Mineralogy of lignites and associated strata in the Mavropigi field of the Ptolemais Basin, northern Greece

Abstract

The mineralogy of five lignite layers and the associated roof, floor and intercalating strata has been analysed using low-temperature oxygen-plasma ashing and quantitative X-ray diffraction techniques. The lignite contains quartz, kaolinite, illite and smectite, and in some cases small proportions of feldspar, thought to represent mainly detrital minerals in the original peat deposits. One sample also contains crystalline calcium oxalates, with weddellite present in the raw lignite and whewellite in the low-temperature ash (LTA) of the same lignite sample. Minor proportions of gypsum are present in some of the raw lignite samples, possibly in part derived from precipitation of dissolved Ca and SO 4 in the pore waters on drying. All the LTAs, however, contain much higher proportions of bassanite and/or gypsum compared to the other minerals present, consistent with production of calcium sulphate artefacts from interaction of abundant organically-associated Ca and S in the lignite during the plasma-ashing procedure. The inorganic strata are mainly carbonate rocks, consisting almost entirely of calcite with minor aragonite, small proportions of illite, traces of pyrite and in some cases siderite. Some non-carbonate rocks are also associated with the lignite seams; these are dominated by plagioclase feldspar and mica, along with illite, kaolinite and a relatively minor proportion of quartz. The mineral assemblages can be related to the palaeoenvironmental conditions under which the various sediments (including lignite) were deposited. Comparison of the mineral abundances in the samples to the chemical composition of the materials shows that the quantitative XRD technique provides results consistent with ash analysis data. Although abundant in the associated rocks, calcite does not occur in the lignite samples. While part of the relatively abundant Ca in the lignite occurs as either gypsum or weddellite, most of it appears to be organically-bound. Mine products that incorporate calcareous rocks of the type associated with the lignite may, however, contain separate calcite phases and display high carbonate CO 2 contents.