Coal spectral features in the mid-infrared range

Rodrigues1, Sandra; Fonteneau2, Lionel; Esterle1, Joan

1The University of Queensland, School of Earth and Environmental Science, St Lucia, QLD 4069, Australia; 2CoreScan Pty Ltd, 1/127 Grandstand Road, Ascot WA 6104, Australia

An increasing rank coal suite (from subbituminous to low volatile bituminous coal, LVB) was used to investigate the spectral features of the organic matter in coal in a wide spectral range from 450 nm up to 14300 nm. Different sensors and equipment were used, covering the visible-near and -shortwave infrared (VNIR-SWIR, 450-2500 nm), the mid-infrared (MIR, 2000-8000 nm) and the thermal infrared (TIR, 6000-14300 nm) regions. The MIR and a small portion of the TIR holds the fundamental vibrations for organic materials, while the SWIR only exhibit the overtones and combinations of those fundamental vibrations. Consequently, spectral features (absorption bands) are better defined in the MIR. The 2900 nm absorption feature is attributed to the fundamental –OH stretch in organic materials. This feature is more prominent in the low rank coal and tends to disappear with increasing rank. Therefore, it must be related with the moisture content in the coal samples. The 3280 nm band corresponds to CH stretch in the aromatic fraction. This band develops with increasing coalification, appearing as a deep absorption feature in the LVB coal. A doublet occurs at 3380 nm and 3420 nm, corresponding to CH3 and CH2 asymmetric stretches, respectively, in the aliphatic fraction. In the subbituminous coal sample, these two bands are hard to decouple appearing as an intense absorption band around 3420 nm. In the medium volatile bituminous coal, these bands are clearly separated but become shallower when the LVB coal rank is reached. The 3500 nm absorption band is attributed to CH3 symmetric stretch in the aliphatic fraction, and becomes shallower with increasing rank. In the region between 3280 nm up to 3500 nm the absorption bands seem to be related with the increasing of the aromaticity in the coal, with the loss of the aliphatic components favouring the development of the aromatic. The absorption features described above are the most prominent bands recognised in the MIR region in the coal samples available for the project (ACARP C28045). Other minor bands also occur in the MIR, including the band at 5250 nm, which is an aromatic related feature that does not occur in the subbituminous coal sample. The 6200 nm absorption band is possibly related to the CC aromatic bonding and it is a deeper feature in the LVB coal. On the other hand, the band at 6860 nm correspond to CH2 asymmetric bending and becomes shallower with increasing rank. The results showed that the MIR spectral range has the potential to characterise the rank of the coal though the development and/or disappearance of the spectral absorption features by evaluating the variations in their area, depth and position.


Sandra Rodrigues is an Organic Petrologist working for the past seven years at The University of Queensland. She has a vast experience working with coal , oil and gas. More recently, Sandra has been applying hyperspectral technologies to the characterisation of the organic matter in the coal.

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