Decomposition kaolinite

FIGURE 5.11 Kaolinite decomposition at 400 C in a vacuum. From Holt et al. [17]. Reprinted by permission of the American Ceramic Society. 

Hence, the rate of kaolinite decomposition depends on both the acidity and the kaoli-nite concentration. As kaolinite decomposes, its rate of decomposition decreases, and complete disappearance should theoretically require infinite time. Indeed, kaolinite is quite resistant to weathering. Kaolinite is nonexpanding so its exposed surface is small. Inasmuch as few soil minerals expand, Eq. 7.8 probably characterizes soil mineral weathering better than does Eq. 7.7. Smaller mineral particles tend to decompose first, leaving behind the larger particles. The weathering rate thus dimin-... 

Gays are an essential component of soils, to which we owe our survival, and they are also the raw materials for some of mankind s most ancient and essential artefacts pottery, bricks, tiles, etc. Clays are formed by the weathering and decomposition of igneous rocks and occur typically as very fine panicles e.g. kaolinite is formed as hexagonal plates of edge. 1-3 p m by the weathering of alkaline feldspar... 

For N-methyl-p-toluyl carbamate, a chemical susceptible to neutral and base-promoted decomposition, the rates of hydrolysis on Na-montmorillonite and kaolinite were slower than in aqueous solution of the same bulk pH. A possible explanation is the lower pH at the clay surface (74). The following hydrolysis mechanism in aqueous solution was proposed ... 

Weber, J.B. and Coble, H.D. Microbial decomposition of diquat adsorbed on montmorillonite and kaolinite clays, J. Agric. 

Surface catalysis affects the kinetics of the process as well. Saltzman et al. (1974) note that in the case of Ca -kaolinite, parathion decomposition proceeds in two stages with different first-order rates (Fig. 16.14). In the first stage, parathion molecules specifically adsorbed on the saturating cation are quickly hydrolyzed by contact with the dissociated hydration water molecules. In the second stage, parathion molecules that might have been initially bound to the clay surface by different mechanisms are very slowly hydrolyzed, as they reach active sites with a proper orientation. 

The value of the reaction enthalpy of decomposition of the kaolinite-DMSO intercalate is -12 kcal/mol [142] and the enthalpy of formation of hydrogen bonds is about -10 kcal/mol [143]. 

Kaolinite is transformed into X-ray amorphous state when activated in air. According to authors [14,15], amorphization involves the destruction of bonds between tetrahedral and octahedral layers inside the package, till the decomposition into amorphous aluminium and silicon oxides. Other researchers [ 16,17] consider that amorphized kaolinite conserves the initial ordering of the positions of silicon atoms while disordering of the structure is due to the rupture of A1 - OH, Si - O - A1 bonds and the formation of molecular water. Endothermic effect of the dehydration of activated kaolinite is shifted to lower temperatures while intensive exo-effect with a maximum at 980°C still conserves. When mechanically activated kaolinite annealed at 1(X)0°C, only mullite (3Al20j-2Si0j) and X-ray amorphous SiOj are observed. In this case, the phase with spinel structure which is formed under thermal treatment of non-activated kaolinite is not observed thus, mechanical activation leads to the formation of other phases. 

Correlation of percent oxygen in phenolic hydroxyl groups in coal, Given (28) (O) this study (7) (+) this study corrected for decomposition of kaolinite (A) Loison and Chauvin (22) ( ) Fitzgerald and Van Krevelen (23) (V) Jiintgen and Van Heek (24) after Hanbaba et al. (25). 

Figure 4.8. Changes in the Si spectra of kaolinite during its thermal decomposition, showing the progressive formation of the broad metakaolinite resonance envelope - 99 to - 102 ppm) at 650-800°C, the sudden appearance of free Si02 (—110 ppm) at 970°C, and the formation of mullite (— 88 to — 92 ppm) above 1100°C. Adapted from Mackenzie et al. (1985a) by permission...

Figure 5.30. Changes in the Al site occupancy of kaolinite during its thermal decomposition, after Temuujin et al. (1998a) by permission of the copyright owner.

Notice that acid, represented here by C02 and water, is consumed and that sodium and bicarbonate ions are produced as albite alters to kaolinite. The material entering the lake as the stratification developed appears to have had two characteristic properties—easily hydrolyzed sodium and a limited content of halogens, particularly chlorine. Decomposition of organic matter furnished carbon dioxide. Concentration of sodium carbonates and other dissolved salts in solution in water of the stratified lake s lower layer built up, raising the water density. The evolution from normal lacustrine sediments to oil shale in the northwestern part of the Piceance Creek Basin is continuous... 

Eremin, N.I. Tkacheva, L.V. Makarenko, V.N. "Investigation of the Kinetics of the Decomposition of Kaolinite in Alkaline and Aluminate Solutions," Soviet Non-ferrous Metals Research 1978, 6(5), 197-199. 

Figure 3.23 TGA and DTA spectra of the decomposition of kaolinite as a function of temperature...

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