Minerals in bituminous coals of the Sydney basin (Australia) and the Illinois basin (U.S.A.)

Abstract

Although other forms of inorganic components may be significant in lower-rank coals, the mineral matter in most bituminous coals is dominated by silicates, carbonates, sulphides, phosphates and other crystalline mineral groups. These may be present in various forms, including thin bands and laminae intimately associated with the organic matter as well as lenticles, nodules, crystal aggregates and cell infillings, all of which indicate approximately contemporaneous formation with the peat bed. Other minerals, however, occur as veins or cleat and fracture fillings, indicating precipitation after most of the compaction and, presumably, after most of the rank advance. ome of the penecontemporaneous mineral matter is of detrital origin, representing epiclastic or pyroclastic particles washed or blown into the peat swamp. Other mineral matter, however, including much of the pyrite, quartz and siderite, as well as abundant well-crystallized kaolinite, appears to have been formed by precipitation processes either within the swamp waters or in the pores of the peat deposit. Pyrite, largely formed by bacterial reduction of dissolved sulphate ions, seems to be related in abundance to marine transgressions, which allowed permeation of sea water into the swamp and through the peat bed beneath. Siderite appears to form in the absence of sulphate-rich sea water, probably from CO 2 released by organic decay. The clay fraction of the noncoal rocks in many coal-bearing sequences is typically dominated by illite or interstratified clay minerals, with kaolinite, chlorite and possibly montmorillonite also present to a variable, though typically minor extent. Although perhaps modified by chemical reactions or ion-exchange mechanisms in the peat swamp, these clay minerals may also be present in the coal as well, especially near the base of top of the seam. The kaolinite in the noncoal rocks is usually poorly crystallized and typically a minor constituent, but well-crystallized kaolinite is characteristically present, and often the dominant component in the mineral fraction of bituminous coal beds, as well as in some of the lutites (including underclays) intimately associated with the seams. A progressive upward gradation to a particularly kaolinite-dominated mineral assemblage occurs in some seams of the Sydney basin, Australia. This gradation is thought to be due to either increased blockage of detrital contaminants as the swamp developed, or to in-situ chemical or biological leaching of the mineral fraction and reconstitution of the residues in the upper parts of the peat bed. Superimposition of such cycles is also noted within the seam in some instances, suggesting repetition of several phases of this type during major peat accumulations.