function[kap] = calc_kappa(T) % calculate diffusivity kappa (due to vein migration) at temperature T (degC) % grain growth constants K0 = 1.68e7; % mm2 a-1 R = 8.314; % J K–1 mol–1 Q = 42.4e3; % J mol–1 % geometric factor c1 = 2.5; % calculate diffusivity, in the unit m2 a-1 kap = 1e-6 * K0 * exp(-Q./(R*(T+273.15))) / (3*c1);