The clay mineralogical composition was examined for 4 profiles of each of saline soils and acid sulfate soils collected from some important river systems of Vietnam. Predominant clay minerals of saline and acid sulfate soils were found to be mica and kaolinite, accompanied with mica/vermiculite/smectite-mica/smectite mixed-layer mineral, vermiculite, chlorite and smectite. The clay mineralogical composition of these soils was compared with that of alluvial soils from the corresponding river systems/agro-ecological regions in order to clarify the effects of the secondary soil forming processes of salinization and acidification (as a result of pyrite oxidization) on the clay mineralogical composition of the soils. It was hardly observed the mineralogical differences based on the distribution pattern of major clay minerals in the clay fraction. Then, examination of correlation between mineral contents was taken for the selected pairs of clay minerals, and the clay mineralogical alteration was somewhat indicated. In saline soils, it was the transformation of mica into expansible 2: 1 layer silicate minerals. In alkaline saline soils, the transformation of mica into vermiculite occurred principally due to presence of a large amount of Ca and Mg salts in soil. In case of acid sulfate soils, the transformation into kaolinite (1: 1 layer mineral) was suggested. However, such clay mineral transformations have occurred limitedly in soils. This is probably due to prevention of the development of salinization and oxidation by the secondary soil forming processes under paddy-rice cultivation and mangrove forestation. Based on these results, it could be asserted that saline soils and acid sulfate soils, in general, still kept the essence of the original alluvial soils.

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Profile samples were collected from the soils on the terrace and hill areas in Bangladesh. Rare earth elements (REE), particle-size distribution, and mineralogical composition of the <2μm clay fraction were analyzed. The terrace soils showed an indication of moving/decomposition of clay particles from/in the surface horizons and the presence of different types of interstratified minerals in addition to mica in the clay fraction. Chondrite-normalized curves of REE indicated increase in the negative Eu anomaly towards the surface horizon. The hill soils were coarse-textured and dominated with kaolinite in the clay fraction, and showed a positive Ce anomaly in the A and B horizons. Among the REE a selective release of Eu and an enrichment of Ce in the process of soil formation was suggested.

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Ammonium exchanged zeolites A were isothermally treated at 400-1000°C for durations up to 100min in the air, and then were leached in 1M NaOH solution at 70°C. The products were characterized by XRD, SEM, ICP-AES and ATR-FTIR. The results from isothermal treatments confirmed that the pseudomorphic transformations took place through the formations of an amorphous phase with the tweed-like patterns on the surface of the cubic solid and a subsequent recrystallization to a cubic-shaped composite of mullite and amorphous material. These characterizations supported the existence of a thin surface layer of zeolite crystals acting as a microvessel, which was consistent with a previous hypothesis. After leaching by hot alkali solution, micrograting patterns appeared on the surfaces of the pseudomorphic amorphous phase which exhibited the tweedlike patterns. A cubic solid with fine voids were obtained from a composite of mullite and an amorphous phase. The observed differences in the surface morphology are closely related to the investigated solids.

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The intercalation of cobalt ions (Co (II)) into the interlayer space of a layered silicate, magadiite, was conducted by ion exchange reactions between magadiite and aqueous solution of cobalt (II) chloride. Thermal treatment of the Co (II)-mgadiites in air at 1000°C resulted in the transformation to cristobalite with retaining the platy particle morphology of the original magadiite.

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Nine profiles of ferralitic soils derived from sedimentary and metamorphic rocks were collected from the hilly and mountainous areas in the Northwest, Northern, Northeast, Red River Delta and North of Central agro-ecological regions of Vietnam, and were subjected to clay mineral analysis. The main clay minerals detected were kaolinite, halloysite (0.7nm), and 2: 1-type silicate minerals such as mica, the mica/smectite mixed-layer mineral, and chlorite-vermiculite intergrade. The difference in the clay mineralogical composition among the soils was controlled by the intensity or advancement of ferralitization, a main soil forming process occurring in those areas. Based on the clay mineralogical composition in the clay fraction, nine profiles were divided into 3 groups representing different levels of advancement in ferralitization. In the group 1, in which ferralitization was less advanced due to the young geologic age or retarded by the site-environmental condition or microrelief, halloysite (0.7nm) and 2: 1-type silicate minerals were predominant in the clay fraction. In the group 2 of the intermediate stage of ferralitization, halloysite (0.7nm) was changed to kaolinite, and 2: 1-type silicate minerals were weathered and the decrease in their contents was corresponding with the increase in the kaolinite content. In the group 3 subjected to the most advanced ferralitization, kaolinite occupied up to more than 90% in the clay fraction. Since ferralitization is a prolonged soil genetic process, soils developed from the same parent rock but differing in the geologic age might have the different clay mineralogical composition. In contrast, soils derived from the different parent rocks might have the same clay mineralogical composition at the advanced stage of ferralitization. Inherent potentiality of the soils was assessed based on the type and amount of clay minerals and its variation among the soils was mainly attributed to the amount of 2: 1-type silicate minerals in the clay fraction.

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Magnesia (MgO) is gaining an interest as an agent for stabilization of some heavy metals in the contaminated soils. MgO also reacts with silica (SiO₂) and water at room temperatures and atmospheric pressures to form layer silicates. The presence of SiO₂ may affect the stabilization of heavy metals by MgO. To investigate the effects of SiO₂, we mixed MgO, SiO₂, and solutions of Cu (NO₃) ₂ or Ni (NO₃) ₂ at a solid: solution ratio of 1kg: 2L. The reaction products were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), and chemical analyses. Suspensions immediately turned alkaline in both systems due to hydration of MgO. XRD and FT-IR results showed that copper nitrate hydroxide (Cu₂NO₃ (OH) ₃) or nickel hydroxide (Ni (OH) ₂) as well as brucite (Mg (OH) ₂) precipitated just after mixing and they gradually disappeared to form poorly ordered layer silicate minerals. The Cu and Ni concentrations were kept low throughout the experiments. We interpreted the experimental results as indicating that a part of the Cu and Ni were incorporated in octahedral sites of the newly formed layer silicate minerals. The sequential extraction with water and MgCl₂ solution indicated that the amounts of extractable Cu and Ni were larger for the products from heavy metal-MgO-SiO₂ systems than for heavy metal-MgO systems. The results show that the presence of active SiO₂ is a little detrimental to the stabilization of Cu and Ni by MgO in a short term but may be advantageous in a long run.

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Various kinds of clay were used to prepare self-standing-films with a simple casting method using aqueous dispersions of the clays. Eleven clays including natural, synthetic, and organophilic ones were selected. These clays were characterized using a laser-scattering size-meter, UV-vis spectrometer, SEM, XRD, and AFM. Saponite, stevensite, montmorillonite, and water-swellable synthetic mica formed self-standing-films with ordered laminated structure. Results confirmed that the swelling and delamination properties are fundamental parameters of film formation; the cation exchange capacity (CEC) was also revealed as an important parameter.

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