Implications of sepiolite dehydration for earthquake nucleation in the Galera Fault Zone: A thermodynamic approach

A new thermodynamic model for the Mg-phyllosilicate sepiolite was developed and used to calculate its P-T stability conditions and water content for different bulk rock compositions. The standard-state thermodynamic properties, entropy (S°) and enthalpy (H°), were initially estimated by oxide summation taking into account the different entropic and enthalpic contributions of the three types of water in sepiolite: zeolitic water, bound water, and structural OH groups. The starting model was then refined with experimental data. The dehydration process follows a step function that allowed us to define “end-members” with decreasing hydration states in a theoretical solid solution. The stability field of sepiolite is ultimately limited by the reaction sepiolite = talc + quartz + H2O, which is located at about 325 °C at 1–500 MPa. The large thermodynamic stability field of this clay mineral suggests that it could control the mechanical behaviour of crustal faults to 325 °C. The refined model was then applied to the natural case of the Galera Fault Zone (SE Spain) using thermogravimetric analysis and X-ray fluorescence data of the bulk rock composition of the fault core gouge, which is mainly composed of sepiolite. The dehydration of sepiolite at T