Fe²⁺-Mg interdiffusion in orthopyroxene: retrieval from the data on intracrystalline exchange reaction

Crystal structural data of orthopyroxene suggest that Fe-Mg interdiffusion should be fastest along the c axis and slowest along the a axis and that the overall kinetics of the Fe²⁺-Mg fractionation, or order-disorder, between the M1 and M2 sites should be dominated by exchange along the c and b directions. Using these crystallographic constraints and a formulation of the diffusion process according to absolute reaction rate theory, an expression of the average Fe²⁺-Mg interdiffusion coefficient along the c and b directions of orthopyroxene in terms of the rate constant of the disordering process the equilibrium intracrystalline distribution of Fe²⁺ and Mg, and the average distance between the neighboring M1 and M2 sites along c and b is developed. -from Authors