be used to study the changes in the molecular structure of clays. And high-resolution TGA (Thermogravimetric analysis), combined with mass spectrometry, enables the correct selection of temperatures for the pillaring of clays. These four are not the only techniques, but illustrate that clay science is advancing and that many new experimental techniques are being used to understand how to develop clays into new materials.

Following are some examples of some new materials that are based on designed synthetic clays and on modification of natural clays:

A first illustration is synthetic hydrotalcite: This material is being explored for use as a catalyst for the wet-catalytic oxidation of organics in aqueous systems and for the removal of undesireable anions -- such as cyanide or arsenate -- from water systems via anion exchange. Key features are the how perfectly the hexagonal crystals of the hydrotalcite form, as well as the size of the particles, which are synthesised on the nano scale.

A second illustration is the use of high-resolution, thermogravimetric analysis to explore the correct temperature for pillaring of a natural montmorillonitic clay taken from Miles in Queensland, Australia. In this case, we are using a mixed, aluminum-gallium pillared clay to synthesize a pillared clay. This new clay may prove suitable as a catalyst - both because the small sizes of its particles result in a high surface area, and also because of the clay's morphology.

One method of producing nano-sized clay minerals is through a grinding method known as mechanochemical activation of clays. If, for example, clay is ground fine enough, the kaolinite particles are reduced to submicron size. But perhaps what is not widely recognized is that the chemistry of the kaolinite particles is changed. This change is clearly evident from the diffuse reflectance infrared spectra, which the loss of intensity of the hydroxyl-stretching vibrations.

Another good example of the use of vibrational spectroscopy is in the use of Raman spectroscopy to follow the changes in the molecular structure of kaolinite upon intercalation with, for example, potassium acetate. What may be clearly observed is that the spectrum of the kaolinite is changed significantly upon intercalation.

Significant advances are being made in synthesizing and characterizing nanocomposites, which can be used to make plastic materials that are fire-proof or that are stable at elevated temperatures. Such materials may also be used to make fire retardant paints or to make high strength materials. The nanocomposites are based on a blend of an organo clay, such as montmorillonite, in which the counter cation is replaced with a an organic cation. The clay particles then form an integral part of the polymeric material. Nanocomposites may be studied by using a combination of Differential Scanning Calorimetry and heat capacity measurements.

Many of the applications of clays , modified clays and activated clays are in the environmental areas. Two major problems facing future society is that of pure water and pure air. It is probable that clean water only exists in Northern Canada and in the Antarctic ice sheets, although even this idea can be argued as nitrate ions may be detected in the Antarctic ice. Pillared clays based on anatase inserted between montmorillonite layers have proven most useful for both the wet catalytic oxidation and the photocatalytic oxidation of organics in water. Fundamentally, the organics are converted to water and carbon dioxide through oxidation.

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Frost teaches at the Queensland University of Technology in Australia and is Secretary General for AIPEA (Association Internationale Pour L'etude Des Argiles, or the International Association for the Study of Clays, affiliated with the International Union of Geological Sciences). For more information, e-mail Frost.

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