Good to Know Series – 03 – Macerals

Macerals – I now propose the new word “Macerals” (from the Latin, macerare, to macerate) as a distinctive and comprehensive word tallying with the word “mineral”…. It is now proposed to give to the individual “macerals” a distinctive set of descriptive names with the termination -inite, which is a parallel to the characteristic termination given to unit minerals in mineral rock sections. (Stopes, 1935)

In 1935, Marie Stopes introduced the term “macerals” to describe the microscopic constituents of coal. She also proposed several individual maceral names, some of which are still in use today (e.g., vitrinite, fusinite, cutinite), while others have fallen out of favor (e.g., xylinite, exinite, micronite). Since then, maceral classification has undergone numerous changes, leading to the development of different international and regional classification systems.

Without delving into exhaustive details, it is important to recognize the three primary maceral groups: Huminite/Vitrinite Group, Inertinite Group, and Liptinite Group. These groups are distinguished based on their reflectance, morphology, structure, and chemical composition.

Huminite/Vitrinite Group

This group originates from plant tissues rich in cellulose and lignin, such as roots, stems, bark, and leaves. Depending on the degree of gelification, the original plant cell walls may or may not be visible under the microscope. In coal samples, vitrinite exhibits intermediate reflectance, appearing lighter than liptinite but darker than inertinite. It also has a relatively high oxygen content compared to the other groups and is rich in aromatic structures.

The naming of this group depends on coal rank: in lignite to subbituminous coals, it is referred to as Huminite, while in bituminous to anthracite coals, it is called Vitrinite. Since vitrinite evolves consistently throughout the coalification process, its reflectance is one of the most reliable indicators of coal rank. Technologically, vitrinite macerals are the most reactive, making them critical for coke production, liquefaction, gasification, and combustion.

Inertinite Group

Inertinite macerals have diverse precursors, including fungal remains, detritus, amorphous gels, and oxidized plant tissues that might have originally contributed to vitrinite formation but underwent oxidation during peatification. In coal samples, inertinite macerals exhibit high reflectance, making them the brightest maceral group. Their high reflectance primarily reflects depositional conditions rather than the degree of coalification.

This group is carbon-rich and low in oxygen and hydrogen, making its macerals less reactive during coking processes. However, depending on their reflectance, inertinite can undergo significant transformations during combustion.

Liptinite Group

Liptinite macerals originate from non-humifiable plant constituents such as resins, waxes, and spores. They have a relatively high aliphatic compound content, resulting in a higher hydrogen content compared to the other two maceral groups. Liptinite macerals are easily identified by their low reflectance and fluorescence properties. As coal rank increases, liptinite macerals gradually lose fluorescence and increase reflectance, becoming indistinguishable from vitrinite at ~1.3% vitrinite reflectance.

However, in some cases, liptinite macerals can still be identified based on their morphology and reflectance, which may exceed that of vitrinite. In such cases, they are reclassified as inertinite, given their optical properties. On a personal note, I believe high-reflectance liptinites (meta-liptinites) are often overlooked. They can provide valuable paleoenvironmental insights and potentially explain localized high gas content in coals. Liptinite macerals are also significant because they produce high volumes of volatile matter and serve as primary precursors for liquid hydrocarbons.

Reids Dome Beds, Australia.

Further Reading & References

To keep this post concise, I will cover individual macerals in future blogs. If you are interested in detailed classification systems, you can refer to the following standards and publications (note that some may require purchase or institutional access):

International Committee for Coal and Organic Petrology (ICCP) publications (Recommended by ISO 7404-3:2009):

• Huminite Classification: ICCP Publication

• Vitrinite Classification: ICCP Publication

• Inertinite Classification: ICCP Publication

• Liptinite Classification: ICCP Publication

  Indiana Geological & Water Survey: ICCP Classification Overview (Note: Liptinite classification may not reflect the latest ICCP nomenclature.)

Australian Classification

• AS 2856.2 – 1998: Coal Petrography, Part 2: Maceral Analysis

Other Regional & International Resources

• U.S. Geological Survey Atlas: Maceral Classification

While multiple classification systems exist, it is important to recognize that coal is a globally traded resource, and regional classifications help facilitate its characterization and commercial applications.

References

Stopes, M.C., 1935. On the petrology of banded bituminous coal. Fuel 14, 4-13.