Silicate minerals tetrahedra

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. 

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 differentiation between cations and anions in regard to size and charge is reflected in the rules markedly different roles are attributed to cations and anions in a crystal. The rules are based upon the concept of the coordination of anions at the corners of a tetrahedron, octahedron, or other polyhedron about each cation, as assumed in the early work of W. L. Bragg on the silicate minerals, and they relate to the nature and interrelations of these polyhedra. 

In all silicate minerals formed under crustal conditions silicon is coordinated to four oxygen atoms. In high-pressure transformations, silicon commonly increases its coordination number. The longer- the Si—O distances in tetrahedral silicates the higher the pressure transformations to phases with octahedral silicon. The average Si—O bond distance for the pressure transformation is 159 pm. This distance is achieved at room temperature at pressures in all measured silicates and may be a minimum for tetrahedryl Si—O bonds 300 kbar is an upper pressure limit for the silicon tetrahedron and SOkbar is a lower pressure limit for octahedral silicon. Temperature has little effect on Si—O bond distances in either tetrahedra or octahedra... 

In order to understand the chemical stmctures and formulas of the silicate minerals, one must begin with the basic bnilding block of all sificates the silica tetrahedron. A sifica tetrahedron is an anionic species, which consists of a silicon atom covalently bound to four oxygen atoms. The silicon atom is in the geometric center of the tetrahedron and at each of the four points of the... 

Let us know first about the silicate minerals and then the cl minerals in the silicate group. All silicate minerals have the anionic group [SiOJ consisting of one Si surrounded by four 0". The tetrahedron obtained by joining the centres of the four O is known as [SiOJ" tetrahedron. 

Silicate Si04" tetrahedron is the basic structural unit of all silicate minerals in which each Si " is surrounded by four 0 ions in tetrahedral coordination. Phyllosilicate In the structure of phyllosilicate, each Si04 tetrahedron is linked to three adjacent tetrahedra to form an infinite sheet of tetrahedra. Each tetrahedron in the sheet thus shares three (out of fom) apical oxygens, having the basic structiual imit Si205. The micas, clay minerals, chlorite, talc and serpentine minerals are examples of phyllosilicates. 

All minerals belonging to silicate class contain the dominant anionic group Si04" , where each Si cation is surrounded by four 0 anions. If the centres of the four 0 are joined by imaginary lines, a tetrahedron is obtained with the Si situated at its centre (Fig. 3.2). This Si04" tetrahedron is the basic structural unit of all silicate minerals. The Si-0 bond is neither completely ionic nor completely covalent it is said to be 50% ionic bond. The bond angle between each Si-0 bond in the tetrahedron is 109.5°. 

Silicates and aluminosilicates are the most abundant minerals, which are main constituents of various rocks in the crust and mantle. Si" " combines with 0 to form silicate anion, SiOa" " in silicate crystal. The SiOa" has tetrahedral form (Fig. 1.11). The SiOa tetrahedron is the basic building block of silicates in which silicon is situated at the center of a tetrahedron of four oxygen atoms. SiOq" tetrahedra exist as discrete units or is joined via the O atoms, sharing their particles with other tetrahedra (Fig. 1.12). Silicate minerals mainly consisting of oxygen and... 

The properties of the silicates depend on the connections between the silicate tetrahedrons. Because of the wide variety of combinations of tetrahedron connections and the many different metal ions that fit within the structure, an enormous variety of different silicate minerals exist in nature, making the silicate materials the most common structures found on Earth. 

The fundamental structural imit of industrial silicate minerals is the silica tetrahedron. Quartz is just a densely packed arrangement of these tetrahedra, as depicted in Figure 3. 

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 ... 

Fig. 2.1 Configurations of the tetrahedral units and chain, double chain, and sheet structures in the silicate and aluminosilicate minerals. (A) Two-dimensional representation of a single silicate tetrahedron. (A ) Two-dimensional representation of an extended silicate chain. (B) Three-dimensional representations of single tetra-hedra in two orientations. The apexes of the tetrahedra point above or below the plane of the paper. (B ) Three-dimensional representations of extended silicate chains showing different orientations of the tetrahedra in two of the many possible configurations. Single chain pyroxenes (C), wollastonite (D), rhodonite (E). Double chains amphiboles (F). Sheets as found in the serpentines, micas, and clays (G).

The crystal structures of all the minerals in the serpentine group contain the same basic building blocks. The basic unit is composed of a silicate sheet of composition (Si205) ", in which three of the O atoms in each tetrahedron are shared with adjacent tetrahedra (Fig. 2.2A), and a nonsilicate sheet of... 

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