Minerals: Building Blocks of Rocks
We begin our discussion of Earth materials with an overview of mineralogy (mineral 5 mineral, ology 5 study of) because minerals are the building blocks of rocks. In addition, humans have used minerals for both practical and decorative purposes for thousands of years. For example, the common mineral quartz is the source of silicon for computer chips (Figure 1).
The first minerals mined were flint and chert, which people fashioned into weapons and cutting tools. As early as 3700 b.c.e., Egyptians began mining gold, silver, and copper. By 2200 b.c.e., humans had discovered how to combine copper with tin to make bronze—a strong, hard alloy. Later, humans developed a process to extract iron from minerals such as hematit —a discovery that marked the decline of the Bronze Age.
During the Middle Ages, mining of a variety of minerals became common, and the impetus for the formal study of minerals was in place.
The term mineral is used in several different ways.
For example, those concerned with health and fitness extol the benefits of vitamins and minerals. The mining industry typically uses the word mineral to refer to anything extracted from Earth, such as coal, iron ore, or sand and gravel. The guessing game Twenty Questions usually begins with the question Is it animal, vegetable, or mineral? What criteria do geologists use to determine whether something is a mineral?
Defining a Mineral
Geologists define mineral as any naturally occurring inorganic solid that possesses an orderly crystalline structure and a definite chemical composition that allows for some variation. Thus, Earth materials that are classified as minerals exhibit the following characteristics:
- Naturally occurring. Minerals form by natural geologic processes. Synthetic materials, meaning those produced in a laboratory or by human intervention, are not considered minerals.
- Generally inorganic. Inorganic crystalline solids, such as ordinary table salt (halite), that are found naturally in the ground are considered minerals. (Organic compounds, on the other hand, are generally not. Sugar, a crystalline solid like salt but extracted from sugarcane or sugar beets, is a common example of such an organic compound.) Many marine animals secrete inorganic compounds, such as calcium carbonate (calcite), in the form of shells and coral reefs.
If these materials are buried and become part of the rock record, geologists consider them minerals.
- Solid substance. Only solid crystalline substances are considered minerals. Ice (frozen water) fits this criterion and is considered a mineral, whereas liquid water and water vapor do not. The exception is mercury, which is found in its liquid form in nature.
- Orderly crystalline structure. Minerals are crystalline substances, made up of atoms (or ions) that are arranged in an orderly, repetitive manner (Figure 2).
This orderly packing of atoms is reflected in the regularly shaped objects called crystals. Some naturally occurring solids, such as volcanic glass (obsidian), lack a repetitive atomic structure and are not considered minerals.
- Definite chemical composition that allows for some variation. Minerals are chemical compounds having compositions that can be expressed by a chemical formula. For example, the common mineral quartz has the formula SiO2, which indicates that quartz consists of silicon (Si) and oxygen (O) atoms in a 1:2 ratio.
This proportion of silicon to oxygen is true for any sample of pure quartz, regardless of its origin. However, the compositions of some minerals vary within specific, well-defined limits. This occurs because certain elements can substitute for others of similar size without changing the mineral’s internal structure.