The dark, solid substance known as coal is often confused with minerals, largely because it is extracted from the earth and appears similar to rock. Geologists, however, do not classify coal as a mineral, a distinction based on strict scientific criteria regarding composition and structure. This article will clarify the scientific requirements for mineral status and explain why coal is instead categorized as a type of rock.
The Five Strict Criteria of a Mineral
For any naturally occurring substance to be classified as a mineral, it must satisfy five specific criteria. First, it must be a naturally occurring solid, meaning it cannot be man-made or a liquid or gas. Second, it must have a definite chemical composition, allowing it to be represented by a specific formula.
Third, the substance must possess an ordered internal structure—a repeating, three-dimensional arrangement of atoms known as a crystalline lattice. The fourth criterion requires the substance to be inorganic, meaning it must not be derived from living organisms. Finally, the material must form through geological processes, such as crystallization from magma or precipitation from water.
Coal’s Origin The Organic Exclusion
Coal’s fundamental disqualification stems from its origin, as it fails the requirement to be inorganic. Coal forms from the accumulation of dead plant matter, primarily in ancient swampy forests, which was buried and compressed over millions of years. This organic material, initially peat, undergoes a process called coalification, where increasing heat and pressure drive off water, methane, and oxygen.
The resulting material is a sedimentary substance composed predominantly of organic carbon, hydrogen, and oxygen compounds. Since coal is derived directly from biological remains, this biological source fundamentally separates it from true minerals.
Coal’s Structure Lacking Crystalline Order
The second major reason for coal’s exclusion is its internal structure, as it fails the requirement for an ordered internal structure. True minerals exhibit a crystalline structure, where atoms are arranged in a highly organized, repeating geometric pattern. Coal, by contrast, is primarily an amorphous material, meaning it lacks this long-range, repeating atomic order.
Even high-rank coal, like anthracite, still consists of a complex, disordered mixture of organic molecules. While it may contain small, localized areas of ordered carbon clusters, the overall structure is non-uniform and random. This disordered arrangement contrasts sharply with the precise atomic lattice that defines a crystalline mineral.
How Geologists Classify Coal
Given its organic origin and lack of crystalline structure, geologists classify coal not as a mineral, but as an organic sedimentary rock. Rocks are defined as aggregates of one or more minerals, or consolidated material derived from organic matter, as is the case with coal. The organic building blocks of coal are called macerals, which are the organic equivalents of the minerals that make up inorganic rocks.
Some scientists also categorize coal as a mineraloid, a term used for naturally occurring, mineral-like substances that lack one or two of the five defining mineral criteria. Other mineraloids include materials like opal and volcanic glass, which lack a crystalline structure. This classification as an organic sedimentary rock or mineraloid allows scientists to accurately describe coal’s unique biological composition and disordered structure.