SODIUM-POTASSIUM INTERDIFFUSION IN POTASSIUM-RICH ALKALI FELDSPAR II: COMPOSITION- AND TEMPERATURE-DEPENDENCE OBTAINED FROM CATION EXCHANGE EXPERIMENTS

Na-K interdiffusion in disordered potassium-rich alkali feldspar was studied experimentally using cation exchange between gem quality sanidine from the Eifel and alkali-halide melt at temperatures of 800 degrees C to 1000 degrees C and at close to ambient pressure. Sodium-potassium interdiffusion coefficient D-NaK was determined for potassium mole fractions in the range 0.65 <= X-Or <= 0.99. At 0.65 <= X-Or <= 0.95 the sodium-potassium interdiffusion coefficient is largely independent of composition. At X-Or >= 0.95, it rises sharply with increasing potassium mole fraction. Diffusion perpendicular to (001) is about one order of magnitude faster and less strongly temperature dependent than perpendicular to (010). The parameters of the Arrhenius equation describing the temperature dependence of the sodium-potassium interdiffusion coefficient D-NaK = D(0)Exp (-E-A/RT) were estimated as D-0(perpendicular to(001))(0.92) = 5.18 . 10(-8) m(2)/s, E-A(perpendicular to(0.92)) = 179kJ/mole D-0(perpendicular to(001))(0.98) = 1.82 . 10(-7) m(2)/s, E-A(perpendicular to(001))(0.98) = 182kJ/mole D-0(perpendicular to(001))(0.92) = 5.81 . 10(-5) m(2)/s, E-A(perpendicular to(001))(0.92) = 272kJ/mole D-0(perpendicular to(010))(0.98) = 1.34 . 10(-4) m(2)/s, E-A perpendicular to(010)(0.98) = 269kJ/mole The results of our direct determinations are compared with theoretical calculations using the corresponding sodium- and potassium tracer-diffusion coefficients, and the processes underlying the observed composition- and temperature dependence of sodium-potassium interdiffusion are discussed.