Ternary diffusion coefficients of the brine systems NaCl (0. 5 M)-Na/sub 2/SO/sub 4/ (0. 5 M)-H/sub 2/O and NaCl (0. 489 M)-MgCl/sub 2/ (0. 051 M)-H/sub 2/O (seawater composition) at 25/sup 0/C

Mutual diffusion coefficients have been measured by Rayleigh interferometry for two ternary brine compositions at 25/sup 0/C: NaCl (0.5 M)-Na/sub 2/SO/sub 4/ (0.5 M)-H/sub 2/O and NaCl (0.489 M)-MgCl/sub 2/ (0.051 M)-H/sub 2/O. The latter is a ternary analog of seawater. Four diffusion coefficients are required to characterize a ternary system. The data show that the cross term diffusion coefficients are significant, and cannot be neglected in accurate geochemical modeling. Since d/sub 21/ for NaCl-Na/sub 2/SO/sub 4/-H/sub 2/O is negative, a sufficient gradient of NaCl can cause Na/sub 2/SO/sub 4/ to move uphill against its own gradient. One of the cross term coefficients for NaCl-MgCl/sub 2/-H/sub 2/O is nearly as large as one of the main term coefficients.