Good to Know Series – 05 – Minerals in Coal
Alongside macerals (organic constituents), minerals are also an integral part of coal composition. Determining the amount, type, and distribution of mineral matter is essential for coal quality control, understanding mineral liberation during beneficiation, and assessing the presence of valuable elements like lithium and rare earth elements (REEs).
Main mineral groups commonly found in coal include:
Silicates – e.g., quartz, clays, feldspar
Carbonates – e.g., siderite, calcite, ankerite
Sulphides – e.g., pyrite, galena, chalcopyrite
Phosphates – e.g., apatite, crandallite, monazite
Sulphates – e.g., jarosite, barite, gypsum
Oxalates – e.g., weddellite, whewellite
Others – e.g., anatase, rutile, boehmite, diaspore.
Among these, clays—especially kaolinite—are typically the most abundant, while other minerals tend to occur in smaller quantities. I recommend looking at Professor Colin Ward’s papers (reference below) for a comprehensive list of minerals found in coal.
Modes of Mineral Occurrence in Coal
Minerals in coal originate through different geological processes, often reflected in their modes of occurrence. Broadly, they can be classified as syn-depositional or authigenic.
Syn-depositional minerals
These form during peat accumulation and include:
Biogenic materials: diatoms, sponge spicules, phytoliths from plant tissues, faecal pellets
Detrital minerals: quartz, clays, volcanic ash-derived minerals transported into the mire by water or wind.
Note: While biogenic minerals are common in peat, they are rarely preserved in coal.
Detrital minerals may appear as intercalated layers or lenses, or be finely dispersed within the coal matrix. Their presence can offer insights into paleoenvironmental conditions and sediment provenance. Volcanic ash layers can be used as chronostratigraphic markers useful for seam correlation.
Authigenic minerals
These are formed in place and include:
Syngenetic (early diagenetic or primary) minerals – precipitated during the peat stage or early coalification (e.g., framboidal pyrite, siderite nodules). Often found dispersed in the coal matrix.
Epigenetic (late diagenetic or secundary) minerals – introduced during later stages by fluid migration. These typically fill cleats, fractures, or pore spaces, forming monomineralic or polymineralic assemblages. Differentiating between late syngenetic and early epigenetic minerals can be challenging, especially when both occur in similar microenvironments.
Fractures in coal may record multiple mineralisation episodes, and epigenetic mineral cements can reduce permeability, impacting gas migration and storage.
Why Minerals Matter
Understanding the mineral content and its relationship to the organic matter is essential for:
Paleoenvironmental reconstruction
Basin analysis
Coal beneficiation strategies
Coal quality assessment
Permeability evaluation (especially for gas-related applications)
Critical element recovery
Know your minerals. Know how they occur.
It makes a difference in both science and industry.
References
Ward, C.R., 2016. Analysis, origin and significance of mineral matter in coal: An updated review. International Journal of Coal Geology 165, 1-27.
https://www.uky.edu/KGS/coal/coal-mineral-matter.php
https://www.tsop.org/coal_minerals.html
Sun, B., Zeng, F., Moore, T.A., Rodrigues, S., Liu, C., Wang, G., 2022. Geochemistry of two high-lithium content coal seams, Shanxi Province, China. International Journal of Coal Geology 260, 104059.
