Atomistic modeling of phengite and molecular mechanisms of NH4+ and K+ diffusional mobility in its structure

Phengite is one of the important hydrous phyllosilicate minerals capable of transporting water and potassium in subducting slabs down to the Earth’s upper mantle. Being able to incorporate significant amounts of ammonium in its structure, phengite is also a major contributor to the nitrogen cycle in subduction zones. Employing classical molecular dynamic simulations with the empirical ClayFF-MOH force field, we studied the behavior of K-phengite and NH 4 -phengite under ambient conditions and at high temperatures and pressures. The calculated structural and elastic properties of K- and NH 4 -phengite agree well with available experimental data. However, the main focus of the simulation is the investigation of molecular scale mechanisms driving the diffusion of K + and NH 4 + cations in the interlayer space of phengite clay. Even though the transport of cations is not observed at room temperature, the simulations reveal that diffusivity of both cations is possible at high temperatures. Moreover, high temperature ensures the diffusional mobility of both cations within phengite crystals at pressures as high as 10 GPa. In addition, the presence of even small amounts of H 2 O in the clay interlayers reduces the activation energy barriers for diffusion, thereby facilitating easier mobility of cations. Despite its larger size, ammonium cation consistently demonstrates a higher diffusional mobility than potassium over the entire temperature and pressure range studied.