The Rock - COAL

  • Sedimentary Rock Type:  Biochemical
  • Related to:  Shale, sandstone and limestone
  • Color:  Generally black to dark brown or gray
  • Texture:  Amorphous and glassy to coarse fragments
  • Origins:  Swampy environments
  • Common Minerals:  Graphite , pyrite and jet (a mineraloid
  • Uses:  Source of energy, lubricant, pigment and source of carbon for industrial purposes

Coal is a sedimentary rock of biochemical origin. It forms from accumulations of organic matter, likely along the edges of shallow seas and lakes or rivers. Flat swampy areas that are episodically flooded are the best candidates for coal formation. During non-flooding periods of time, thick accumulations of dead plant material pile up. As the water levels rise, the organic debris is covered by water, sand and soils. The water (often salty), sand and soils can prevent the decay and transport of the organic debris. If left alone, the buried organic debris begins to go through the coal series as more and more sand and silt accumulates above it. The compressed and/or heated organic debris begins driving off volatiles, leaving primarily carbon behind. The sand and soils form the rocks sandstone, shale, and if the soils are limey, limestone.

Great deposits of coal, sandstone, shale and limestone are often found together in sequences hundreds of feet thick. The key to large productive coal beds or seams seems to be long periods of time of organic accumulation over a large flat region, followed by a rapid inundation of sand or soil, and with this sequence repeating as often as possible. The best time for this to have happened in the past has been during the Carboniferous Period, named for the large world wide occurrences of carboniferous (coal) beds. This period is recognized in the USA as the Mississippian and Pennsylvanian time periods due to the significant sequences of these rocks found in those states. Coal is mined in these states and in many others that have rocks of this age found in them. Other good coal bearing ages are the Cretaceous, Triassic and Jurassic Periods. The more recently aged rocks are not as productive for some reason, but lignite and peat are common in younger deposits. The older the deposit, the better the grade of coal (in general). 

Some people consider coal to be a metamorphic rock, the result of heat and pressure on organic sediments such as peat. But most sedimentary rocks undergo some heat and pressure, and coal's intimate association with "normal" sedimentary rocks and its mode of formation usually keep low grade coal in the sedimentary classification system. Anthracite, on the other hand, undergoes more heat and pressure and is associated with low grade metamorphic rocks such as slate, quartzite and low grade marbles. Eventually enough heat and pressure can be applied and the carbon converts to graphite. Subducted coal may become graphite in igneous rocks or even the carbonate rich rock called carbonatite.

The degree of processing results in differing qualities of coal:

PEAT - is not actually a rock yet, but no longer just organic matter, either. Peat is a major source of energy for many non-industrialized people of the world. The not-quite-consolidated plant matter is a precursor of true coals which is what it would have become had the material been left buried for a few million years more.

LIGNITE (or brown coal) -  is the least mature of the true coals and the most impure. It provides the least yield of energy of the true coals and burns the dirtiest. It is often crumbly, relatively moist and powdery.

SUB-BITUMINOUS - is still poorly indurated and brownish in color, but more like bituminous than lignite.

BITUMINOUS - is the coal most people are used to. The black, soft, slick rock is the most common coal used around the world.

ANTHRACITE - is usually considered to be the highest grade of coal and is actually considered to be metamorphic. Compared to other coals it is much harder, has a glassy luster, and is denser and blacker with few impurities. It is largely used for heating domestically as it burns with little smoke.

GRAPHITE - is essentially pure carbon and would actually be a higher grade of coal than anthracite, but is difficult to ignite and is rarely used for this purpose. Graphite forms from intense metamorphism and even igneous processing.

JET - is the gem variety of coal. Generally derived from anthracite, jet is where the phrase "jet black" comes from. Because it lacks a crystalline structure, jet is sometimes treated asa mineraloid.

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