Like petroleum, coal is an enormously valuable mineral resource, used for hundreds of years in industry. And also like petroleum, the Web has lots of information about producing it, using it, buying and selling it and dealing with its wastes, but little about what the stuff actually is. But every rock is a story, and here's the story that coal tells.
Coal, the Organic Rock
Coal differs from every other kind of rock in that it is made of organic carbon: the actual remains, not just mineralized fossils, of dead plants. Today the vast majority of dead plant matter is consumed by fire and decay, returning its carbon to the atmosphere as the gas carbon dioxide—it is oxidized. The carbon in coal, however, was preserved from oxidation and remains in a chemically reduced form, available for oxidation.
Coal geologists study coal just the way other geologists study other rocks. But instead of talking about the minerals that make up the rock (because there are none, just bits of organic matter), coal geologists refer to the macerals, of which there are three groups: inertinite, liptinite, and vitrinite. To oversimplify a complex subject, inertinite is generally derived from plant tissues, liptinite from pollen and resins, and vitrinite from humus or broken-down plant matter.
Where Coal Formed
The old saying in geology is that the present is the key to the past. Today we can find plant matter being preserved in anoxic places, without oxygen: peat bogs like those of Ireland or wetlands like the Everglades of Florida. And sure enough, fossil leaves and wood are found in some coal beds. Therefore geologists have long assumed that coal is a fossilized form of peat created by the heat and pressure of deep burial. But coal beds are much, much larger than peat bogs, some of them tens of meters in thickness, and they occur all over the world. This says that the ancient world must have had enormous and long-lived anoxic wetlands when the coal was being made. What do we know about the geologic history of coal?
While coal has been reported in rocks as old as Proterozoic (possibly 2 billion years) and as young as Pliocene (2 million years old), the great majority of the world's coal was laid down during the Carboniferous Period, a 60-million-year stretch (359-299 m.y.a.) when sea level was high and forests of tall ferns and cycads grew in gigantic tropical swamps.
The key to preserving the forests' dead matter was burying it. We can tell what happened from the rocks that enclose the coal beds: there are limestones and shales on top, laid down in shallow seas, and sandstones beneath, laid down by river deltas.
Obviously the coal swamps were flooded by advances of the sea, which allowed shale and limestone to be deposited on top of them. The fossils in the shale and limestone change from shallow-water organisms to deep-water species, then back to shallow forms. Then sandstones appear as river deltas advance into the shallow seas and another coal bed is laid down on top. This cycle of rock types is called a cyclothem. Hundreds of cyclothems occur in the rock sequence of the Carboniferous. Only one cause can do that—a long series of ice ages raising and lowering the sea level. And sure enough, in the region that was at the south pole during that time, the rock record shows abundant evidence of glaciers.
That set of circumstances has never recurred, and the coals of the Carboniferous (and the following Permian Period) are the undisputed champions of their type. It has been argued that about 300 million years ago, some fungus species evolved the ability to digest wood, and that was the end of the great age of coal, although younger coal beds do exist. A genome study in Science gave that theory more support in 2012. If wood was immune to rot before 300 million years ago, then perhaps anoxic conditions were not always necessary.
Grades of Coal
Coal comes in three main types, or grades. First the swampy peat is squeezed and heated to form a brown, soft coal called lignite. In the process, the material releases hydrocarbons, which migrate away and eventually become petroleum. With more heat and pressure lignite releases more hydrocarbons and becomes the higher-grade bituminous coal. Bituminous coal is black, hard and usually dull to glossy in appearance. Still greater heat and pressure yields anthracite, the highest grade of coal. In the process, the coal releases methane or natural gas. Anthracite, a shiny, hard black stone, is nearly pure carbon and burns with great heat and little smoke.
If coal is subjected to still more heat and pressure, it becomes a metamorphic rock as the macerals finally crystallize into a true mineral, graphite. This slippery mineral still burns, but it is much more useful as a lubricant, an ingredient in pencils and other roles. Still more valuable is the fate of deeply buried carbon, which at conditions found in the mantle is transformed into a new crystalline form: diamond. However, coal probably oxidizes long before it can get into the mantle, so only Superman could perform that trick.