Unique but diverse: some observations on the formation, structure, and morphology of halloysite

New insights from the recent literature are summarised and new data presented concerning the formation, structure and morphology of halloysite. Halloysite formation by weathering always requires the presence of water. Where substantial drying occurs, kaolinite is formed instead. Halloysite formation is favoured by a low pH. The octahedral sheet is positively charged at pH ~2. The opposing sheet charge would facilitate interlayer uptake of H₂O molecules. When halloysite intercalates certain polar organic molecules, additional (hkl) reflections appear in the X-ray diffractogram, suggesting layer re-arrangement which, however, is dissimilar to that in kaolinite. Associated oxides and oxyhydroxides of Fe and Mn may limit the growth of halloysite particles as does incorporation of Fe into the structure. Particles of different shape and iron content may occur within a given sample of halloysite

Filtration and Breakdown of Clay Clusters during Resin Transfer Molding of Nanoclay/Glass/Epoxy Composites

Dispersion of nanoclay clusters during resin transfer molding of nanoclay/glass/epoxy disks is investigated. In addition to a center-gated disk containing only 14% glass fibers, three nanocomposite disks are fabricated with the addition of 2, 5 or 10 wt% Cloisite® 25A nanoclay. The spatial distribution of nanoclay clusters along the radial axis of the nanocomposite disks are characterized at two length scales. Clusters larger than 1.5 μm are characterized by performing image analysis on the SEM micrographs whereas smaller nanoclay clusters are identified by wavelength dispersive spectrometry. Results obtained from image analysis indicate that nanoclay clusters are filtered out by as much as 50% in the flow direction by the glass fiber preforms. In addition, increasing nanoclay content led to higher filtration, suggesting that cluster formation is more prominent at higher nanoclay loadings. Cluster size distribution analyses revealed that the outer edges of the disks, on average, contain finer nanoclay particles. For instance, the outer edge of the nanocomposite with 2% clay contains 22% more small nanoclay clusters compared to center of the disk. Glass transition temperature, Tg, of four specimens obtained from each molded disks is characterized under oscillatory shear. Glass transition temperature of the samples are shown to increase with the nanoclay content, yielding a 40% higher Tg at 10% nanoclay loading compared to glass/epoxy composite without clay. Increasing glass transition temperature with increasing nanoclay content may be an indication of intercalation of nanoclay within the epoxy matrix.Yeshttps://us.sagepub.com/en-us/nam/manuscript-submission-guideline

Soil with variable charge

xv.448 hal.;ill.;21 cm

L'échange d'ions organiques dans les zéolites synthétiques du type faujasite : effets stériques

Synthetic zeolites of faujasite type constitute the most open mineral structures. Their alumine silicate framework is composed of small cavities (6. 6 Å) and large holes 13 Å) interconnected by rings of six oxygen ions with a free diameter of 2. 2 Å. Small ions can pass through all these cavities and an ion exchange similar to that pointed out in classic clay minerals can take place. The exchange of alkyl and arylammonium ions which is studied here is restricted to large cavities. But even in these cavities, the available volum is not infinite and steric effects could limit exchange. The authors show that the upper limits of exchange with large ions are not governed by the available volum but rather by factors influencing also the selectivity of exchange. Selectivity is more important in the case of ions with aromatic character, which have a rather strong acidity. This very marked selectivity is explained by the affinity of zeolitic framework for protons.Les zéolites synthétiques du type faujasite constituent les structures minérales les plus ouvertes. Leur réseau alumino-silicique comporte des petites cavités (Ø 6,6 et des grandes cavités ((Ø 13 Å) interconnectées par des anneaux de 6 ions d'oxygène ayant un diamètre libre de 2,2 Å. Les ions à faibles dimensions (K* inclus) peuvent migrer à travers toutes ces cavités et exécuter un échange ionique semblable à celui observé dans les argiles classiques. L'échange des ions alkyl-et arylammonium, étudié dans ce travail, est restreint aux grosses cavités. Même dans ces cavités le volume disponible n'est pas infini et des effets stériques pourraient limiter l'échange. Nous démontrons que les limites supérieures de l'échange avec des ions volumineux ne sont pas déterminées par le volume disponible, mais plutôt par des facteurs influençant également la sélectivité de l'échange. La sélectivité est plus grande dans le cas des ions à caractère aromatique qui ont une acidité assez prononcée. Cette sélectivité très marquée est expliquée par l'affinité du réseau zéolitique pour les protons.Vansant E., Theng B.K.G., Maes A., Uytterhoeven J. L'échange d'ions organiques dans les zéolites synthétiques du type faujasite : effets stériques. In: Bulletin du Groupe français des argiles. Tome 21, fascicule 1, 1969. pp. 46-65