Silicate Class

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°. 

In some silicate minerals, some of the tetrahedra may have Al at their centres instead of Si . This is commonly referred as substitution of Si by Ar. It necessitates inclusion of some cation in the mineral structure, or substitution of a low-valency cation by a high-valency cation, for balancing the excess negative charge. 

Each Si04 tetrahedron has an imbalanced charge of -4, which can be neutralized in the following two ways  

Bonding with cations An Si04 tetrahedral group can be bonded to cations. Generally bivalent cations like Mg, Fe, Ca etc. are bonded to the tetrahedral group in common silicate minerals. The monovalent cations like Na, and the bivalent cations like Fe , Al etc. are also common. These cations are generally surrounded by six O to form an octahedral coordination site. 

Different arrangements of linking of apical oxygens give rise to the different silicate subclasses, as discussed here. 

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In the previous paragraph, it has been stated that minerals have the same structure but different compositions (phenomenon of isomorphism of minerals) while some minerals have the same composition but different structures (phenomenon of polymorphism of minerals). Mineral composition and structure are both important in studying and classifying minerals. The major class of minerals - based on composition and structure - include elements, sulfides, halides, carbonates, sulfates, oxides, phosphates, and silicates. The silicate class is especially important, because silicon makes up 95% of the minerals, by volume, in the Earth s crust. Mineral classes are divided into families on the basis of the chemicals in each mineral. Families, in turn, are made of groups of minerals that have a similar structure. Groups are further divided into species. 

The Silicate Class. Amethyst Galleries, Inc., http //mineral.galleries.com/minerals/silicate/class.htm... 

About a third of all minerals belong to the silicates class, which is divided into five subclasses. Thirty-five other elements participate in the formation of various silicates which form about 95% of the rocky crust of the earth. Most of these (72%) belong to the subclass of tektosilicates called framework silicates. Feldspar and quartz are the most prominent species in this group. 

The Si04 " tetrahedron is the basic building block of the silicate class of minerals. The bonding in this structure is mesodesmic because the charge remaining on each O atom following coordination to the Si ion at the center of the tetrahedron is exactly one-half the charge on the oxide ion. 

The silicate class of minerals is a perfect example of Pauling s first three rules at work. The radius ratio rule (Rule I) predicts that each Si + ion will be four-coordinate (because 41/140 = 0.29). Because the electrostatic bond strength (Rule 2) of is 4/4, or -l-l, each will contribute -1 to the Si-O bonding, as shown in Figure 12.14. 

Cl minerals belong to the phyllosilicate subclass of silicate class. They have been abundantly studied by X-ray diffraction, which is the basic tool for then-identification as it is difficult to observe without using electron microscope. 

Some minerals of silicate class and oxide and hydroxide class are stable in the environment prevailing at or near die Earth s surface, and they are resistant to chemical weathering. These minerals, when pulverised to very fine grain size by natural processes, classified as in Table 2.3, can be present as associated minerals in the clays. These minerals are not plastic when wet or harden by drying or firing neither do they impart these properties to the clay. On the contrary, the plasticity of clay decreases as the proportion of associated minerals increases in them. Some associated minerals, especially the oxides and hydroxides of iron, imparts to clay some red or reddish-brown colour, which even persists after firing of those clays. Thus the increase of associated minerals decreases the economic value of clay. Some common associated minerals are listed below. 

About one third of all minerals belong to the silicate class, but only three types of natural silica are in popular use, and these are included in the table above. Diatomaceous earth is one of the most frequently used antiblocking agents. Its range of applications has recently decreased, when it was discovered that diatomaceous earth absorbs process additives used in extrusion. 

Nanoscale Bioactive Silicate Classes in Biomedical Applications... 

I 6 Nanoscale Bioactive Silicate Classes in Biomedical Applications 6.3.3.2 Antimicrobial Effects... 

Silicate Unit Ideal Formula Silicate Class... 

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