Components, rocks, soil silicates

The feldspars are aluminosilicates in which up to half the Si44 ions have been replaced by Al3+ ions. They are the most abundant silicate materials on Earth and are a major component of granite, a compressed mixture of mica, quartz, and feldspar (Fig. 14.45). When some of the cations between the crystal layers are washed away as these rocks weather, the structure crumbles to clay, one of the main inorganic components of soil. A typical feldspar has the formula KAlSi3Og. Its weathering by carbon dioxide and water can be described by the equation... 

If the linking of silicate chains continues in two dimensions, sheets of SiO tetrahedral units result (Table 18.4). Various clays and mica have this sheetUke structure. Clays, which are essential components of soils, are aluminosilicates— some Si + ions are replaced by Al ions plus other cations that take up the additional positive charge. Feldspar, a component of many rocks and a network silicate, is weathered in the following reaction to form clay. 

Optical identification of kaolinite is very difficult. Biaxial (-), a= 1.556, P = 1.563, 7=1.565, 5 = 0.007,2H= 40° It is common rock-forming secondary mineral, forming after aluminous silicates, component of soils and replaces feldspar in rocks undergoing weathering. Associated mineral is quartz. 

The main problems with field analysis are sample preparation for solids and lack of sensitivity for liquids. One option for converting solid matrices (rock, soil, concrete etc.) into solution is to utilise equipment for the preparation of XRF bead samples. Portable versions are available commercially and use lithimn borate fusion at 1000°C to solubilise siliceous and other refractory matrices in a single step, rather than repeated extraction with aggressive and toxic reagents, such as hydrofluoric and/or perchloric acids. The beads can be analysed non-destructively for major components by portable XRF and then dissolved for alpha spectrometry or HazWAC, as required. 

As part of a study of the secondary chemistry of members of Cistus (the rock-rose) in France, Robles and Garzino (1998) examined the essential oil of C albidus L. Plants were sampled from two areas in Provence characterized by different soil types, calcareous sites west of Marseille, and siliceous sites near Pierrefeu-du-Var and Bormes les Mimosas (PF and BM, respectively, in Fig. 2.23), which lie about 60 km and 80 km to the east, respectively, in the Massif les Maures. Regardless of the soil type, a-zingiberene [88] (Fig. 2.24) was the dominant component. Concentrations of other major components of the plants varied between the two soil types, as summarized in Table 2.6. Many other compounds were present in lesser amounts, but varied little between the two areas. A more recent paper by the same workers (Robles and Garzino, 2000) described an analysis of C. monspeliensis L. leaf oils, the results of which are summarized in Table 2.7. 

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 a result of the interfacial processes on rocks and soils, the structure and chemical bonds of the sorbed compounds can be changed. For this reason, different chemical reactions can be initiated in which the components of rocks or soils act as catalysts. The most important mineral catalysts are zeolites and clay minerals. Naturally, the different oxides also have catalytic effects, and nowadays some of them are being artificially produced for catalytic purposes such as framework silicates (zeolites), the most effective and selective catalysts in organic syntheses. The catalytic applications of zeolites are too wide to summarize in this book, so we deal with the catalytic effects of clay minerals. 

Pathfinder), 4000 km apart in the northern hemisphere gave similar basaltic compositions for the fine-grained soils that were analyzed the fine material will contain a significant component from the ancient cratered terrain that dominates the southern hemisphere, where most of the global dust storms originate. However, no component more siliceous than basalt appears in the Martian compositions. At the Pathfinder site, more siliceous rock... 

Silicon dioxide is the main component of the crust of the earth. Combined with the oxides of magnesium, aluminum, calcium, and iron, it forms the silicate minerals in our rocks and soil. 

Layer silicates, sheet-like phyllosilicates such as the familiar micas, are in primary rocks and in soils. The soil minerals are often called clay minerals. Since other components can also be in the clay fraction, layer silicates is a mom accurate term. A typical layer silicate is a combination of a layer of Al-, Mg-, or Fe(II)-0 octahedra plus one or two layers of Si-0 tetrahedra. The tetrahedral and octahedral sheets bond together by sharing oxygens at the corners of the tetrahedra and octahedra. Layer silicate minerals are differentiated by (1) the number and sequence of tetrahedral and octahedral sheets, (2) the layer charge per unit cell, (3) the type of interlayer bond... 

Because of the mainly detrital character of soil micas, all mica species known as rock components may occur in soils. Quantitatively, however, the proportion of different mica species within particular soils varies under the influence of environmental conditions. On a large scale, this is shown by the fact that although in the earth s crust trioctahedral micas are more abundant than dioctahedral micas, the reverse is true in soils. This is usually ascribed to the greater susceptibility of trioctahedral micas to weathering. Trends of weathering in soil profiles show the change in predominance from trioctahedral to dioctahedral layer silicates as the soil surface is approached. 

Diatoms can be expected in soils derived from parent materials of a siliceous aquatic nature. One rock type, diatomite, which occurs widely in California, is composed almost entirely of diatom frustules. Diatom remains are also common in alluvium, loess, lacustrine deposits, and in glacio-lacustrine sediments. Soils derived from any of these materials may be expected to have a rich diatom component. 

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