A comparative study on structural differences of xylite and matrix lignite lithotypes by means of FT-IR, XRD, SEM and TGA analyses: An example from the Neogene Greek lignite deposits

• Study of xylite vs matrix lignite by means of FT-IR, SEM, XRD, and TG/DTG/DTA.
• FTIR confirms major differences between the xylite and the matrix lignite lithotype.
• Vibrations due to clay minerals are almost absent in xylite FTIR spectra.
• SEM confirms major differences in mineral content between xylite and matrix lignite.
• TGA curves of the studied lithotypes show different cellulose and lignin content.

The FT-IR spectra for both Neogene xylite and matrix lignite samples from six different Greek lignite deposits (NW Greece) show significant differences. In particular in the aliphatic stretching region (3000–2800 cm− 1) the intensities of the vibrations are more prominent in the xylite lithotype as in the matrix lignite lithotype. The intense bands in the region 3402–3416 cm− 1 are attributed to − OH stretching of H2O and phenol groups for both xylite and matrix lignite lithotypes. The bands at ~ 3697 cm− 1 and ~ 3623 cm− 1 as well as at ~ 538 cm− 1 and 470 cm− 1, which are more evident in the FT-IR spectra of the matrix lignite, are attributed to a higher clay content in the samples of this lithotype. Data resulting from X-ray diffraction analysis (XRD), scanning electron microscopy (SEM) and thermoanalytical methods (TG/DTG and DTA) indicate main differences of the xylite and matrix lignite lithotype as well. Typical peaks in X-ray diagrams confirm the high content of clay minerals in matrix lithotype, and their minor contribution in the xylite lithotype. Scanning electron microscopy (SEM), combined with the FT-IR and XRD results, reveals prevalence of gypsum, pyrite/marcasite, quartz, and clays in the matrix lignite lithotype as compared with the xylite lithotype and also textural differences such as the heterogeneity in the mass of matrix lignite and the homogeneity of the xylite. Compositional differences of the examined lignite materials, as identified by FT-IR, were also confirmed by TG/DTG/DTA. While cellulose decomposition was observed by an endothermic peak of DTA curves, at around 360 °C, in the xylite samples, lignin degradation became a prominent process observed in the temperature range from 200 °C to 400 °C in the matrix lignite samples. This was also observed by an exothermic peak on DTA curves. Each of the heat effects was accompanied with a partial mass loss registered on TG curves. As a whole, the data resulting from the combined research by means of FT-IR, XRD, SEM, and TG/DTG/DTA of both xylite and matrix lignite lithotypes confirm the significant differences between these two lignite lithotypes reflecting their different structure, texture, and maturity stage.