Deccan Traps-associated obsidian glass: a nuclear waste containment

Alteration of obsidian collected from Osham Hill, Gujarat after treatment under hydrothermal-like conditions is compared with the naturally altered obsidian for its assessment as a nuclear waste glass. Experimental data have been obtained for ionic release, glass alteration and its retention in the residue. Geochemical evolution of obsidian shows partial to complete leaching of all the ions, but profusely of Si and Na ions. The ionic release is found in the order of Na > Si > K > Ca > Al = Mg > Fe > Mn > Ti. SEM-BSE images show distinct microstructures of smectite, montmorillonite and illite inside as well as outside the secondary layers, resulting from paragenesis of alteration products at various temperatures (100–300°C) and pressures (50, 250 and 1260 psi). It has been found that the octahedral cation occupancies of smectite are consistent with the dioctahedral smectite. The secondary layer composition shows retention for Si, Al and Mg ions, indicating their fixation in the alteration products, but remarkably high retention of Ti, Mn and Fe ions suggests release of a very small fraction of these elements into the solution. Devitrification of glass along the cracks, formation of spherulite-like structures, yellowish-brown palagonite, chlorite, calcite, zeolite and finally white-coloured clays that yielded after experiments, largely correspond to the minerals which are found in the residual soil profile (developed over fresh obsidian outcrops), formed as a result of weathering in the natural environment.