  Smectite clay minerals have an outsize impact on the response of clay-rich
media to common stimuli, such as water imbibition and ion exchange, motivating
extensive effort to understand microscopic behaviors resulting from these
processes such as swelling and exfoliation. Nonetheless, there is no general
consensus about the microscopic forces that govern smectite properties, which
are model systems for understanding colloidal and interfacial phenomena more
generally. We find that the complex free energy surface arising from the
interplay of at least four intermolecular forces and their nonlinear couplings
that control local particle-particle interactions leads to dynamic, unstable
equilibria between distinct phases. Mechanical disequilibrium arising from
osmotic gradients between curved or interacting interfaces drive the dynamic
exchange of layers and ions between dense and dilute phases via avalanche
transitions that are sustained by thermal fluctuations. We suggest that the
surprising interfacial dynamics displayed by smectite minerals, arising from
the vastly different mobilities of water, ions and mineral, makes them
fundamentally distinct from non-clay minerals because their structures are
easily perturbed away from simultaneous chemical and mechanical equilibrium.
