The low to medium-rank Tertiary coals from Meghalaya, India, are explored for the first time for their comprehensive microstructural characterization using the FTIR and Raman spectroscopy. Further, results from these coals are compared with the Permian medium and high-rank coals to understand the microstructural restyling during coalification and its controls on hydrocarbon generation. The coal samples are grouped based on the mean random vitrinite reflectance values to record the transformations in spectral attributes with increasing coal rank. The aliphatic carbon and the apparent aromaticity respond sharply to the first coalification jump ($${\bar{\rm R}}$$ R ¯ r: 0.50%) during low to medium-rank transition and anchizonal metamorphism of the high-rank coals. Moreover, the Raman band intensity ratio changes during the first coalification jump but remains invariable in the medium-rank coals and turns subtle again during the onset of pregraphitization in high-rank coals, revealing a polynomial trend with the coal metamorphism. The Rock–Eval hydrogen index and genetic potential also decline sharply at the first coalification jump. Besides, an attempt to comprehend the coal microstructural controls on the hydrocarbon potential reveals that the Tertiary coals comprise highly reactive aliphatic functionalities in the type II-S kerogen, along with the low paleotemperature (74.59–112.28 °C) may signify their potential to generate early-mature hydrocarbons. However, the presence of type II-III admixed kerogen, a lesser abundance of reactive moieties, and overall moderate paleotemperature (91.93–142.52 °C) of the Permian medium-rank coals may imply their mixed hydrocarbon potential. Meanwhile, anchizonal metamorphism, polycondensed aromatic microstructure, and high values of paleotemperature (~ 334.25 to  ~ 366.79 °C) of the high-rank coals indicate a negligible potential of producing any hydrocarbons.

No abstract

No abstract

A suite of sheared syenites occurring along the western margin of the Eastern Ghats Belt, India have developed extensive flame perthite in K-feldspar. Albite flames show large variation in size, shape and abundance. Field, petrographic and chemical evidence suggests complex interplay between differential stress, recycling of K-Na-Ca and supply of Na by infiltration for the development of flame perthite. Partial replacement of pyroxenes, plagioclase and alkali feldspar by amphibole, biotite, nepheline and calcite causes internal recycling of Na-Ca-K in a closed system. Representative compositions of the minerals are used to constrain the model dissolution–reprecipitation ion-exchange reactions involving Na and K either as reactants and/or as products. A substantial proportion of Na+ required for the development of the albite flames, originates from Na metasomatism accompanied by ductile shearing in the feldspathic rocks, providing an ideal open system wherein both the differential stress and Na+ are made available for the development of the flame perthites. This process probably augmented the replacement of K-feldspar grains by flame albite and the K+ released was carried away by the fluid or, possibly, augmented the biotite-forming reactions in the associated quartz-poor syenites and, hence, trigger the Na-K cycle in these rocks.

No abstract