Final answer:
Coal is not classified as a mineral because it is organic in origin, does not have a definite chemical composition, and lacks a uniform crystalline structure.
Step-by-step explanation:
Coal is classified as a non-mineral substance. While coal is indeed a naturally occurring, solid substance derived from plants and is used as a fuel, it does not meet the criteria for a mineral because it is not inorganic.
Minerals are defined as naturally occurring, inorganic solids with a definite chemical composition and a characteristic crystalline structure.
Coal, however, is derived from organic plant materials that have undergone transformation over millions of years due to heat and pressure. Given that coal is composed of complex carbon molecules that are characteristic of once-living organisms, it does not fit the description of a mineral. Additionally, coal lacks a definite chemical composition and a uniform crystalline structure.