In this chapter, we introduce various fundamental aspects of the architecture of crystal structures of minerals. We begin with their building blocks, which are the naturally occurring chemical elements, be they atoms or ions. We then discuss the size of the atoms and ions, and we point out that the size of ions is influenced by the number of an ion's closest neighbors. This leads to the concepts of coordination number, C.N., and coordination polyhedron. We then discuss the five rules, known as Pauling's rules, that relate to how polyhedra stack together to form a crystal structure. We also address the forces that hold crystal structures together, known as bonding forces. Finally, we address the various factors that influence the variability in the chemical composition of many mineral groups.
Crystal structures represent the most fundamental aspects of minerals. In classifications, minerals are organized by chemical groups (see Sec. 2.2) as well as structure types (see Sec. 7.4). Because there are about 1140 known silicates, this large chemical group is subdivided by structure type.
Many of the minerals treated in this text are silicates, and gaining an understanding of their various structure types is essential. This text is accompanied by CrystalViewer, which incorporates all the crystal structures of the minerals that are systematically treated in Chapters 7, 11, 14, and 16. Furthermore, each systematic mineral description is accompanied by an illustration of its structure, its mineral name, its chemical formula, and a shorthand notation known as the space group. This chapter provides you with the basic aspects of crystal structures, which will provide you with a good understanding of the crystal structures both in CrystalViewer and in the crystal structure illustrations in this text.
Naturally Occurring Chemical Elements
In Section 2.1, we introduced the definition of mineral in which one of the clauses states with a definite, but commonly not fixed, chemical composition. That led to the concept of minerals with a fixed composition such as quartz, SiO2, and minerals with a variable composition, such as members of the olivine series, ranging from Mg2SiO4 to Fe2SiO4, with the general formula (Mg,Fe)2SiO4.
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