Sepiolite associations

FLEISCHER (P,), 1972. Sepiolite associated with Miocene diatomite, Santa Cruz Basin, California. Amer. Min. 57. 903-13. 

Both sepiolite and palygorskite contain tetrahedral silicate sheets (with a variety of substitutions for the Si " ), but the apicies of the tetrahedra are thought to point up or down with the transition areas containing Ca and Mg ions, and bound or associated H2O. The arrangement produces a continuous basal oxygen plane that is compartmentalized. Laths or ribbons three chains wide in sepiolite and two chains wide in palygorskite are separated by discontinuous octahedral areas (Fig. 2.16A and B). 

Substitution and variations in the tetrahedral sites change the manner of side linkages for the ribbons, effecting the octahedral cation and water associations. In addition, different ribbon widths can lead to different numbers of octahedral cations. Variation in the width of chains and substitution of cations and water are easily accomplished, which means that accurate and consistent chemical and crystal structural data on these minerals are difficult or, at best, approximate. However, the minerals do form fibers with a consistent fiber axis repeat of about 0.512 nm (Preisinger, 1959 Rautureau et al., 1972). Sepiolite and palygorskite represent the widest possible structural and chemical diversity among fibrous silicate minerals. 

Fig. 2.16 Schematic representations of the structures suggested for sepiolite and palygorskite. The ribbonlike arrangement of silicate chains alternates with hydroxyl and water areas. (A) Sepiolite, the (001) projection, showing the cross section of three 2 1 silicate chains and associated water and hydroxyl groups. (B) Palygorskite, the (100) projection, showing the cross section of two silicate chains and associated water and hydroxyl groups.

Sepiolite and palygorskite are frequently associated In natural deposits. They are both fibrous in form, a characteristic dictated by their chain-type (linear) structure. They contain hydroxyls, zeolitic... 

Sepiolite and palygorskite have a rather special composition and seem to be related to specific mineral parageneses. They appear to be stably associated with montmorillonite, corrensite, serpentine, chert, sulfates, carbonates and various salts. They are found in deposits typified by processes of chemical precipitation or solution-solid equilibria (Millot, 1964) and are therefore rarely associated in sediments with large quantities of detrital minerals. Their chemical environment of formation is in all evidence impoverished in alumina and divalent iron. Their frequent association with evaporites, carbonates and cherts indicate that they came from solutions with high chlorinity. 

The present-day sediments of the Atlantic do not contain a substantial amount of zeolite (Elderfield, 1976). Clinoptilolite has been found, however, in association with sepiolite, quartz, and mont-morillonite (Hathaway and Sachs, 1965 Bonatti and Joensuu, 1968). The clay minerals palygorskite and sepiolite are usually minor constituents of marine sediments (Hathaway, 1979), and may be detrital (Weaver and Beck, 1977) or authigenic. Their hydrogenous occurrences are usually in basal sediment sections exposed to fluids of elevated temperarnres (Bonatti and Joensuu, 1968 Church and Velde, 1979 see below). 

Palygorskite and sepiolite are magnesium-rich fibrous aluminosilicates that have been identified in basal deep-sea sediments (Table 4 e.g., Hathaway and Sachs, 1965 Bowles et al., 1971 Bonatti and Joensuu, 1968 Church and Velde, 1979 Jones and Galan, 1988 Velde, 1985). These phases are commonly associated with smectite, and it has been suggested that they originate by alteration of montmorillonite by low-temperature, magnesium-rich, hydrothermal solutions (Bonatti and Joensuu, 1968), e.g.. 

Another system for which both Ha and O2 production has been observed involves the use of RUO2 and [Ru(bipy)3] " cosupported on sepiolite combined with platinum deposited on Eu " embedded in alumina. The two solids, alumina and the clay, associate in solution to give a system as shown in Figure 12. Photolysis of this system produces hydrogen and oxygen in an oscillatory... 

Similar features were obtained for sepiolite-based specimens, but no comparable variation of the electrochemical response was obtained for indigo associated to laminar clays such as kaolinite and montmorillonite or for alizarin-palygorskite... 

From the MIP curves of support A heat treated at 500 and 800°C (not shown), although the total pore volume was maintained, the mesopores associated with sepiolite (18 nm) were lost and a shift in the pore size of the wider pores due to titania (65 to 90 nm) was observed. An associated fall in the MIP and BET surface areas from 95 to 46 m /g and 133 to 60 m /g, respectively, was found. These changes in the support would explain the decrease in activity, pointing out the importance, not only of the surface area, but also of the pore size distribution and the presence of sepiolite mesopores. A change in the available titania surface in the support should not affect the ft dispersion as indicated by the results with different titania contents in Section 3.1. 

Clay constitutes the most abundant and ubiquitous component of the main types of marine sediments deposited from outer shelf to deep sea environments. The clay minerals are conventionally comprised of the <2 pm fraction, are sheet- or fiber-shaped, and adsorb various proportions of water. This determines a high buoyancy and the ability for clay to be widely dispersed by marine currents, despite its propensity for forming aggregates and floes. Clay minerals in the marine environments are dominated by illite, smectite, and kaolinite, three families whose chemical composition and crystalline status are highly variable. The marine clay associations may include various amounts and types of other species, namely chlorite and random mixed layers, but also ver-miculite, palygorskite, sepiolite, talc, pyrophyllite, etc. The clay mineralogy of marine sediments is therefore very diverse according to depositional environments, from both qualitative and quantitative points of view. 

Hydrophilic polymers such as polyvinyl alcohol (PVA) exhibit high affinity for association with sepiolite and palygorskite via interaction with the silanol groups at the external surface of the fibrous clays. Thus, a simple mixture of both components... 

Focusing on reinforced polysaccharide films, positively charged chitosan (a linear polysaccharide mainly extracted from shrimp and other crustacean shells) was assembled with sepiolite clay, giving rise to self-supported bionanocomposite films with interesting structural and functional properties. In this study, Darder and co-authors [113] revealed that the association of sepiolite and chitosan resulted in a threefold improvement in the mechanical properties as compared with unmodified... 

Polyacrylonitrile (PAN) is produced inside the sepiolite tunnels by in situ polymerization of previously adsorbed acrylonitrile molecules [46, 249]. Carbonization of PAN-sepiolite nanocomposites at temperamres superior to 500°C under inert gas flow give carbon-sepiolite nanocomposites showing electrical conductivity, probably associated with the formation of graphene-hke species that remain strongly assembled with the silicate support. These materials have been studied to assess their suitability for applications in diverse electrochemical devices such as... 

In all those circumstances, sepiolite, which, by the way, is often associated with opal, might be the result of weathering under the influence of prevailing climatic conditions. 

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