Why is coal an unusual sedimentary rock?

Answer 1

First of all coal is not an unusual sedimentary rock, it is very abundant for the time being. The following is a tutorial from the supplied web site:

Coal is clearly made up of lots of compressed bits of dead plants. But when we walk around in the world we usually see old bits of wood and leaves decaying and rotting away (that is, getting eaten by bacteria, insects, and all sorts of organisms involved in decay and recycling of organic matter). For thick layers of plant stuff to get fossilized as coal, something must be going on.

Coal Formation starts with accumulation of organic matter (bits of dead plants) in a low oxygen setting such as a peat bog. The organic matter accumulates and forms a bed of peat. The peat bed gets buried by other sediments and under heat and pressure begins to transform to a low grade coal - a Lignite. More heat and pressure further metamorphose the lignite into Bituminous coal. Even more heat and pressure metamorphose the bituminous coal into a nice hard shiny Anthracite.

Coal is usually classified into three grades: Lignite, brown coal; Bituminous coal, soft coal; and Anthracite, hard coal. Anthracite is dense, nice and hard, and shiny.

The first step in the formation of coal is the accumulation of plant debris in low oxygen conditions, such as in this damp low spot on a moor. Peat exposed to heat and pressure from burial beneath other sediments becomes compressed and chemicaly changes into low grade coals such as this lignite, and under further heat and pressure is converted to higher grade coals. The pressure from overlying sediments that bury a peat bed will compact the coal. Peats transform to low grade lignites when they are compressed to about 20% of their original thickness. Lignite typicaly transforms to bituminous coal as it is compressed further and heated to between 100 and 200°C. This drives much of the water and other volitiles from the coal. Longer exposure to elevated temperature will further drive volatiles from the coal, and drive chemical reactions that produce anthracite. Anthracite coals are typicaly compressed to 5-10% of the orginal thickness of the peat bed, and contain less than 10% water and other volatiles.

Answer 2

Coal is unusual because it is composed mostly of organic material, primarily plant matter that has been compressed and altered over hundreds of millions of years. Unlike most sedimentary rocks that are formed from mineral deposits, coal is a fossil fuel that forms in swampy environments where plant material is abundant and can accumulate without decomposing.