Bentonite deposits intercalated with the Rajmahal volcanic rocks of eastern India

Bentonite is distributed widely in the Rajmahal Hills of eastern India, usually in association with volcaniclastic deposits. Although bentonite occurs between the successive flow units, it always forms pockets, rather than a continuous bed. The occurrence of bentonite in the lower part of the Rajmahal lava pile (characterised by fossiliferous sedimentary interbeds) is consistent with its formation in a subaerial or lacustrine environment. The bentonite appears to have formed as a result of alteration of basaltic pyroclastic rocks. The Rajmahal bentonite is of non-swelling type. The exchangeable base is Ca, which is substituted by Na. Petrographic studies reveal that montmorillonite is the predominant mineral in the bentonite, together with minor kaolinite and illite. Cristobalite constitutes the main non-clay mineral. Nontronite, saponite, dickite, nacrite, anauxite and beidellite have also been identified. The bentonite is completely devoid of attapulgite, dolomite and organic material. Rajmahal bentonite with high $\text{Fe}{}_2\text{O}{}_3\text{CaO}$ ratio is gritty in texture and low plasticity, whereas bentonite with high $\text{Al}{}_2\text{O}{}_3\text{Fe}{}_2\text{O}{}_3$ and $\text{CaO}\text{Al}{}_2\text{O}{}_3$ and low $\text{Fe}{}_2\text{O}{}_3\text{CaO}$ ratios is grit-free and of high plasticity. The latter material is superior in quality when compared to the former. Based on surface and drill core data, a total economic reserve of ∼177,000 t of bentonite is estimated in the Rajmahal Hills. Of this total, 82,300 t is Grade I quality and 94,400 t is Grade II quality.