Good to Know Series - 01 - What is coal and how is it formed?

I recently gave a short talk for a general audience, where I asked if they knew how coal is formed. Lo and behold, no one knew how to answer — or perhaps they were just too shy to respond. If you’ve read my blog episode (Things About Me Series - 01), you’ll know that sometimes, you simply stumble upon coal or don’t even realise its presence in our daily lives, hidden in plain sight. Either way, I thought this would be a great way to kick off the “Good to Know Series”: What is coal, and how is it formed?

What is coal?

By definition, coal is a sedimentary organic-rich rock composed of macerated plant remains. Let’s break that down.

·       It is a sedimentary rock because it forms from the deposition of sediments. In the case of coal, these sediments are particulate organic matter.

·       Although minerals are also part of coal, more than 50% of its weight or 70% of its volume must be organic matter— otherwise, it’s classified as a carbonaceous mudstone.

·       The term macerated comes from the Latin verb macerare, meaning “to soften.” This refers to the first process organic matter undergoes during the peat phase.

Before coal becomes coal, it starts as peat. Plant remains accumulate in mires (wetland environments) and partially decompose through biochemical processes aided by bacteria, forming peat. Over time, this peat becomes buried by other sediments such as sand and silt.

As burial depth increases, so do temperature and pressure. Gradually — on a geological time scale — the peat transforms into coal through geochemical processes. And that’s how coal is formed.

If you want to dive deeper into this topic, I highly recommend the Kentucky Geological Survey website (https://www.uky.edu/KGS/coal/index.php) — it’s clear, concise, and accessible to everyone and the figure are awesome.

For more technical readings, my go-to references include:

·       Chapter 2 from Taylor et al. (1998) Organic Petrology — my personal bible.

·       Teichmüller and Teichmüller (1979) Diagenesis of Coal — though much of this content has been incorporated into Taylor’s book.

·       A recent open-access paper: Dai et al. (2020) Recognition of peat depositional environments in coal: A review (https://www.sciencedirect.com/science/article/pii/S0166516219312236) — a comprehensive look at coal formation environments and processes.

Whether you're new to the world of coal or looking to refresh your knowledge, these resources offer valuable insights. Stay tuned for more posts in the “Good to Know Series” as we continue exploring the science behind carbon materials!

If you have any questions or comments, feel free to email me at Sandra.Rodrigues@carbonmat.com.au