Geochemistry and nano-mineralogy of two medium-sulfur northeast Indian coals

• High-S Indian Cenozoic coals were investigated.
• The vitrinite-rich coals are subbituminous to hvCb rank.
• Potentially toxic trace elements are associated with Fe-bearing minerals.

The petrology, mineralogy, and inorganic geochemistry of two contrasting high-sulfur, vitrinite-rich subbituminous to bituminous coals from the North Eastern Region (NER) of India have been studied using a combination of optical microscopy, quantitative X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HR-TEM/SAED), and chemical analysis techniques. The study reveals that the Fe nano-particles are present in Indian high-sulfur Tertiary coals including nano-pyrite, Fe-oxyhydroxides and a small amount of sphalerite. The pyrite is present as framboids and unsymmetrical cubic crystals (rhombohedral). They contain potentially hazardous elements viz. As, Pb, and Se measured by EDS techniques. Mineral-matter residues isolated from the coals by low-temperature oxygen-plasma ashing are dominated by carbonate minerals (calcite, dolomite, ankerite), pyrite and pyrite oxidation products (jarosite and coquimbite), kaolinite, illite, and quartz. Sulfate-bearing phases such as tschermigite, bassanite and gypsum are also present, probably as artifacts of the plasma-ashing process.