Stability of lanthanum-saturated montmorillonite under high pressure and high temperature conditions

Smectite has been used to capture radioactive cations through adsorption in deep radioactive waste repositories in various parts of the world. Smectite is also important in the transport of water and some trace element cations such as rare earth elements (REE), which are captured in its structure, back to the mantle in subduction environments. Such captures are based on the ionic strength of the surrounding solution and the adsorption coefficient of smectite. However, captured cations can be released from the smectite structure once the ionic strength of the solution changes. In this work, the stability of a particular smectite (montmorillonite) structure saturated with lanthanum was verified at high pressures (up to 12 GPa) and room temperature and at high pressure and high temperature (HPHT) concomitantly. La3+-montmorillonite remains stable up to 12 GPa at room temperature with a small variance in its vibrational mode. At HPHT, however, the structure becomes muscovite-like and rich in La3+. When in contact with a Ca2+-enriched solution, La3+ is partially replaced by Ca2+ in the new phase, returning to its original Ca2+-montmorillonite phase, whereas another part remains La-muscovite-like. These results were confirmed by X-ray diffraction and scanning electron microscopy with energy dispersive X-ray spectroscopy.