X-ray-diffraction Study of Fe-2+ -Mg Order-disorder In Ortho-pyroxene - Some Kinetic Results

Kinetic rates of Fe2+-Mg disordering in three orthopyroxenes (mean value of XFe = Fe2+/(Fe2++Mg) = 0.175,0.482,0.770 respectively) have been determined employing heating experiments and single crystal X-ray structural refinements. Disordering rate constants (K) (550800° C) for two pyroxenes are given by: ln (K) = 27.107(±5.177)–32062(±783)T–1(XFe = 0.175) ln (K) = 16.142(±0.057)–18227(±423)T–1(XFe = 0.770) The distribution coefficients KD (representing a steady state of disordering FeM2 + MgM1 lrhar FeM1 + MgM2) are given by: ln KD = 5.016(±0.223)-7033(±1473) T–1(XFe = 0.175) ln KD = 1.988(±0.122)-3809(±913)T–1(XFe = 0.770) These distribution coefficients provide the constraint of the disordering reaction on the value of the equilibrium constant for Fe2+-Mg order-disorder. Until the low temperature dependence of KD is well constrained, the calculation of cooling rates of pyroxenes and host rocks cannot be done reliably.