Petrographic and Raman spectroscopic characterization of coal from Himalayan fold-thrust belts of Sikkim, India

The present study focuses on the investigation of the optical anisotropy, optical sign, textural heterogeneity and deformational features of the maceral grains along with the Raman spectral characteristics of the seven coal samples collected from the Himalayan fold-thrust belts of Sikkim State, India. The coal samples were extremely fragile and pulverized due to the intense tectonic deformation. The maceral composition revealed the dominance of semifusinite over collotelinite grains. The calculated maximum vitrinite reflectance (5.94-8.66%) and mean random reflectance (4.11-5.36%) suggest anthracite A rank of the coal samples following ISO 11760:2005. The proximity of the intermediate reflectance axis value (R-INT) to maximum reflectance axis value (R-MAX) as well as the range of Reflectance Indicating Surface (RIS) style (R-st) values (-9.98 to -19.37) indicates the biaxial negative optical texture of the vitrinite grains. The augmented bireflectance values due to enhancement of the R-MAX associated with strong decline in R-MIN may suggest the commencement of pregraphitization. In addition, the strong linear correlation (r = 0.94) of the RIS-anisotropy (R-am) parameter with the bireflectance values may imply the role of tectonic stress on the optical transformations of the samples. The range of the peak temperature (334.94-369.01 degrees C) calculated from mean random vitrinite reflectance may suggest the effect of thermo-stress coupling on the metamorphism of these coal samples. Microlithotype study combined with deformational aspects of macerals shows the presence of "deformed", "sheared" and "smashed" grains within each sample, which may, additionally, document the tectonic stress influence on the coal particles. Moreover, relatively, larger area of 'defect band 1 (D-1)' than that of 'graphitic band (G)' along with the broad G band in the first order Raman spectra may indicate the considerable presence of structural dislocations and aromatic compounds with disordered bond angle within the microstructure induced by the tectonic deformation. The lowest intensity of the 'defect band 4 (D-4)' may suggest the preferential removal of aliphatic compounds from the samples in response to the tectonic stress degradation. In addition, the relative area ratio calculated from the D-1 and the G bands (AD(1)/(AD(1) + AG)) may indicate that the studied anthracite samples would have attained the metamorphic temperature ranging from 325.12-387.89 degrees C.