Kaolinite amorphous silica

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 ordering at the kaolinite surface is striking when compared with similar calculations for the amorphous silica surface (Figure 2c) which shows the ordering is substantially reduced relative to kaolinite. It is tempting to associate this disorder with the disorder at the silica surface this is consistent with the snapshot in Figure 4. 

Figure 2 (opposite) (a) One-dimensional density profile of the centres of mass of water molecules in bulk water, b) Density profile of the centres of mass of the water molecules along the z-axis (perpendicular to the surface) for kaolinite. (c) Density profile of the centres of mass of the water molecules along the z-axis (perpendicular to the surface) for amorphous silica... 

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. 

Assuming a correlation between surface complexation and aqueous hydrolysis exists, the trend in strengths of surfaces complexes for An in different oxidation states onto a given mineral would be in the order An4+ > AnC>2+ > An3+ > AnOj. Several authors have provided evidence for linear relations between the first hydrolysis constant of metals and the intrinsic constant associated to the formation of surface species of metals as S-OMamorphous silica (Schindler Stumm 1987), hydrous ferric oxides (Dzombak Morel 1990), aluminum (hydr-)oxides and kaolinite (Del Nero et al. 1997, 1999a). 

The major aluminous clay minerals, alkali zeolites and feldspars which are most commonly associated in nature can be considered as the phases present in a simplified chemical system. Zeolites can be chemiographically aligned between natrolite (Na) and phillipsite (K) at the silica-poor, and mordenite-clinoptilolite at the silica-rich end of the compositional series. Potassium mica (illite), montmorillonite, kaolinite, gibbsite and opal or amorphous silica are the other phases which can be expected in... 

Figure 36. Representation of the zeolite-clay mineral assemblages found in a systeirf at 25°C and atmospheric pressure where Na is an intensive variable (perfectly mobile component) whereas A1 and Si are extensive variables or inert components of the system. G = gibbsite kaol = kaolinite Mo = montmorillonite Si = amorphous silica Anal = analcite.

Figure 37. Phases found in zone one of the alkali lake (Figure a) and in zone two of the lake (Figure b). Mi mica Mo = montmorillonite Kaol = kaolinite F = feldspar Op = opal amorphous silica.

However, before considering such a complex system of four independent variables, which is represented in planar perspective, let us first take the variables as they can be represented in a sequence of change from inert components which, one by one, become "perfectly mobile" or intensive variables of a thermodynamic system. We will first assume that the phases which will be present in some portion of the system are gibbsite, kaolinite, crystalline or amorphous silica, mica, illite, mixed layered illite-montmorillonite (beidellite), K-feldspar (no pure potassium zeolite is present). Initially we will simplify the mineralogy in the following way ... 

Figure 4. Silicate stability. KF, KM, G, K, and Q are K+-feld-spar, K+-mica, gibbsite, kaolinite, and amorphous silica, respectively. M and AB are montmorillonite and albite. W, S, FW, and SW represent areas of winter lake data, summer lake data, extracted fresh water sediments, and extracted sea water sediments, respectively...

Chemical dissolution techniques indicate kaolinite from Cornwall contains 3.1-4.9% of easily soluble Si02 and 1.5-5.9% of easily soluble A1203 (Follett et al., 1965). Most of this material is presumably present as amorphous material. Experiments (by the senior author) with Georgia kaolinite indicate the amount of amorphous material varies as a function of particle size and preparation (Table LX). Amorphous silica and alumina is a common constituent of kaolinite and considerable care must be taken in determining and interpreting the significance of the Si/Al ratio of kaolinites. 

Solubility calculations were added for two allophanes, for which the equilibrium constants and formulae are a function of pH. Paces (74) found cold ground waters collected from springs in granitic rocks of the Bohemian Massif of Czechoslovakia to be supersaturated with respect to kaolinite while being unsaturated with respect to amorphous silica. He interpreted this as an indication that a metastable aluminosilicate more soluble than kaolinite was controlling the concentrations of alumina and silica in these waters. This aluminosilicate was further hypothesized to be of varied chemical composition, controlled by the mole... 

Figure 3.9 FTIR and FTFIR skeletal spectra of amorphous silica (aerosil), a-quartz, silicalite-1 and kaolinite.

Zeolites, particularly zeolite A, can be manufactured from kaolinitic clays, which as particularly found in Central Europe, Great Britain, Japan, China and USA. To transform kaolin into zeolite, it has to be thermally converted, e.g. by shock heating to > 550°C, to metakaolin. The metakaolin is then su.spended in sodium hydroxide solution and converted at 70 to 100°C into zeolite A. Some of the impurities contained in the natural raw material are retained in the final product. If amorphous silica is added, Si02-rich zeolites are produced. This process enables the transformation of preformed bodies into zeolite materials. 

However, the synthesis process most extensively studied by solid-state NMR is that of carbothermal reduction of aluminosilicate minerals such as kaolinite, which are mixed with finely divided carbon and heated in nitrogen at > 1400°C (Neal et al. 1994, MacKenzie et al. 1994a). Under carbothermal conditions the clay decomposes to a mixture of mullite and amorphous silica (MacKenzie et al. 1996b), the latter forming SiC which reacts with the mullite to form P-sialon, in some cases via other sialon phases such as X-sialon (see below). The precise reaction sequence and the nature of the intermediates has been shown by the NMR studies to depend on various factors including the nature of the aluminosilicate starting mineral (MacKenzie er a/. 1994a). 

