Aluminosilicate minerals

The approximate elemental composition of the earth s inorganic mineral surface is reported in Table 3,2. Elemental composition alone, however, cannot justify the unique properties of soil and how such properties influence the soil and water environment. The component that explains many of the physical and chemical properties of soil is the molecular arrangement of elements, forming structures with unique physicochemical properties. Soil mineral structures are briefly discussed below. 

The inorganic minerals of soil are classified into (a) primary minerals and (b) secondary minerals (Table 3.3). Primary minerals are minerals with the chemical composition and structure obtained during the crystallization process of molten lava, whereas secondary minerals are those that have been altered from the original structure and chemical composition by weathering, a process referred to as the geomorphic cycle (Fig. 3.2). Generally, the size of soil mineral particles varies from clay-sized colloids ( 2 pm) to gravel ( 2 mm) and rocks. 

Aluminosilicates or phyllosilicates are inorganic crystalline structures which make up a large part of the 0.2 mm soil-sized particles. These minerals, commonly referred to as clay minerals, consist of Si-0 tetrahedrons, in which one silicon atom (Si4+) is 

TABLE 3.2. Approximate Elemental Composition of the Earth s Outer-Surface Layer 

The amount of substitution in the tetrahedral and octahedral sheets and the ratio of octahedral to tetrahedral sheets are the primary differentiating characteristics between the many clay minerals (Fig. 3.6). For example, clays that have one tetrahedral sheet and one octahedral sheet are known as 1 1 clay minerals (e.g., kaolin group) (Fig. 3.7) clays that have two tetrahedral sheets and one octahedral sheet are known as 2 1 clay minerals (e.g., smectite group) (Fig. 3.8) or mica and vermiculite (Fig. 3.9), while clays that have two tetrahedral sheets and two octahedral sheets are known as 2 2 clay minerals (e.g., chlorite) (Fig. 3.10). These sheet arrangements give rise to various mineral surface identities such as magnitude (specific surface), functional groups, and interactions with solution species. 

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The metals which form silicate and aluminosilicate minerals are the more electropositive metals, i.e. those in Groups 1, 2 and the 3d transition series (except Co), together with Y, La and the lanthanoids, Zr, Hf, Th, U and to a much lesser extent the post-transition elements Sn", Pb", and Bi ". 

The migration of heavy metals into mineral lattices is very slow. Ahnstrom and Parker (2001) observed the slow migration of luCd into the residual fraction in arid soils with a weeks-to-years reaction half-time. Theoretically, the residual fraction is comprised of very refractory Cd bound in the lattices of aluminosilicate minerals. Lattice diffusion, a process necessary for isotopic exchange, can require years. 

Error in the input data can also be significant. The saturation state calculated for an aluminosilicate mineral, for example, depends on the analytical concentrations determined for aluminum and silicon. These analyses are difficult to perform accurately. As discussed in the next section, the presence of colloids and suspended particles in solution often affects the analytical results profoundly. 

If pH at a point in the domain drops, for example, an aluminosilicate mineral... 

From a plot of the saturation states of the silica polymorphs (Fig. 23.7), the fluid s equilibrium temperature with quartz is about 100 °C. Quartz, however, is commonly supersaturated in geothermal waters below about 150 °C and so can give erroneously high equilibrium temperatures when applied in geothermometry (Fournier, 1977). Chalcedony is in equilibrium with the fluid at about 76 °C, a temperature consistent with that suggested by the aluminosilicate minerals. 

Houdry The first catalytic petroleum cracking process, based on an invention by E. J. Houdiy in 1927, which was developed and commercialized by the Houdry Process Corporation. The process was piloted by the Vacuum Oil Company, Paulsboro, NJ, in the early 1930s. The catalyst was contained in a fixed bed. The first successful catalyst was an aluminosilicate mineral. Subsequently, other related catalysts were developed by Houdry in the United States, by I. G. Farbenindustrie in Germany, and by Imperial Chemical Industries in England. After World War II, the clay-based catalysts were replaced by a variety of synthetic catalysts, many based on alumino-silicates. Later, these too were replaced by zeolites. U.S. Patents 1,837,963 1,957,648 1,957,649. 

Jambor, J.L., Dutrizac, J.E., Groat, L.A., Raudsepp, M. 2002. Static tests of neutralization potentials of silicate and aluminosilicate minerals. Environmental Geology, 43, 1-17. 

Kaolin Minerals. The 1 1 structures include a group of aluminosilicate minerals which are termed collectively the kaolin minerals specifically these are kaolinite, dickite, nacrite, and halloysite. The basic 1 1 layer for all of these minerals has the composition AlgSigOj-fOHJj, there is a small amount of substitution of iron for aluminum, ana fluoride for hydroxyl ion. All, except halloysite, are normally anhydrous and do not expand (as do the smectites) upon exposure to water and most organic molecules. As a result, they generally have a rather small surface area, on the order of 10 nr... 

Potassium-Calcium Exchange Equilibria in Aluminosilicate Minerals and Soils... 

Adsorption of Metal Ions and Complexes on Aluminosilicate Minerals... 

As far as adsorption properties are concerned, aluminosilicate minerals may be divided into 3 general groups -... 

The surface charge of aluminosilicate minerals may arise either from isomorphous cation substitution within the structure,... 

In addition to stabilizing organic products by reaction with metal-exchanged clays, as indicated above, aluminosilicate minerals may enable the preparation of metal organic complexes that cannot be formed in solution. Thus a complex of Cu(II) with rubeanic acid (dithiooxamide) could be prepared by soaking Cu montmorillonite in an acetone solution of rubeanic acid (93). The intercalated complex was monomeric, aligned with Its molecular plane parallel to the interlamellar surfaces, and had a metal ligand ratio of 1 2 despite the tetradentate nature of the rubeanic acid. 

Chemical reactions of adsorbed species are of importance in vast areas of science, the involvement of adsorbed metal ions in catalysis being one example of great economic value. In addition reactions involving adsorbed species can sometimes produce products that may be either difficult or impossible to prepare away from the mineral surface. Therefore, an understanding of the chemical processes that occur in such systems is of potential economic benefit to industrial operations. Such knowledge is also of much wider significance, however, because the movement of ions in most environmental situations is controlled by sorption processes, and aluminosilicate minerals play a major role in many situations. 

Sorption processes are influenced not just by the natures of the absorbate ion(s) and the mineral surface, but also by the solution pH and the concentrations of the various components in the solution. Even apparently simple absorption reactions may involve a series of chemical equilibria, especially in natural systems. Thus in only a comparatively small number of cases has an understanding been achieved of either the precise chemical form(s) of the adsorbed species or of the exact nature of the adsorption sites. The difficulties of such characterization arise from (i) the number of sites for adsorption on the mineral surface that are present because of the isomorphous substitutions and structural defects that commonly occur in aluminosilicate minerals, and (ii) the difference in the chemistry of solutions in contact with a solid surface as compound to bulk solution. Much of our present understanding is derived from experiments using spectroscopic techniques which are able to produce information at the molecular level. Although individual methods may often be applicable to only special situations, significant advances in our knowledge have been made... 

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