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Coal Through a Microscope
It's far more than a black rock!
Article by: Hobart M. King, PhD, RPG
If you think that coal is a boring black rock, then you have never seen it through a transmitted light microscope. The microscope reveals coal's hidden beauty as well as its organic composition.
Coal seams form from thick accumulations of plant debris, usually deposited in a swamp. The tiny particles of plant debris and swamp sediment give a spectacular show of color when viewed through the microscope. Well-preserved woody material is bright red, spores are brilliant yellow, algal material is yellow-orange, charcoal and opaque minerals are black, and grains of many transparent minerals are white. It's hard to believe that coal can be so colorful!
Let's Start With a Hand Specimen
To best understand coal we should start with a hand specimen of coal like the one shown below. If we look closely we will see that it is not a uniform black. Instead there are different shades of black and also different lusters.
The coal shown below is a "bright banded" coal. It is dominated by bright shiny bands which represent well-preserved woody material. Between those bright bands are some thin dull bands. Those dull bands are composed of substances such as mineral material, degraded woody material, and charcoal. Charcoal? Yes, some coals contain lots of charcoal. There were fires in the swamps just like there are fires in the Everglades.
Thin Sections
"Transmitted light" means that light is passing through the coal, just like light passes through a stained glass window. This requires a very thin slice of coal called a "thin section." The slice must be so thin that light easily passes through it. Here's how a thin section is made.
First a small block of coal is cut with a saw to form a flat surface. The surface is then ground and polished until it is exceptionally smooth. That flat surface is then glued to a microscope slide.
The block is then cut again - parallel to the face of the microscope slide. This yields a thin slice of coal glued to the glass slide. It's not thin enough yet, so the thin slice of coal is ground thinner and thinner. The craftsman grinds a little and checks the thinness, grinds a little more and checks the thinness. When it is almost the right thinness, grinding is stopped and the slice of the coal is polished to remove the tiny scratches of sawing and grinding. A perfectly smooth surface is needed. If scratches are not removed, a uniform passage of light through the thin section will not occur. (Note the few tiny scratches visible in the thin section of humic coal. It has a very uniform thinness.)
Many Types of Coal
As mentioned above, coal forms from plant debris that accumulates in an environment such as a swamp. There are many possible situations of plant types and environmental conditions that produce a variety of coal types.
Within a swamp some areas might be shallow and other areas deep. Some areas might have woody plants and other areas grassy. The environment might be changing over time, making the bottom (older part) of the coal seam very different from the top.
These variations result in the formation of many different types of coal - all within the same coal seam.
You have seen how plant types and environmental conditions can produce a variety of types of coal within a single swamp. Now imagine two different coal swamps, in different parts of the world and at different points in geologic time. They might have coals of even greater diversity.
Coal is a very complex rock, and that complexity is part of what makes it so interesting.
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