When the clay mineral kaolinite is heated to about 980°C, it transforms to an intimate mixture of amorphous silica and an aluminium-rich spinel phase, the latter having potentially useful catalytic properties if the silica can be removed. Selective leaching... 

Strontium adsorption onto soil minerals is an important retardation mechanism for Sr " ". Chen et al. (1998) investigated the adsorption of Sr " " onto kaolinite, illite, hectorite, and montmorillonite over a range of ionic strengths and from two different electrolyte solutions, NaNO3 and CaCb- In all cases, the EXAFS spectra suggested Sr adsorbed to clay minerals as an outer-sphere mononuclear complex. Sahai et al. (2000) also found that on amorphous silica, goethite, and kaolinite substrates, Sr"+ adsorbed as a hydrated surface complex above pH 8.6. On the other hand, Collins et al. (1998) concluded from EXAFS spectra that Sr " " adsorbed as an inner-sphere complex on goethite. 

Electroacoustic studies of silica at high ionic strengths produced controversial results similar to those discussed in Section 4.3.2. Amorphous materials [1813,1870] showed a shift in the IEP to high pH at 1-1 electrolyte concentrations of 0.1 M or higher. The shift was more substantial in the presence of Cs than in the presence of other monovalent cations. This result is in line with the cation specificity series reported in Section 4.1.5. However, the IEP of quartz [1813] was not shifted in the presence of 1-1 electrolytes. Montmorillonite and kaolinite [2271] showed shifts in the IEP and similar cation affinity series as amorphous silica. Contradictory results are reported for goethite [76,1318]. 

Like calcium, strontium has moderate mobility in soils and sediments, and sorbs moderately to metal oxides and clays (Hayes and Traina 1998). The Sr2- ion is strongly hydrated and is firmly coordinated with six or more water molecules in aqueous solution. When Sr2- ions sorb on negatively charged mineral surface sites, the hydration sphere is retained (O Day et al. 2000). Strontium sorbs as hydrated ions on the surface of clay minerals (kaolinite), weathered minerals (amorphous silica), and iron oxides (Sahai et al. 2000). Sorbed carbonate on iron oxides enhances the sorption of Sr2- and permits the nucleation of Sr2- as strontium carbonate (Sahai et al. 2000). On calcite (calcium carbonate), Sr2 sorption occurs by electrostatic attraction as hydrated ions. However, at higher concentrations, precipitation of strontianite (strontium carbonate) occurs and strontium is likely to be less mobile (Parkman et al. 1998). 

Secondary silicates form as clay minerals in soils after weathering of the primary silicates in igneous minerals. The secondary silicates include amorphous silica (opal) at high soluble silica concentrations and the very important aluminosilicate clay minerals kaolinite, smectite (montmorillonite), vermiculite, hydrous mica (il-lite), and others. Kaolinite tends to form at the low silicate concentrations of humid soils, whereas smectite forms at the higher silicate and Ca concentrations of arid and semiarid soils. The clay fraction of soils usually contains a mixture of these day minerals, plus considerable amorphous silicate material, such as allophane and imogolite, which may not be identifiable by x-ray diffraction. 

HCOs, Cr, F, and N03 The initial pore water chemistry is based on a sample that was ultracentrifuged from the proposed repository host rock (Tptpmn). Minerals considered include silica phases (a-cristobalite, quartz, tridymite, amorphous silica, and opal-CT), calcite, feldspars, smectites, illite, kaolinite, sepiolite, zeolites, fluorite, hematite, and gypsum. Treatment of CO includes gas-water equilibration, diffusion, and advection. 

When kaolinite is heated (Figure 2.5) to above 500 °C it dehydroxylates endothermically, that is, it loses its water of crystallisation forming metakaolinite. This is then stable up to 980 °C, when a defect spinel structure, which is virtually amorphous, forms exothermically. Above 1100 °C there is a slow transformation of the defect spinel with mnllite forming in an amorphous silica matrix. 

In contrast to the relatively pronounced effects of aqueous speciation on actinide sorption, the similarity in the pH-dependence of actinide sorption on a wide variety of minerals such as quartz, a-alumina, clinoptilolite, montmorillonite, amorphous silica, kaolinite, and titanium oxide. suggests a relative insensitivity to surface charge characteri.stics of the sorbent as compared with (he effect of changing the total number of available sites. For example, the data in Fig. 10 3 demon-... 

Ceramists are particularly interested in reactions that occur in the firing range of ware, i.e., above 1200°C. It has been observed that in the range of 1200-1250°C, secondary mullite and amorphous silica are present, The silica changes to cristobalite in the range of 1240-1350°C. Primary mullite is formed below 1200°C. The kaolinite-mullite transition involves metakaolin transformation to a spinel phase and then to mullite of possible composition 3Al203 2Si02. 

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