Silicate Minerals structural unit

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 ... 

The fundamental structural unit of all crystalline silicates is a tetrahedron that has an anion at each of its four corners and a Si" cation in the center. The silicate tetrahedra can be arranged in a variety of ways giving rise to a great variety of crystalline silicate minerals. Most of these arrangements involve some degree of sharing of the corner 0 anions between adjacent silicate tetrahedra. 

Many mineral species have the same or similar chemical basic units within their atomic structure. All common silicate minerals, for example, are characterized by the association of four large oxygen ions (0 ) bonded to a small silicon ion (Si ). The shape of the complex ion is a tetrahedral unit, with the composition (Si04) . The two- and the three-dimensional expressions of the silicate ion are presented in Fig. 2.1, parts A and B, respectively. The three-dimensional figures emphasize the potential variations in orientation between the ions as they have been observed in minerals. 

Some of the diversity that characterizes the properties and compositions of the silicate minerals stems from the ability of the aluminum ion (Al ) to substitute for silicon in the tetrahedral unit. When silicate tetrahedra in a mineral are replaced by aluminum-containing tetrahedra, concomitant changes occur in the size of the tetrahedron (usual Si—O bond length = 0.160 nm. A1—O bond length = 0.178 nm) and in the cations or protons that balance the tetrahedral unit charge. Regular substitutions with distinct chemistries and structures lead to the formation of groups of discrete minerals called aluminosilicates. 

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... 

Silicate ceramics are well suited for structural applications because of their strength, which originates in the partially ionic, strong silicon-oxygen bonds in the tetrahedral orthosilicate anion. This structural unit appears in naturally occurring minerals and clays, which are fashioned into ceramic pieces through sintering and densification processes. 

Using Table 22.1, predict the kind of structure formed by manganpyrosmalite, a silicate mineral with chemical formula Mni2FeMg3(Sii203o)(OH)ioClio. Give the oxidation state of each atom in this formula unit. 

In an average upper-crustal granodiorite, it is mainly feldspars that weather to form clay minerals (eqns. 4.13 4.14). Since feldspars are framework silicates, the formation of clay minerals (sheet silicates) must involve an intermediate step. This step is not at all well understood although it has been proposed that fulvic acids, from the decay of organic matter in soil, may react with aluminium to form a soluble aluminium-fulvic acid complex, with aluminium in six-fold coordination. This gibbsitic unit may then have Si04 tetrahedra adsorbed on to it to form clay mineral structures. 

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