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Department of Geology, Mappin Street, University of Sheffield, Sheffield S1 3JD, England
The minerals present in coals are grouped as either detrital or diagenetic. The detrital minerals undergo diagenetic modifications however, and provide one of the sources for elements incorporated into diagenetic minerals. The detrital minerals are present either dispersed or concentrated in the coal seams. The overall composition of the detrital minerals is a function of grain size and thus of current velocity. Clay mineral composition varies with the maturity of the sediment, and in particular whether this is reworked from within the basin or is more representative of the upland source. Dirt partings (intraseam mudrocks) represent either an increase in detrital sediment input or a reduction in the rate of organic accumulation, or even loss of organic matter. Wind blown sediment in coals are best represented by altered volcanic ashes, tonsteins, providing a means of correlation and minerals for absolute age determinations. The alteration of ash to kaolinite both in the peat and in the associated sediments demonstrates which clay mineral is stable in the pore waters. Diagenetic kaolinite, probably formed from elements organically derived, is also recognized, particularly in low ash coals. The elements present in diagenetic silicates, sulphides and carbonates originated from within the peat and also from mudrocks in the sequence. The diagenetic sequence of minerals in the cleat is equated with that of other clastic sediments and is similarly interpreted as a burial depth sequence. The residual, diagenetic porefluids resulting from burial diagenesis are the source of economically important, high Cl concentrations in the coal.
Key Words: mineral matter • coal • England • clay minerals • pyrite • siderite diagenesis
