Coal is not considered a mineral because it does not have a solid, uniform crystal structure, meaning it doesn't fit the primary definition of a mineral. Coal is primarily made of carbon, but its properties can vary even within the same deposit.
There are four major types of coal found in the world: anthracite, bituminous, sub-bituminous and lignite. Of the four, anthracite has the highest energy content and is also the rarest. Lignite has the lowest energy content and is the most basic type of coal, which forms when peat moss is buried and begins to dry and harden.
Sub-bituminous coal occurs when lignite is exposed to a greater amount of heat and pressure, creating a slightly harder coal with a dull, blackish color. When the pressure rises even more, the sub-bituminous coal can be turned into bituminous coal. Commonly known as soft coal, this is the type of coal most commonly used to generate electricity in the United States.
Anthracite only occurs in certain geographical areas, as it can only be formed when bituminous coal is subjected to extreme pressures during the creation of mountain ranges. In the United States, the only significant deposit of anthracite is in the Appalachian mountains of Pennsylvania.