Silicate-based minerals

Silicon being the second most abundant element in Earth s crust, it is no surprise that most minerals are based on this element. There are several major classes of silicate-based minerals, including quartz (which is considered in Chapter 9), simple silicates, and more complex compounds. The largest and more important class is constituted by those which can be grouped, in a general way, as aluminosilicates, comprising many types of structures, including both primary and secondary minerals. 

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Before we examine the structures and properties of metallic classes in further detail, it is useful to consider the natural sources of the metals, generally as oxide and/or silicate-based mineral formations. If the mineral deposit contains an economically recoverable amount of a metal, it is referred to as an ore. The waste material of the rock formation is known as gangue, which must be separated from the desired portion of the ore through a variety of processing steps. 

It follows that the oxides, nitrides, borides, carbides, and silicides (not to be confused with silicates) of all metals and NMESs are ceramics which, needless to say, leads to a vast number of compounds. This number becomes even more daunting when it is appreciated that the silicates are also, by definition, ceramics. Because of the abundance of oxygen and silicon in nature, silicates are ubiquitous rocks, dust, clay, mud, mountains, sand — in short, the vast majority of the earth s crust — are composed of silicate-based minerals. When it is also appreciated that even cement, bricks, and concrete are essentially silicates, the case could be made that we live in a ceramic world. 

Figure 2.92. Molecular structures of silicate-based minerals. Shown are (a) chain structures of chrysotile (Mg3Si205(0H)2 - a common member of the asbestos family) and pyroxene (XSiOs, X = Mg, Na, etc.), and (b) sheet/layered structures exhibited by various clays. Reproduced with permission from AUcock, H. R. Introduction to Materials Chemistry, Wiley New York, 2008. Copyright 2008 John WUey and Sons, Inc.

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