What is a Coal Maceral?

A maceral is to coal what a mineral is to rock. Rocks are composed of minerals; coal is composed of macerals. However, minerals often have reasonably well defined chemical and physical characteristics.
Macerals can't quite be put into such neat little boxes. Here's a killer definition of macerals:

It takes awhile for this definition to sink in. Macerals are the remains of plants and degraded plant materials; they also have some characteristic chemistries and physical attributes. Unfortunately, these characteristics have to be more loosely defined than for minerals. In the coalification section some of the factors that affect the transformation of plant material to coal were given. One was how the type of plant influences the type
of peat. But there is a much finer point to this, which is at the heart of the maceral concept. Not only is the type of plant important, but the type of plant part and how nature modifies it is an important aspect of coal structure. For instance, spores, pollens, and resins often do not degrade as rapidly in a peat as
other organic materials, such as leaves or roots. They are often seen intact, but chemically changed in coal. Depending on microenvironmental conditions, some parts of a fallen log may degrade extensively in a swamp becoming gel-like, while other parts may retain their original cellular structure. All this can be seen in coals. It is these organic remains which we call macerals.

Now we're ready to get down to the heart of what coal is, and one of its big surprises. At the left is a microphotograph of a coal thin section taken in white light. The bar on the bottom right is ten micrometers long. Coal is not just a black featureless rock. In fact, most coal in the world, when cut thin enough, will show predominantly red colors. There certainly is material that is black though. There are also yellows and all the intermediate shades up to red. Sorry, there are no greens or blues (not when observing the coal in white light anyway). Want a hint to tell you if a coal will have a lot of red material? When you break it, does it have areas that have a very shiny texture? Those will be red under the microscope. By the way, the white areas in the picture are pores, or holes were coal was lost making the thin section.

Okay, so what do all these pretty different colors mean"? All the different colored materials are the macerals. Take a close look at the small, short, yellow "lines", blobs, or dumbbells in some of the photos. Those are coalified spores. In the coal thin section, do you see any long orange squiggly lines? Those are coalified plant cuticles. Cuticles are a plant's idea of skin. They prevent moisture loss and resist damage by microorganisms
to the underlying material. It is often found on the leaves of plants. The red material is the remains of the woody tissues of plants, such as the trunk and roots of trees. The black material can be two things:
either minerals or coalified charcoal. The type of material you see in the photos can be found in any coal chunk that you pick up. What will vary is the amounts and types of the different macerals.

Unfortunately, thin sections are not often used in analyzing the types and amounts of macerals in coal. It is very difficult to make thin sections of coal. Coal scientists need some way of rapidly preparing samples for routine examination. An easier technique is to work with polished sections of coal. The surface of many
coals can be polished until it resembles a black mirror. Then reflectance microscopy is used to examine the coal. This type of microscopy is commonly used with opaque samples. Unfortunately, pretty colors are not what you see. The coal is mostly shades of grey. Some macerals do show shades of red and orange, but this
is not in the same sense as with thin sections.