Silicates point defects

We have Investigated the structure of solids In the second chapter and the nature of point defects of the solid in the third chapter. We are now ready to describe how solids react. This will Include the mechanisms Involved when solids form by reaction from constituent compounds. We will also describe some methods of measurement and how one determines extent and rate of the soUd state reaction actually taking place. We will also show how the presence and/or formation of point defects affect reactivity In solid state reactions. They do so, but not In the memner that you might suspect. We will also show how solid state reactions progress, particularly those involving silicates where several different phases appear as a function of both time and relative ratios of reacting components. 

Schmalzried H. (1978). Reactivity and point defects of donble oxides with emphasis on simple silicates. Phy.. Chem. Minerals, 2 279-294. 

These results show that changes in the anion to cation ratios of the building components of the linked octahedra and tetrahedra which form the core of these silicates is accommodated structurally. There is no evidence to suggest that substantial stoicheiometric variation is accomplished by point-defect populations, and indeed, mineralogists have never had recourse to the point-defect model to account for such changes in stoicheiometry. The brief account above could be greatly expanded and many more examples will be found in the review article previously cited. ... 

We have seen above that the kinetics of mineral dissolution is well explained by transition-state theory. The framework of this theory and kinetic data for minerals have shown that dissolution is initiated by the adsorption of reactants at active sites. Until now these active sites have been poorly characterized nevertheless, there is a general consensus that the most active sites consist of dislocations, edges, point defects, kinks, twin boundaries, and all positions characterized by an excess surface energy. Also these concepts have been strongly supported by the results of many SEM observations which have shown that the formation of crystallographically controlled etch pits is a ubiquitous feature of weathered silicates. 

Localisation of electron den ty over a small region in a crystal lattice occupied by 02 leads to the increased importance of electron correlation which cannot be tackled by traditional one-electron or HF theories[61]. Here we build upon our previous studies in which we used ab initio, semi-empirical and semi-classical approaches to study 0) ionic crystalline peroxides (e.g. Si and Ba02 [70,71]), (ii) point defects in the bulk and on the sui % of ionic and semi-ionic materials (e.g. corundum, silica and aluminium silicates [72,73]), and (iii) bipolaron formation in lanthanum cuprate (a superconducting material [74]). 

With decreasing temperature, as we have seen, the intrinsic defect population decreases exponentially and, at low T, extrinsic disorder becomes dominant. Moreover, extrinsic disorder for oxygen-based minerals (such as silicates and oxides) is significantly alfected by the partial pressure of oxygen in the system (see section 4.4) and, in the region of intrinsic pressure, by the concentration of point impurities. In this new region, term Qj does not embody the enthalpy of defect formation, but simply the enthalpy of migration of the defect—i.e.,... 

When clay minerals are treated with dilute acids ( activation process), protons may attack the silicate layers via the interlayer region and exposed edges. Octahedral ions such as AP and Mg are extracted into the interlayer which promotes a rapid decomposition. However, many mont-morillonites resist such a treatment, even when using concentrated acids . Formation of layer defects allows the number of anchoring points for new cations at the surface layer to increase. 

The rate and degree of crystallization may also be affected. Any interface, any surface, any defect in any system may act as a nucleation point, and the addition of more of these nucleation points wiU necessarily produce systems with finer-grained crystalline micro-structures. In nanocomposites, the material is full of nucleation sites as long as silicate dispersion is favorable, materials containing finer-grained polymer crystallites (versus the... 

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