Defect silicate

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. 

Besides the multiplicity of defects that can be envisaged, there is a wide range of host solid phases within which such defects can reside. The differences between an alloy, a metallic sulfide, a crystalline fluoride, or a silicate glass are significant from... 

As Ti is incorporated in the silicate lattice, the volume of the unit cell expands (consistent with the flexible geometry of the ZSM-5 lattice) (75), but beyond a certain limit, it cannot expand further, and Ti is ejected from the framework, forming extraframework Ti species. Although no theoretical value exists for such a maximum limit in such small crystals, it depends on the type of silicate structure (MFI, beta, MCM, mordenite, Y, etc.) and the extent of defects therein, the latter depending to a limited extent on the preparation procedure. Because of the metastable positions of Ti ions in such locations, they can expand their geometry and coordination number when required (for example, in the presence of adsorbates such as H20, NH3, H2O2, etc.). Such an expansion in coordination number has, indeed, been observed recently (see Section II.B.2). The strain imposed on such 5- and 6-fold coordinated Ti ions by the demand of the framework for four bonds with tetrahedral orientation may possibly account for their remarkable catalytic properties. In fact, the protein moiety in certain metalloproteins imposes such a strain on the active metal center leading to their extraordinary catalytic properties (76). 

The incompletely condensed silsesquioxanes can be considered as intermediates in the synthesis of the whole family of silsesquioxanes, but at the same time they are the building blocks (e.g., initiators) for the new polymeric architectures.66 67 The importance of this class of materials in model studies of silica surfaces and silicate defects,68 and in creating various oligosilanols with novel frameworks,69 70 makes them the subject of extensive research.71 The use of the incompletely condensed silsesquioxanes in forming dimeric silsesquioxanes and metallasilsesquioxanes in attempts to design new routes to the well-defined Si-O assemblies has recently been reviewed in a well-illustrated research report.72... 

A common phenomenon in the dissolution of silicate minerals is the formation of etch pits at the surface (90-91.,93-94). When this occurs, the overall rate of mineral dissolution is non-uniform, and dissolution occurs preferentially at dislocations or defects that intercept the crystal surface. Preferential dissolution of the mineral could explain why surface spectroscopic studies have failed... 

The organization of this book follows the various states of aggregation of the earth s materials, in an order that reflects their relative importance in geology. Five chapters deal with the crystalline state. The first chapter is preparatory, the second and third are operative. The fourth summarizes some concepts of defect chemistry, the role of which in geochemistry is becoming more and more important as studies on kinetics and trace element applications advance. The fifth chapter is a (necessarily concise) state-of-the-art appraisal of the major silicate minerals. 

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.,... 

Lasaga A. C. (1980). Defects calculations in silicates Olivine. Amer. Mineral, 65 1237-1248. 

Morlotti R. and Ottonello G. (1982). Solution of rare earth elements in silicate solid phases Henry s law revisited in light of defect chemistry. Phys. Chem. Minerals, 8 87-97. 

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

No other silicates with arrays 1,2") are known , except as defects in leucophoenicite crystals but other compounds are known. The cation array in magnesium fluorobo-... 

It is now well established that surface defects and imperfections determine the chemical reactivity of solid surfaces, making defect engineering an important branch of modern solid state chemistry and physics. However, this reactivity can in turn be responsible for the toxicity of different silicate dusts. A number of interesting studies have appeared which correlate the surface properties of different silicates to their genotoxicity and potential health hazards.190 192 In particular the potential of a surface to generate and release free radicals via the... 

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