We begin our discussion of Earth materials with an overview of mineralogy because minerals are the building blocks of rocks. Humans have used minerals for both practical and decorative purposes for thousands of years. Consider several examples. In the Neolithic period, more than 6000 years before present, flint and chert were mined and traded, and humans fashioned the minerals 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, a process was developed to extract iron from minerals, such as hematite—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.

Defining a Mineral

In everyday conversation, 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, sand, and gravel. What criteria do geologists use to determine whether something is a mineral ▼?

Geologists define a 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:

1. Naturally occurring. Minerals form by natural geologic processes. (Synthetic materials, meaning those produced in a laboratory or by human intervention, are not considered minerals.)

2. Generally inorganic. Inorganic crystalline solids, such as ordinary table salt (halite), that are found naturally in the ground are considered minerals. (Sugar, a crystalline solid like salt but extracted from sugarcane or sugar beets, is an organic compound and, thus, not a mineral.) Many marine animals secrete inorganic compounds, such as calcium carbonate (calcite), in the form of shells and coral reefs. If such inorganic materials are buried and become part of a rock, geologists consider them minerals.

3. Solid substance. Only solid crystalline substances are considered minerals. Ice (frozen water) fits this criterion and is considered a mineral. (Liquid water and water vapor do not fit the criterion.)

4. Orderly crystalline structure. Minerals are crystalline substances made up of atoms (or ions) that are arranged in an orderly, repetitive manner ▼. This orderly packing of atoms is reflected in regularly shaped objects called crystals. (Some naturally occurring solids, such as volcanic glass (obsidian), lack a repetitive atomic structure and are not considered minerals.)

5. Definite chemical composition that allows for some variation. Most 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 can 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.

What Is a Rock?

In contrast to minerals, rocks are more loosely defined. Simply, a rock is any solid mass of mineral or mineral-like matter that occurs naturally as part of our planet. Most rocks, like the sample of granite shown in ▼, are aggregates of several different minerals. The term aggregate implies that the minerals are joined in such a way that their individual properties are retained. Note that the different minerals that make up granite can be easily identified. However, some rocks are composed almost entirely of one mineral. A common example is the sedimentary rock limestone, which is an impure mass of the mineral calcite. In addition, some rocks are composed of nonmineral matter. These include the volcanic rocks obsidian and pumice, which are noncrystalline glassy substances, and coal, which consists of solid organic debris.

Although this chapter deals primarily with the nature of minerals, keep in mind that most rocks are simply aggregates of minerals. Because the properties of rocks are determined largely by the chemical composition and crystalline structure of the minerals contained within them, we will first consider these Earth materials before discussing rocks later this semester.

• In Earth science, the word mineral refers to naturally occurring inorganic solids that possess an orderly crystalline structure and a characteristic chemical composition. The study of minerals is called mineralogy.

• Minerals are the building blocks of rocks. Rocks are naturally occurring masses of minerals or mineral-like matter, such as natural glass or organic material.

• mineral: A naturally occurring, inorganic solid that possesses an orderly crystalline structure and a definite chemical composition that allows for some variation.

• mineralogy: The scientific study of minerals.

• rock: A naturally-occurring solid mass of mineral or mineral-like matter.