Silicate clays dissolution

Fixation by silicate clays Dissolution of AF+ can take place as a result of kaolinite breakdown, followed by the precipitation of AIPO4 (Figure 9.17). 

Silicate and clay dissolution and transformation by fungi The silicates are the largest class of minerals comprising 30% of all minerals and making up 90% of the Earth s crust (Ehrlich, 1996). Microorganisms including fungi play fundamental roles in the dissolution... 

The solid phase of soils can consist of many Al-bearing materials, including organic matter oxides and hydroxides of Al noncrystalline aluminosilicates layer silicate clays and various primary minerals. Any model of Al solubility that is based on the dissolution reaction of only one of these materials is likely to be too simple. Nevertheless, some of these models will now be considered in turn. 

The AF thereby generated from mineral dissolution becomes available to displace Ca from soil exchange sites. Unfortunately, the mass-action cation exchange equation (5.21) does not adequately describe Ca -AF exchange on layer silicate clays (see Chapter 3). Instead, a Langmuirlike function is found to be useful ... 

The reviewed processes of dissolution and minerogenesis are capable of noticeably changing the composition of the host rocks. As a regular rule, relic rock-forming minerals are removed, and the secondary ones can both form and dissolve. An exception among the latter ones are secondary aluminum silicates, clay minerals, which practically insoluble in water. They as if replace relic minerals. 

While rapid burial enhances preservation, the type of sediment produced is determined by the relative particle composition of the deposit. For example, rapid burial of biogenic silicate by clay minerals helps protect the shells against dissolution, but the resulting deposit is classified as an abyssal clay, rather than a siliceous ooze, if the sediment is less than 30% by mass BSi. Thus, prediction of the sediment type likely to be found at a given location requires knowledge of the relative magnitudes of the accumulation rates of all particle types. 

Ca (aq), Mg (aq), and HCOjCaq). Silicate weathering is an incongruent process. The most important of these reactions involves the weathering of the feldspar minerals, ortho-clase, albite, and anorthite. The dissolved products are K (aq), Na (aq), and Ca (aq), and the solid products are the clay minerals, illite, kaolinite, and montmorillonite. The weathering of kaolinite to gibbsite and the partial dissolution of quartz and chert also produces some DSi,... 

The results of this reaction are precipitation of smectite clay and release of silicic acid, sodium ions, and bicarbonate ions to solution. The actual dissolution reaction is significantly more complex than indicated here, as discussed in Section 2.4. 

The complete disintegration and dissolution of the mineral material. This method frees the largely intact kerogen. Concentrated hydrofluoric acid is the most widely used reagent in this class (6,7) since it is still the only reagent that can effectively solubilize the silicate mineral (clay and quartz) that is resistant to most chemical treatments. 

Some silicate minerals are also formed in a similar manner. The process is very slow, slower than even carbonate formation, because of the very low solubility of silicate minerals. In clay minerals, or in lateritic soils, silicates dissolve very slowly to form an intermediate product, silicic acid (H4Si04), which subsequently will react with other sparsely soluble compounds and form silicate bonding phases. Thus, a dissolution-precipitation process seems to be crucial to forming some silicate minerals. 

This process comprises dissolution of alumino-silicate minerals, and is the basis for much of the discussion concerning the development of saprolites and associated weathering products. Although conditions at the weathering front may initially favour the formation of micaceous clays (e.g. chlorite, vermiculite) and smectite, small quantities of gibbsite may also be formed, and, eventually, kaolinite can come to dominate those free-draining profiles where there is an adequate water supply. 

Little dissolution of quartz and a fortiori of the siliceous constituents (micas, phytolites, etc.) is possible, in the top layer of the soil. On the other hand, kaolinite which is the exclusive clay mineral in all the soils of the bioclimatic sequence, must be stable, since at pH 6—7 and for dissolved silica near 10 " mol/1, the solutions are saturated with respect to this mineral, when dissolved Al exceeds 10 mol/1, i.e. 10 mg/1 (Gardner, 1970). The addition of Al in the chemical charge of drainage waters might result from the colloidal mobility of this element. 

Rubber compounds can contain a range of fillers such as silicates, sulphates, oxides, carbonates, phosphates, nitrates, titanium dioxide, barium sulphate and various clays. Analysis of mixtures of these additives is complicated and it is necessary to separate the constituents by dissolution in suitable inorganic solvents and/or filtration of insoluble substances such as silicates. The residues can be examined in several ways ... 

This case is used when clay and silica dissolution/precipitation is important and different sodium silicates are injected. Compared with Example 10.4, additional elements or pseudoelements are aluminum and silicon. Additional independent aqueous species include Al and 11(8104. Additional dependent aqueous species are Al(OH) +, Al(OH)2, Al(OH), 1138104, H28iO, and H8i206T Additional solid species are 810a (silica), Al28i205(OH)4 (kaolinite), and NaA18i0206-H20 (analcite). 

Finally, the dissolution of a 2 1 layer silicate such as beidellite can also be involved in controlling the solubility of A1 and Si. If is assumed to occupy the exchange sites of this clay, the dissolution reaction is... 

The acid attack on the clay structure progresses inwards from the edge of the clay platelets leaching cations, particularly Mg if present, from the octahedral sheet. Acid activation causes little damage to the silicate layer and consequently the structure in the center of the platelet remains intact. The rate of dissolution of the octahedral sheet is a first order process which increases not only with increasing concentration of acid, temperature and contact time, but also increasing Mg content in octahedral sheet [4,8]. 

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