erty of quartz-based glass is that

erty of quartz-based glass is that it is highly transparent (i.e., readily transmits light). Consequently, modern societies use large amounts of quartz for making thin sheets of crystal-clear window glass. The fact that quartz-based glass is also quite hard gives it a distinct advantage in certain applications over other transparent materials that are relatively soft. For example, plexiglass (made from plastics) is rather soft and is never used for automotive windshields since it would become so heavily scratched that it would be difficult to see through. Note that quartz-based glass is also chemically inert, which means that glass products do not break down when exposed to most chemicals. Note that many of the minerals and properties listed in Table 12.2 will be referred to throughout this chapter.

Economic Mineral Deposits
In Chapter 3 you learned that there are over 4,000 known minerals on our planet, but only a dozen or so minerals make up the bulk of Earth’s crust and mantle. Even more surprising is that 98.3 % of the crust by weight is composed of just eight elements (oxygen, silicon, aluminum, iron, calcium, sodium, potassium, and magnesium). All the remaining elements and their compounds therefore are actually quite rare in terms of the rocks we find near Earth’s surface, Fortunately, most of these less abundant elements are not evenly dispersed in trace amounts throughout the crust, but rather are often found concentrated in mineral deposits that are formed by geologic processes. Because it takes energy to extract earth materials, the concentration of a particular deposit is a major concentrated deposit means there is less waste material to remove, which in turn makes it more profitable since less energy is required to extract the resource. Geologists use the term enrichment factor to describe the degree to which a mineral resource is concentrated above its average concentration in the crust. A deposit may be deemed economical to mine if it reaches a certain concentration factor. Table 12.3 lists the average crustal composition of several important metals and the amount of enrichment typically required to create an economical deposit. Notice how an element such as gold can be mined profitably at a much lower concentration than aluminum. The different enrichment factors reflect both the value of these resources to society and the energy required to extract them.
Since society has developed applications for a relatively small number of Earth’s 4,000 or so minerals which are economically valuable. Although the term ore is often associated with metals, for our purposes we can define ore minerals as those which contain an element or compound that has some value to society. Likewise, an ore deposit will be defined as a body of rock or sediment whose concentration of ore minerals is sufficiently high so that it is economically feasible to extract. Note that the terms lowgrade and high-grade refer to the enrichment level of ore deposits. Later in this chapter we will examine how geologic processes cause crustal elements to become concentrated and form ore deposits.