Dissolution Rates of Actinolite in Water and Its Modified Mineral Surface Across the Critical State

Dissolution kinetic experiments of actinolite in water were performed using a flow-through reactor at temperatures from 20 to 400?°C and 23.5±0.5?MPa. The results indicate that the steady-state release rates of the different elements of actinolite vary with temperature. Generally, Ca, Mg, Fe, and Al dissolve more quickly than Si at temperatures from 20?°C to near 300?°C, but slower from 300 to 400?°C. The Si release rate increases with temperatures from 20 to 300?°C and then decreases from 300 to 400?°C. Si release rate reaches the maximum at 300?°C. Amorphous Si-rich surface layers occur at temperature <300?°C. Hydrations of actinolite are relatively faster, and proton–metal exchange reactions are weakening across the critical point. M_i-rich (Fe, Ca, Mg) and Si-deficient surface layers form at temperatures ≥300?°C. XPS, TEM, and SEM observations indicate that the hydrated silicate occurred at surface as temperature >300?°C. Water property variations within the critical region strongly affect the dissolution rates and the modification of surface.