Associated Minerals and Phases

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. 

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. 

Kaolinite Smectite Illite Vermiculite Palygorskite- Mixed 

Kaolinite, Montmorillonite, Illite, Trioctahedral Sepiolite, Aliettite, 

Dickite, Beidellite, Glauconite, vermiculite, Loughlinite, Kulkeite, 

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