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

Abstract

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 ℃) 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 ℃.

Evaluation of terrestrial carbonaceous matter aromatization by Raman spectroscopy and its application to C chondrites

“…The data from Bonoldi et al (2016) deviate from trend because they used additional bands (1693 and 1547 cm ‐1 ) in the G‐band region from sedimentary samples, altering the G‐band width significantly as discussed by Lupoi et al (2017). The discrepancy of the data from Ghosh et al (2018) in the range Rmax 5.94–8.66% is explained by the very high laser power of 18 mW or their option to suppress the additional D2‐band for deconvolution, which leads to larger G‐band width. Data from Kelemen and Fang (2001) diverge possibly due to laser power and/or fitting procedures.…”

“…The authors who studied OM/CM by Raman spectroscopy and reported the G‐band width along with vitrinite reflectance are: Ammar and Rouzaud (2012); Bonoldi et al (2016); Ghosh et al (2018); Guedes et al (2010); Henry et al (2018, 2019b); Hinrichs et al (2014); Jubb et al (2018); Kelemen and Fang (2001); Kwiecinska et al (2010); Li et al (2019); Lupoi et al (2017); Quirico et al (2005); Rantitsch et al (2004, 2016a); Schmidt et al (2017); Spötl et al (1998); and Wilkins et al (2014). The acquisition protocols of all authors are described in Table in supporting information, the full raw data sets are available at Table in supporting information.…”

“…It is noteworthy that they are unreliable above Rr of 5–6%, due to the increasing anisotropy in vitrinite reflectance values (Kilby 1988, 1991). Data sets that measured both Rr and Rmax (Lupoi et al 2017; Ghosh et al 2018; Li et al 2019) were used to test the conversion functions, and the best agreement in the range between Rr 0.5% and Rr 7% was achieved using the inclination of the linear relationship Rmax = 1.155*Rr ‐ 0.1 from Kilby (1991).…”

Potassium-metasomatism in Anthracite: A Cluster of Geochemical Evidences

“…CG3300, CG3301, and CG3302 samples were picked up from the Jorethang area and CG3303 was collected near Namchi at the south Sikkim. Sample CG3304 was sampled from the north of the Kamling village (Ghosh et al 2018(Ghosh et al , 2020. Furthermore, CG3305 and CG3306 were collected near the Reshi Khola in the Reshi town (Ghosh et al 2018(Ghosh et al , 2020.…”

Geochemical Alteration and Mineralogy of Coals under the Influence of Fault Motion: A Case Study of Qi’nan Colliery, China

“…For instance, fluids migrating through brecciated coals in fault zones probably lead to cleat mineralization and elemental enrichment [58,59,61]. Specifically, the concentrations of Ti, Sc, Ta, U and rare earth elements all increase as the coal deformation increases, while the concentration of Mo decreases with increasing deformation intensity, which can be mainly ascribed to the coal dynamic metamorphism accompanied by frictional thermal effects [62][63][64][65]. Analogously, concentrations of rare earth elements in fault-related boreholes are higher than those in unrelated boreholes in specific coal seams.…”