Thermogravimetric and Raman spectroscopic investigations on different coals in comparison to dispersed anthracite found in permineralized tree fern Psaronius sp.

Abstract

Permineralized samples of the tree fern Psaronius sp. belong to the spectacular discoveries of the Lower Permian Rotliegend ‘Petrified forest of Chemnitz’, one of the most famous palaeontological sites in the world. The cellular tissues of the 280 million years old fossil fern show a delicate three-dimensional preservation by permineralization. Moreover, anatomical detail can be distinguished very well because of the black coloured former cell walls. By means of Raman spectroscopy this fossilised tissue of the adventitious roots is revealed to be carbon which structure seems to correspond to anthracite. The aim of this work is the experimental confirmation of the carbon structure by means of thermogravimetry and Raman spectroscopy on coal samples of different rank, especially anthracite samples from different times and localities. Coal maturation involves the sequence peat–lignite–coal–anthracite-microcrystalline graphite by incresasing the density and specific energy. The loss of in situ moisture (typically up to 200 °C) and volatile organic matter (up to 600 °C) has been acquired by thermogravimetry. The increasing carbon content, density resp. structure results in Raman spectra with typical band shape of the D- and G-peaks. We used these methods to characterise and compare the separated coal samples to the dispersed carbon inclusions in permineralized Psaronius.