Mineralogy and geochemistry of ammonian illite in intra-seam partings in Permo-Carboniferous coal of the Qinshui Coalfield, North China

Ammonian illite is observed to be presented in the intra-seam coal partings of the Permo-Carboniferous coal seams from the Qinshui Coalfield, North China. This paper provides new insights into its geochemical and mineralogical characteristics, as well as the factors influencing its formation and its nitrogen isotope ratios. The interlayer cations of ammonian illite in the samples presented in this study were dominated by NH4+ with a certain amount of K+ and distributed homogeneously in each illite layer. Ammonian illite has an NH4+/(NH4+ + K+) ratio of 0.87 on average, with a basal spacing (d001, 10.267 Å on average) greater than those of potassian illite and muscovite, but lower than that of tobelite. The average stoichiometric formula of the ammonian illite was inferred as (NH40.67,K0.11)(Al1.90,Fe0.06,Mg0.04)(Al0.68,Si3.32)O10(OH)2, and the average Si/AlIV ratio (4.88) was found to be higher than that of tobelite. This indicates that the conversion of ammonian illite to tobelite includes the expulsion of Si from the tetrahedral sheets and the incorporation of Al into the octahedral and tetrahedral sheets. The d005 is influenced not only by the NH4+/(NH4+ + K+) ratio but also, to some extent, by the entrance of AlIV into tetrahedral sites. It is inferred that the ammonian illite was formed by the incorporation of Si into pre-existing kaolinite during diagenesis, and that its formation was influenced not only by the diagenetic temperature but also by the depositional environment. Brackish water favors the formation of ammonian illite during deposition. The NH4+ of the ammonian illite was mainly derived from pyrrolic (N-5) and pyridinic (N-6) nitrogen groups at the margins of the coal's carbon matrix during diagenesis. On average, it has a δ15N value of + 8.0‰, much higher than that of total coal nitrogen (+ 3.9‰ on average). This is related mainly to the higher δ15N values of N-5 and N-6 than the quaternary nitrogen group (N-Q) within the carbon matrix.