ABSTRACT
Minerals are naturally occurring inorganic materials. They have crystalline structures which are not always externally evident, but their regular internal structure and constancy of interfacial angles can be shown using X-ray crystallography techniques. Some minerals show polymorphism, that is, they have the same chemical formula but different crystalline structures, e.g. silicon dioxide, SiO2, can exist as quartz or cristobalite, and carbon can occur as graphite (Figure 7.1) and diamond. Minerals can be elements, e.g. gold, Au, but most are found as chemical compounds. Minerals have either a fixed chemical composition or chemical formulae which vary between limits, e.g. olivine varies from Fe2SiO4 via (Fe,Mg)2SiO4 to Mg2SiO4.
