Kaolinite precipitation

Further conversion of mixed-fay-er clays towards illite authigenesis. Authigenic kaolinite precipitation. Authigenic chlorite development. Mixed-layer s ordering corrensite,allevardite. Feldspar dissolution.Significant quartz overgrowths development, chert dissolution. Carbonate cement dissolution/predpi-tation. Some secondary porosity development. 

Figure 18.6 Reaction path model for the reaction of K-feldspar with water. Gibbsite-dissolving, kaolinite-precipitating is represented by path B C in Figure 18.7.

For kaolinite the sample permeability was very low and the solution was poorly removed. The spectra (Figure 3C) are consequently complex, containing peaks for inner and outer sphere complexes, CsCl precipitate from resMual solution (near 200 ppm) and a complex spinning sideband pattern. Spectral resolution is poorer, but at 70% RH for instance, inner sphere complexes resonate near 16 ppm and outer sphere complexes near 31 ppm. Dynamical averaging of the inner and outer sphere complexes occurs at 70% RH, and at 100% RH even the CsCl precipitate is dissolved in the water film and averaged. 

CaC03 CaCOg precipitation CaCOg Fe-hydroxide kaolinite... 

The alkali process uses sodium hydroxide and is well known as Bayer s process. It involves relatively simple inorganic and physical chemistry and the entire flowsheet can be divided into caustic digestion, clarification, precipitation and calcination. Although mineral assemblage in bauxites is extensive, processing conditions are primarily influenced by the relative proportions of alumina minerals (gibbsite and boehmite), the iron minerals (goethite and hematite), and the silica minerals (quartz and clays-usually as kaolinite). 

Smectite is the first secondary mineral to form upon rock weathering in the semi-arid to sub-humid tropics. Smectite clay retains most of the ions, notably Ca2+ and Mg2+, released from weathering primary silicates. Iron, present as Fe2+ in primary minerals, is preserved in the smectite crystal lattice as Fe3+. The smectites become unstable as weathering proceeds and basic cations and silica are removed by leaching. Fe3+-compounds however remain in the soil, lending it a reddish color aluminum is retained in kaolinite and A1-oxides. Leached soil components accumulate at poorly drained, lower terrain positions where they precipitate and form new smectitic clays that remain stable as long as the pH is above neutral. Additional circumstances for the dominance of clays are ... 

As an example, we consider the reaction path traced in the previous section (Fig. 13.1). To extract the overall reaction for each segment of the path, we construct a plot as described above. The result is shown in Figure 13.2. There are three segments in the reaction path the precipitation of kaolinite, the transformation of kaolinite to muscovite, and the continued formation of muscovite once the kaolinite is exhausted. There is a distinct overall reaction for each segment. 

Below about 10 cm, as a result, kaolinite becomes saturated and precipitates, and below about 25 cm the mineral replaces gibbsite as the sink for alumina. The reactions here are,... 

The rate of quartz precipitation, however, is insufficient to consume all the excess silica released by the conversion of feldspar to kaolinite, so silica continues to accumulate in the migrating fluid. At a depth of about 50 cm, tridymite, a proxy in the calculation for opal CT, becomes saturated and begins to form,... 

Hersh and coworkers (16) examined electrostatic precipitator samples using petrographic microscopy. The minerals along with their chemical formula eind mode of occurrence are given in Table VIII. Kaolinite and limonite were by far the most abundant minerals detected. 

In acids soils, particularly those with kaolinite clay minerals, soluble Fe + concentrations tend to rise to high levels because of low CEC and because conditions do not favour precipitation of Fe(II) oxides or carbonates or synthesis of silicates. 

Eordham, A.W. (1970) Sorption and precipitation of iron on kaolinite. III. The solubility of iron(III) hydroxides predpitated in the presence of kaolinite. Aust. J. Soil Res. 8 107— 122... 

N. Carroll, S.A. (2000) X-ray absorption spectroscopy of strontium coordination. II Sorption and precipitation at kaolinite, amorphous silica and goethite surfaces. J. Coll. 

The marked difference in the relaxation times for the kaolinite and silica may be attributed to the nature of the surface. Intuitively, the hydrogen bonding which influences the increased structure at the kaolinite surface would be expected to give shorter values for the relaxation time. However this is not observed in the simulations. Instead, shorter values are seen for the silica surface which is a result of water molecules becoming trapped in the cage-like amorphous silica surface. This reflects experimental results where precipitated silica surfaces are microporous and water inclusion in the surface is common. 

Nagy, K. L. Blum, A. E. Lasaga, A. C. 1991. Dissolution and precipitation kinetics of kaolinite at 80°C and pH 3. The dependence on solution saturation state. American Journal of Science, 291, 649-686. 

Alternatively, several workers have shown that not only is the soluble, zero-charged hydrolysis product considerably more surface active than the free (aquo) ion but also a polymeric charged or uncharged hydrolysis product may be formed at the solid-liquid interface at conditions well below saturation or precipitation in solution. Hall (5) has considered the coagulation of kaolinite by aluminum (III) and concluded that surface precipitates related to hydrated aluminum hydroxide control the adsorption-coagulation behavior. Similarly Healy and Jellett (6) have postulated that the polymeric, soluble, uncharged Zn(OH)2 polymer can be nucleated catalytically at ZnO-H20 interfaces and will flocculate the colloidal ZnO via a bridging mechanism. 

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