Geochemical and mineralogical characteristics of a power station feed-coal, Eggborough, England

Abstract

Chemical and mineralogical analyses are presented for a power station feed-coal in the Yorkshire Coalfield, U.K. The mineralogy of the whole coal consists of quartz, kaolinite, illite, mixed-layer clay, chlorite, pyrite and carbonates. Pyrite, carbonates and some of the kaolinite are diagenetic. In separated, low-density fractions of the coal both pyrite and kaolinite are concentrated relative to the detrital minerals. Compared with the high density fractions the detrital clay mineralogy of these low density fractions contains less chlorite, and the mixed-layer clay to illite ratio is higher, which is consistent with a grain size control during sedimentation. The major element analyses are comparable with mudrocks in the stratigraphic sequence, but there is evidence of Na 2O enrichment related to connate brinnes and the Fe 2O 3 content shows significant enrichment, particularly in the ashes of the low density fractions. The dissolution of oxhydroxide grain coatings associated with detrital sediment in the coal, and also in the adjacent strata, are probable sources of the iron in the diagenetic pyrite. There is evidence of iron mobility in the sedimentary sequence and introduction into the coal itself during diagenesis. Within the coal, trace elements are also enriched (on an ash basis) relative to the mudrocks and their origin is also probably very similar to that of iron. A noteworthy feature of the trace element enrichment is that the enrichment factors for a number of elements are comparable with those determined for marine black shales in the sequence. This suggests a common origin for the elements during the diagenetic processes. Analysis of the coal density fractions, after correcting for detrital mineralogy, suggests Pb, Cu, Ni, Zn and possibly Mn are associated with pyrite and V, Sr, Ba, Zr, and Nb are either directly or indirectly associated with organic components. Possibly Zr and Nb originate from dispersed volcanic ash.