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Montmorillonite, a ubiquitous clay mineral, has been engineered into a variety of porous and nanostructured catalysts, owing to its peculiar layered structure with characteristic surface and intercalation chemistry. Such a class of catalysts have continuously received considerable attention in both science and industry. This review examines recent advance in engineering montmorillonite into metal ion-exchanged montmorillonite catalysts, acid-activated montmorillonite catalysts, organo-montmorillonite catalysts, pillared interlayered montmorillonite catalysts and montmorillonite-supported catalysts. The former two types of montmorillonite-based catalysts have been used in the catalytic cracking, dehydration, hydrolysis, esterification, polymerization, alkylation, and Fischer-Tropic synthesis. The introduction of oxide pillars in the interlayer space of montmorillonite and the immobilization of metal oxides or zero-valent metals on the surface of montmorillonite allow montmorillonite to be used in the catalytic oxidation, reduction, isomerization and coupling reactions. Currently, the role of montmorillonite in acid catalysis, redox catalysis, and free radical catalysis have been partly revealed. The effect of the structures and the active sites of modified montmorillonite on its catalytic performance need to be investigated in more detail. Future work is suggested to focus on solving engineering problems and exploring novel catalytic applications of montmorillonite, in particular exfoliated montmorillonite nanolayers. [ABSTRACT FROM AUTHOR]

Scanning transmission electron microscopy images of the 00/ white mica planes in crystals from central Switzerland and Crete, Greece reveal that domains of paragonite, margarite, and muscovite are ordered within the basal plane. Energy dispersive X-ray analyses show that both cations in the interlayer and in the 2:1 layer have ordered on the scale of tens to hundreds of nanometers. Domain boundaries can be both sharp and crystallographically controlled or diffuse and irregular. A model outlining the domain formation process is presented that is consistent with X-ray powder diffraction and transmission electron microscopy data. The domain model incorporates aspects of a mixed-layered and a disordered compositionally intermediate phase models. The main feature of the model is the formation of mica species that segregate within the basal plane and contradict the notion of homogeneous layers within mixed-layer phases. Implications for the formation of all diagenetic and very low-grade metamorphic 2:1 sheet silicates are discussed. Keywords: Muscovite, paragonite, margarite, TEM, STEM, low-grade metamorphism, white mica formation

Thermodynamic calculations of ion-exchange reactions were applied for clinoptilolite in a natural state and after irradiation with three doses of [beta]-radiation ([10.sup.12], [10.sup.15], x [10.sup.16] e/[cm.sup.2]) and [gamma]-radiation (70 Mrad). Samples were equilibrated with binary systems of [K.sup.+] [left and right arrow] [Na.sup.+] and [Cs.sup.+] [left and right arrow] [Na.sup.+] at 25[degrees] and a total normality of 0.025 N. Selectivity for K was not affected after [beta]-radiation with doses of [10.sup.12] and [10.sup.15] e/[cm.sup.2] ([DELTA]G[degrees] = -6.37 kJ/equiv, lnK[alpha] = 2.58 for the original clinoptilolite), whereas it increased considerably after 70 Mrad of [gamma]-radiation ([DELTA]G[degrees] = -7.88 kJ/equiv, lnK[alpha] = 3.18). Selectivity for [Cs.sup.+] increased for the clinoptilolite irradiated with [beta]-radiation ([10.sup.12], [10.sup.15], 3 x [10.sup.16] e/[cm.sup.2]) and [gamma]-radiation (70 Mrad). [DELTA]G[degrees] and lnK[alpha] for original sample and [Cs.sup.+] [left and right arrow] [Na.sup.+] were -7.33 kJ/equiv and 2.96, respectively. Irradiated samples with [beta]-radiation [10.sup.12], [10.sup.15], 3 x [10.sup.16] e/[cm.sup.2] and 70 Mrad [gamma]-radiation yielded [DELTA]G[degrees] and lnK[alpha]-7.41, -8.83, -8.60, -8.25 kJ/equiv and 2.99, 3.57, 3.47, 3.33 for [Cs.sup.+] [left and right arrow] [Na.sup.+], respectively. Remarkable amorphization of clinoptilolite was observed after exposure at the highest dose of [beta]-radiation (3 x [10.sup.16] e/[cm.sup.2]) with a concomitant decrease in cation-exchange capacity (CEC). Crystallographic parameters and especially exchangeable cation site coordinates were refined for all samples with the Rietveld method. Cesium-saturated samples exhibited changes in the cation sites Cs2 and Cs3, which are next to clinoptilolite channel walls with lower [Al.sup.3+] for [Si.sup.4+] substitution. The observed changes include a shift in cation sites Cs2 and Cs3 toward channel walls and occupancy decrease in site Cs2. Keywords: Clinoptilolite, ion exchange, [beta]-radiation, [gamma]-radiation, thermodynamics, Rietveld refinement

Upper Cretaceous basic pyroclastic rocks, which overlie the Upper Pillow Lavas of the Troodos ophiolitic complex, Cyprus have been altered to bentonites. The resulting smectite is Fe-rich montmorillonite and Fe-rich beidellite, with moderate Mg contents. The smectite is trans-vacant and contains abundant exchangeable K. The presence of K is linked with hydrothermal alteration, which affected the higher members of the Troodos ophiolitic suite. The smectite displays significant compositional heterogeneity, which involves substitution of Fe for Al and to a lesser degree substitution of Mg for Al, and that reflects the influence of microenvironmental conditions on smectite formation. The layer charge of the smectite is controlled mainly by the tetrahedral charge, whereas the influence of octahedral charge is of lesser importance, because of Fe for Al substitutions, which does not create a charge deficit. Although the parent pyroclastic rocks were basic, the bentonites contain abundant Sipolymorphs and Si-rich zeolites, from dissolution of abundant radiolarian frustules, which increased the Si-activity of the pore waters, and also produced the partial replacement of smectite by palygorskite at a later stage. Dissolution of frustules was facilitated by the high heat flow from the ocean floor and by the circulation of hydrothermal fluids. The crystal chemistry of smectite and the bulk mineralogy of the bentonites influence the physical properties and industrial applications of the Cyprus bentonites, as well as their response to acid treatment. Keywords: Troodos ophiolite, Fe-rich montmorillonite, Fe-rich beidellite, layer charge, charge heterogeneity, hydrothermal alteration, bentonite, clinoptilolite