The K isotopic signatures of coals: A reconnaissance study

• Published in: International journal of coal geology Vol. 304; p. 104770
• Main Authors: Ji, Zhihan, Mu, Jun, Yao, Suping, Li, Weiqiang
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 08.05.2025
• Subjects: Coal; Illite; K isotopes; Low temperature ashing; Organic matter; Pore fluids; Stepwise leaching; Organic matter; Coal; Low temperature ashing; Illite; K isotopes; Pore fluids; Stepwise leaching
• ISSN: 0166-5162
• DOI: 10.1016/j.coal.2025.104770

Potassium (K) isotopes are significantly fractionated in sediments, with biological samples exhibiting even greater K isotopic variability than geological samples. Coal is a unique type of sedimentary rock composed of organic matter and minerals, both of which are K-rich. Therefore, K isotopes hold the potential for tracing the origin of coal and understanding coal-forming processes. However, our knowledge of K isotopes in coals is limited due to the scarcity of K isotopic studies on coals. In this study, we conducted the first detailed investigation of K isotopes in 12 coal samples of varying ranks by analyzing the abundance and isotopic compositions of K in different phases. We ashed the coals and sequentially leached K from the ashes with deionized water (DW), ammonium acetate (AA), HCl, and HF, followed by elemental and isotopic analyses. The majority of K occurs in DW-leached and HF-leached fractions of these coal samples, representing the K associated with organic matter and the K that exists in the silicate phase, respectively. The bulk K content of the twelve studied coals ranges from 0.004 to 1.308 wt%, with the HF-leached K holding the dominant K budget. The δ41K values vary from −1.19 ‰ to 0.52 ‰ in the DW-leached phase, and from −1.27 ‰ to −0.22 ‰ in the HF-leached phase of the low temperature coal ashes. The large variations in δ41K values of DW leachate are attributed to the plants decay and diffusion in pore fluids in the early stage of diagenesis. The K isotope compositions of the HF-leached K in coals are variable and lighter than average igneous rocks (δ41KBSE = −0.43 ‰). Pore fluid exchanges likely caused the preference for 39K in the silicate phase of coal. The K isotope compositions of coals overall reflect 39K enrichment during diagenesis or organic matter inheritance.