The vast majority of Earth’s crust is composed of rocks, which are essentially aggregates of one or more minerals. Minerals are the fundamental building blocks of the solid Earth. Understanding the distinction between a mineral and a rock is foundational to geology, defining the composition and structure of materials that make up mountains, continents, and ocean floors. This relationship allows geologists to classify and understand the formation processes of different rock types.
The Defining Characteristics of a Mineral
A mineral is defined by specific criteria. First, it must be naturally occurring, formed by geologic processes rather than being produced in a lab or factory. Second, it must be an inorganic solid, excluding liquids, gases, and materials derived from living organisms, such as wood or shells.
Third, a mineral must possess a definite chemical composition, expressed by a specific chemical formula (e.g., quartz is silicon dioxide). Fourth, it must have an ordered internal structure, also known as a crystalline structure.
This crystalline structure means the atoms are arranged in a precise, repeating, three-dimensional pattern. This internal order often causes minerals to display characteristic crystal shapes when they have space to grow. Without this specific atomic arrangement, the substance does not qualify as a mineral.
The Relationship Between Minerals and Rocks
Rocks are a cohesive mass of one or more minerals, or mineral-like matter, held together. Unlike minerals, rocks do not have a fixed chemical formula; their composition is variable, reflecting the mixture of the various mineral grains they contain. For example, granite is an aggregate typically composed of several distinct minerals, including quartz, feldspar, and mica.
Some rocks are monomineralic, meaning they are almost entirely made up of a single mineral species. Quartzite, a metamorphic rock, is an example, often consisting of 90 percent or more quartz grains that have been cemented and recrystallized under pressure. The way these mineral grains are formed and bound together dictates the ultimate rock type, which falls into one of three main categories: igneous, sedimentary, or metamorphic.
Igneous Rocks
Igneous rocks, such as basalt, form from the cooling and solidification of molten material, where minerals crystallize out of the melt.
Sedimentary Rocks
Sedimentary rocks, like sandstone, are formed when mineral fragments and other materials are cemented together.
Metamorphic Rocks
Metamorphic rocks, such as marble, result when existing mineral aggregates are transformed by intense heat and pressure, causing the minerals to recrystallize or align in new patterns.
Notable Exceptions to the Mineral Rule
Although most rocks are mineral aggregates, a few exceptions exist that are still classified as rocks. These non-mineral rocks, or “mineraloids,” violate one of the defining mineral criteria. The most common exception is obsidian, a type of volcanic glass.
Obsidian forms when lava cools so rapidly that atoms cannot arrange themselves into a crystalline structure. Because it lacks the ordered internal structure, obsidian is an amorphous solid, violating the crystalline criterion. Another exception is coal, which is classified as a rock but is derived from compressed plant matter. Since minerals must be inorganic, coal’s organic origin excludes it from being a mineral aggregate.