Structural palygorskite

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.

The term fibrous, as applied to zeolites, does not necessarily indicate flexibility. Flexibility, however, is one of the physical properties of cotton stone found on the Isle of Skye, Scotland. This locality provides a feathery sample of mesolite, (Na2Ca2Al6SigO30 8H2O), whose composition and structure place it in the same group as natrolite. The similarity between appearance of this sample and that of the mountain flax variety of palygorskite emphasizes the difficulty of identifying fibrous minerals based on morphology alone. 

There are several other types of minerals commonly found in clay particle size mineral assemblages (i.e.,< 2 microns diameter, Krumbein and Pettijhon, 1938). Aside from quartz and amorphous materials, the two most important mineral groups are sepiolite-palygorskite and zeolites. These two groups are similar in that they both contain free 1 0 molecules in their structure. However the Si-0 linkage is quite different in each case. 

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... 

High pressure studies using natural sepiolite and palygorskite (Frank-Kameneckiji and Klockova, 1969) indicate that these minerals can contain variable quantities of silica because they exsolve quartz while retaining their basic structural and mineral identity. These experiments also demonstrate that the natural minerals are compositionally intermediate between talc or montmorillonite and quartz. These latter phases are formed upon the thermal breakdown of sepiolite and palygorskite under conditions of 1 and 2Kb total pressure. Both sepiolite and palygorskite appear to remain stable in sequences of buried rocks, at least up to the depth where fully expandable dioctahedral montmorillonite disappears (Millot, 1964). 

Palygorskile and Sepiolite. Palygorskite (anapalgite) and sepiolitc arc day minerals in which the 2 1 layers are linked together in chain-likc or a combination of chain-sheet structures. 

Palygorskite and sepiolite are different from other clay minerals in the manner in which the 2 1 layers arc joined Ralher than being joined in a ennlinuous manner, the tetrahedral sheets are joined to an adjacent inverted tetrahedral layer, making the octahedral layers noncontinuous and leaving an open channel in the mineral structure. 

Attapulgite and palygorskite have a fibrous texture and a chain structure. The structure proposed by Bradley (1940) is that of a 2 1 layer structure with five octahedral positions (four filled) four Si tetrahedra occur on either side the octahedral sheet with their apices directed towards the octahedral sheet. These structural units alternate in a checkerboard pattern leaving a series of channels between the structural units. These channels contain water molecules. 

Christ et al. (1969) found that X-ray diffraction powder data for palygorskite samples show both orthorhombic and monoclinic structures and suggested that the variations in symmetry reflect variations in chemical composition. The present data suggest the most likely difference is octahedral Fe. 

Fig. 19, Histograms showing the distribution of the cations of fifteen palygorskite structural formulas.

Abstract. A variety of pyrocarbon/silica gel adsorbents were prepared using commercial mesoporous silica gels Si-40, Si-60, and Si-100 as matrices modified by carbon deposits from pyrolysis of several organic precursors. The second type of hybrid carbon-mineral adsorbents was synthesized using spent natural palygorskite utilized in paraffin purification. The adsorbents were then heated, hydrothermally treated, or modified by additional deposition of carbon. Changes in the structural and adsorption characteristics of hybrid adsorbents before and after treatments were analyzed by microscopy, p-nitrophenol and nitrogen adsorption isotherms, and TG, TEM, XRD, and XRF methods. 

Similarly pure beds of kerolite and sepiolite are found in the modern and Pleistocene groundwater wetlands of Amboseli, Kenya (Stoessell and Hay, 1978 Hay and Stoessell, 1984 Hay et al, 1995). The concentration of dissolved silica is also important at higher ratios of Si02 to Mg, chain-structure clay (sepiolite-palygorskite) can precipitate directly from solution, as is also the case at Amargosa. 

Natural layer silicates represent the mixed type porous adsorbents with structure consisting of mesopores, and micro- and macropores [16]. With respect to the character of their porous structure, the layer silicates can be divided into two classes those with the expanding structural cell (montmorillonite, vermiculite), and with the rigid structural cell (kaolinite, hydromica, palygorskite). 

Like layer silicates, the porous palygorskite can also be organophilized. X-ray studies, however, do not reveal any structural changes in the organocomplexes, since cationic surfactants are adsorbed only on the external surfaces. The amount of surfactant bound by ion-exchange adsorption and the extent of organophilicity can be quantified by the liquid sorption studies and microcalorimetry [19-21]. 

We shall next examine how these surface structural characteristics affect liquid sorption in the alcohol(l)+benzene(2) mixtures. Figure 17 shows the excess isotherm determined on palygorskite. In the case of methanol(l)+benzene(2) mixtures the isotherm is of type II, indicating that methanol is preferentially adsorbed and penetrates into the micropores of the zeolite-like channels as well. With increasing the number of carbon atoms in the alcohol, preferential adsorption is repressed and isotherms of types III and IV are obtained. The reason for this is that, due to steric hindrance, alcohols of larger size cannot enter the micropores. 

A model of the crystal structure of kalifersite has been obtained [47] (Figure 17-13) after realizing the following modular relationships with sepiolite and palygorskite. 

Raite [55] Na3Mn3Tio,25[Si802o](OH)2.10H20 C2/m, a=15.1, fc=17.6, c = 5.290 A, 13= 100.5° has a crystal structure that consists of a palygorskite-like framework, but the channel content differs substantially from that of palygorskite... 

Figure 17-13. Perspective view along [100] of crystal structures of sepioUte (a) palygorskite (b) and kaUfersite (c). The content of the channels is not shown...

The palysepiole polysomatic series PpSs (Ferraris et al. 1998) includes minerals whose structures contain one or both the types of TOT ribbons (modules) which are present in palygorskite and sepiolite these ribbons are reminiscent of the TOT modules occurring in amphiboles (Fig. 19). The two modules are ... 

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