Classification silicate structures

A. Determination of silica in a soluble silicate Discussion. Most of the silicates which come within the classification of soluble silicates are the orthosilicates formed from SiO units in combination with just one or two cations. More highly condensed silicate structures give rise to the insoluble silicates. 

Liebau, F. (1985). Structural Chemistry of Silicates. Structure, Bonding and Classification. Berlin Springer. 

F. Liebau, Structural Chemistry of Silicates Structure, Bonding, and Classification, Springer-Verlag, Berlin, 1985. 

Classification Silicate minerals Definition Any of several silicates of varying chem. composition, but similar physical props, and crystalline structure... 

F Liebau. Structural chemistry of silicates. Structures, bonding and classification. Berlin Springer Verlag, 1985. 

In order to study in more detail the clay minerals, it is first helpful to review briefly the basic structural classification of the silicates in general. Although ultimately complicated, the general progression is logical, and is based on the degree of polymerization of the basic structural unit which is the Si04 tetrahedron (see below). The sequence runs as follows ... 

Each of these solid phases can be described in terms of their mineralogy. This classification scheme is based on crystal structure and chemical composition. The most common minerals found in marine sediments are listed in Table 13.2. Most are silicates in which Si and O form a repeating tetrahedral base unit. Other minerals common to marine sediments are carbonates, sulfates, and oxyhydroxides. Less common are the hydrogenous minerals as they form only in restricted settings. These include the evap-orite minerals (halides, borates, and sulfates), hydrothermal minerals (sulfides, oxides, and native elements, such as gold), and phosphorites. 

The chemical classification of silicates is based on their multiplicity and type of structural arrangement. It partly differs from the classification commonly adopted in mineralogy (table 5.2). 

We have already learnt that the building unit of the silicates is the Si04 pyramid. By linking these pyramids in various ways the different kinds of silicate anions are formed. The classification of the silicates is based on these kinds of ions. The silicate anions of clay minerals have a layered structure, the top view of which is again represented in figure 8.5. 

Quartz is the most common mineral in the Earth s crust. It occurs in a wide variety of forms, colors, and lusters, but other properties are generally consistent for all specimens. All quartz has the same basic chemical formula, silicon dioxide (SiC ). It has a hardness of 7, and a distinctive conchoidal fracture. The color and translucency of quartz can be affected by a disruption of the molecular structure, as in smoky quartz, or by the inclusions of tiny amounts of other elements or minerals. In some classifications, quartz is listed with the oxides, but it is most often placed with the silicates. 

TABLE 3E. Classification of Silicate Minerals Based on Structural Arrangements in Silicate Crystal Lattices... 

The structural classification of silicates is based on the polymerization of silicon tetrahedra (Si04) and on the proportion of silicon-to-oxygen atoms (Si 0) as follows ... 

The silicate minerals are, of course, characterized by the presence of the tetrahedral Si04 cluster unit and the crystal chemistry and classification of silicates dominated by the structures built up by the linking together... 

The principal primary mineral groups are the silica minerals (including quartz), feldspars, feldspathoids, olivines, pyroxenes, amphiboles, and micas. All are silicates and can be classified structurally based on the arrangement of connected silica (Si04) tet-rahedra. Table 2.2 summarizes the classification system for these common rockforming minerals. 

Whereas soil scientists would classify clay by considering the particle size, a chemist describes it as part of the family of silicates and more precisely as hydrated silicates of aluminium. However, the classification of minerals with a stoichiometric formula does not reflect the complexity of their structure. In order to explain the properties of clay minerals in an adequate way, the coordination of silicon has to be described first, before looking at a long-range order in the crystals. 

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