The question of whether coal qualifies as a mineral is a common point of confusion that separates the popular understanding of Earth materials from the scientific definition used by geologists. The definitive answer is that, under the strict criteria of Earth science, coal is not a true mineral. While it is a naturally occurring solid substance excavated from the ground, its origin and internal structure prevent it from being classified alongside quartz, calcite, or other true minerals. This distinction rests entirely on a set of precise, non-negotiable scientific requirements.
Defining a Mineral: The Five Key Criteria
To be officially classified as a mineral, a substance must meet five specific requirements established by geological science:
- It must be naturally occurring, meaning it is formed by natural geological processes and not created in a laboratory or factory.
- It must exist as a solid under normal surface conditions.
- It must be inorganic, meaning it cannot be derived from a living organism or the remains of an organism.
- It must possess a definite chemical composition, which means its chemical formula is fixed or varies only within a specified, predictable range.
- It must have an ordered internal structure, which translates to a crystalline lattice where atoms are arranged in a specific, repeating pattern.
The Organic Origin of Coal
Coal fails to meet two of these criteria, the most significant being the requirement for an inorganic origin. Coal is a biogenic material, meaning it is formed from the remains of ancient plant matter that accumulated in swampy environments millions of years ago. This process begins with peat, a spongy accumulation of partially decayed vegetation, which is then buried by sediment. As the peat is subjected to increasing amounts of pressure and heat over geologic time, it undergoes a transformation called coalification. Because coal originates from plants, it is considered an organic substance, which disqualifies it from the strict definition of a mineral.
Coal also fails the requirement for a definite chemical composition and an ordered internal structure. Its composition is not a fixed chemical formula but rather a complex mixture of carbon, hydrogen, oxygen, nitrogen, and sulfur atoms. The exact percentages of these elements vary significantly depending on the type, or rank, of the coal, such as lignite versus anthracite. This variability means coal lacks the consistent, predictable structure required of a true mineral.
Coal’s Official Classification
Since coal does not satisfy the requirements for a true mineral, geologists classify it differently. Coal is officially classified as a sedimentary rock, specifically a biochemical sedimentary rock. This classification is given to rocks that form from the accumulation and lithification of organic debris. The building blocks of coal are not minerals but organic molecules called macerals, which are the preserved remnants of plant tissues.
Because coal is a naturally occurring solid that fails only a few of the mineral criteria, it is also sometimes referred to as a mineraloid. A mineraloid is a natural solid that displays mineral-like properties but lacks either the required crystalline structure or the definite chemical composition. This dual classification as a biochemical sedimentary rock and a mineraloid reinforces the scientific distinction between coal and the earth’s true mineral species.