Chapter 5. Minerals

Introduction

Learning Objectives

After reading this chapter, completing the exercises within it, and answering the questions at the end, you should be able to:

  • Describe the nature of atoms and their constituents, particularly the behaviour of electrons and the formation of ions
  • Apply your understanding of atoms to explain bonding within minerals
  • Describe mineral lattices and explain how they influence mineral properties
  • Categorize minerals into groups based on their compositions
  • Describe a silica tetrahedron and the ways in which tetrahedra combine to make silicate minerals
  • Differentiate between ferromagnesian and other silicate minerals
  • Explain some of the mechanisms of mineral formation
  • Describe some of the important properties for identifying minerals

What Is a Mineral?

Minerals are all around us: the graphite in your pencil, the salt on your table, the plaster on your walls, and the trace amounts of gold in your computer. Minerals can be found in a wide variety of consumer products including paper, medicine, processed foods, cosmetics, and many more. And of course, everything made of metal is also derived from minerals.

The definition of mineral is very specific.  A mineral is a naturally occurring combination of specific elements arranged in a particular repeating three-dimensional structure.

“Naturally occurring” implies that minerals are not artificially made, although many naturally occurring minerals (e.g., diamond) are also made in laboratories. That doesn’t disqualify them from being minerals.

“Specific elements” means that most minerals have a specific chemical formula or composition. The mineral pyrite, for example, is FeS2 (two atoms of sulphur for each atom of iron), and any significant departure from that would make it a different mineral. But many minerals have variable compositions within a specific range. The mineral olivine, for example, can range all the way from Fe2SiO4 to Mg2SiO4. Intervening compositions are written as (Fe,Mg)2SiO4 meaning that Fe and Mg can be present in any proportion. This type of substitution is known as solid solution.

Most important of all, a mineral has a specific repeating three-dimensional structure or lattice, which is the way in which the atoms are arranged. This regular structure means that all minerals are classified as crystals. The mineral halite is shown as an example in Figure 5.1.

In halite atoms of sodium (Na, purple) alternate with atoms of chlorine (Cl, green) in all three dimensions, and the angles between the bonds are all 90°. Even in a tiny crystal, like the ones in your salt shaker, the lattices extend in all three directions for thousands of repetitions. Halite always has this composition and this structure. The structure of halite happens to be about the simplest mineral lattice of all. Most mineral lattices are much more complicated, as we’ll see.

The lattice structure and composition of the mineral halite (common table salt) [SE]

Figure 5.1 The lattice structure and composition of the mineral halite (common table salt) [Steven Earle CC-BY 4.0]

Note: Element symbols such as Na and Cl are used extensively in this book. In Appendix 1 you can find a list of the symbols, the names of the elements common in minerals, and a copy of the periodic table of elements.

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