The mineralogy of the individual coal plies and intra-seam claystone bands in the Great Northern seam of the Sydney Basin has been evaluated using optical and scanning electron microscopy, and quantitative X-ray diffraction techniques. The uppermost two claystone bands are tonsteins, consisting mainly of well-ordered kaolinite with graupen to vermicular textures. Idiomorphic crystals of K-feldspar within these tonsteins may represent members of the anorthoclase-sanidine series or a sodic sanidine, and indicate an acid to intermediate volcanic ash input. In contrast, the lowermost parting was largely derived from epiclastic sediment, admixed with minor volcanic material such as high-temperature quartz and a different type of K-feldspar component. The mineral fraction of the coals, especially in the middle and upper parts of the seam, is dominated by authigenic kaolinite with a very low abundance of quartz. Apart from the tonsteins, authigenic processes therefore appear to be the dominant mechanism of mineral matter formation. Authigenic K-feldspar also occurs in the lower few metres of the seam, with a variety of modes of occurrence including cell and cleat innings, cross-cutting veins, and thin laminae parallel to the organic matter and detrital clay bands. A late syngenetic low-temperature hydrothermal fluid injection process is suggested for formation of the feldspar veins. The origin of the fluid is uncertain, but is most likely associated with contemporaneous volcanic activity. (C) 2011 Elsevier B.V. All rights reserved.