Phyllosilicate, dissolution

The literature data for ortho-, soro-, ino-, and phyllosilicate dissolution at 25 °C derived from long duration dissolution experiments (> a month except for wollastonite and forsterite. Tables 3 and 5) bracket the value of the order with respect to H, n (see Equation (17)), between 0 and 0.85 at 25 °C. The higher values of n from the literature tend to be for silicates containing iron. Extrapolating the rate constant for the proton-promoted dissolution rate constant to other temperatures, h(T), can be accomplished with the Arrhenius equation ... 

Figure 7 Log (rate constant for dissolution) versus the connectedness, or average number of bridging oxygens per tetrahedrally coordinated atom. A connectedness of 0 reflects an orthosilicate, 2 reflects a pyroxene, 2.5 reflects an amphibole, and 3 reflects a phyllosilicate. Data are reviewed in the text.

Framework - phyllosilicate/microcrystalline quartz fault rocks This class of fault rock is introduced here to describe fault rocks which form in sediments with concentrations (>20%) of dissolvable sponge spicules and varying amounts (15-40%) of phyllosilicates (Fig. 2d). Although not common, sediments with high sponge spicules contents are prone to the development of fault rocks by the cataclasis, dissolution and reprecipitation of silica. The process may be initiated by the collapse of secondary pores created by spicule dissolution and induce the redistribution of more soluble material by mixing. 

Fig. 2. Photo-micrograph of fault structures from the Njord Field, Haltenbanken. (a) Micro-fault zone characterized by dense packing of grains only, (b) Micro-fault zone characterized by dense packing of grains and phyllosilicate enrichment. Dissolution of quartz occur at grain contact with mica (arrow), (c) Micro-fault zone with abundant stylolites.

All studies concerning the weathering of phyllosilicates provide clear evidence that the dissolution of clay minerals and micas is very slow under natural conditions (Correns, 1963 Polzer and Hem, 1965 Calvera and Talibudeen, 1978 t Serstevens et al., 1978 Lim and Clemency, 1981 Carroll-Webb and Walther, 1988). Hence they are characteristic secondary weathering products occurring in soils and sediments. 

Reacted samples remeasured 3 months later showed that all of the reduced Fe had subsequently reoxidized to Fe(III). Previous studies have demonstrated that little clay dissolution occurs during redox cycles of phyllosilicate clays (18, 36, 44). This permits the clay to be defined as a catalyst in the truest sense. The clay facilitates the oxidation of TPB but the clay itself is not destroyed. The redox of Fe in smectites is essentially reversible (15, 38), hence the reactivity of this system is in contrast to the widely studied Fe-oxide and Mn-oxide facilitated reactions in which the oxide typically reductively dissolves and transforms during the reaction (19,20). 

The deposition-precipitation method using a base has been applied for the preparation of various catalysts. Upon raising the pH of the solution, the precipitation of a hydroxide onto the support is expected. In fact, it was shown in several cases (Table 14.2) that mixed compounds such as phyllosilicates for silica support or hydrotalcite for alumina support formed, involving support dissolution and neoformation of a mixed compound with a layered structure. 

It can be noted that the brucitic layer of Ni(II) bonded to silica acts as nuclei for the growth of supported 1 1 nickel phyllosilicate or supported nickel hydroxide. The heterocondensation reaction is faster than the olation one, but it is limited by the concentration and diffusion in solution of silicic acid arising from silica dissolution, which itself depends on the silica surface area, i.e., on the extent of support-solution interface. 

There are several possible origins for smectites in soils inherited from parent materials, formed by weathering of other 2 1 phyllosilicates (especially micas), or precipitated from soil solutions after dissolution of other primary minerals. 

Among the techniques used to characterize silica-supported Ni phases, FTIR spectroscopy is shown to be well adapted to identify ill-crystallized phases generated during the preparation by the competitive cationic exchange method. FTIR spectroscopy permits to discriminate a phyllosilicate of talc-like or serpentine-like structure from a hydroxide-like phase. Samples submitted to hydrothermal treatments have also been characterized by other techniques such as EXAFS and DRS spectroscopies. The pH and the specific surface area strongly influence the nature of the deposited phase, since they control the solubility and the rate of dissolution of silica. The results are discussed in terms of the respective amounts of soluble Si(OH>4 monomers and NP+ complexes at the interface. The relevant parameter as the Ni/Si ratio at the solid-liquid interface is assumed to control the routes to Ni-Si (Ni-Ni) copolyinerization (polymerization) reactions leading to supported Ni phyllosilicates (Ni hydroxide). 

The formation of silica supported phases is more complex since it first implies the dissolution of silica to form Si(OH)4 monomers which react with the Ni2+ complexes, and a rearrangment of the geometry of silicon tetrahedra of the support (which differs from that of the tetrahe al sheet in a bulk phyllosilicate) to accomodate the supported clays. 

The dissolution of silica is catalysed by OH ions and the rate of dissolution increases with the specific surface area [28,29]. The Ni/Si ratio of soluble species available near the interface controls the nature of the deposited phase (Ni hydroxide and/or phyllosilicate) which grows via polymerization of Ni-O-Ni (Ni-O-Si) monomers. 

The quartz, albite and anorthite samples underwent dissolution, often along preferred crystallographic directions to form etch pits visible by SEM. The phyllosilicate reactants were too fine grained for any dissolution textures to be visible by SEM. New calcium aluminosilicate phases precipitated abundantly in the anorthite, chlorite and muscovite samples. The resolution of the SEM analytical facility was not sufficiently high to obtain quantitative chemical data for these phases, whereas good quality chemical data were obtained for them by ATEM (Tables 3 and 4). 

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