Ionic diffusion in naturally-occurring aqueous solutions: use of activity coefficients in transition-state models

The model of Lane and Kirkaldy (Can. J. Chem. 43, 1812–1828, 1965; 44, 477–485, 1966) for estimating diffusion coefficients in aqueous electrolyte solutions from limiting ionic conductances is rederived for a system of n salts with a common anion, by utilizing the equations of Miller (J. Chem. Phys. 71, 616–632, 1967b; 71, 3588–3592, 1967c). Salt and water activities are used, and it is of necessity assumed that the diffusion mechanism does not change with concentration.The revised model predicts the on-diagonal D^v_ik of a 3 M solution in NaCl-KCl-H₂O to within 5%, compared with errors of 20–30% for the original model. Errors remain constant or decrease as concentrations increase to 3 M, so that predictions of these D^v_ik at even higher concentrations appear promising. Relatively large errors persist in our estimates of the small, off-diagonal coefficients in this system.Measured diffusion coefficients in MgCl₂-NaCl-H₂O extend over only a limited concentration range and are of only moderate accuracy. Nevertheless, the revised model predicts D^v₁₁D^v₂₂, and D^v₂₁, the larger of the off-diagonal coefficients, with errors of only 5–20%.     

Testing models for the development of spiral-shaped inclusion trails in garnet porphyroblasts: to rotate or not to rotate, that is the question

Abstract The formation of spiral-shaped inclusion trails (SSITs) is problematical, and the two viable models for their formation involve opposite shear senses along the foliation in which the porphyroblasts are growing. One model argues for porphyroblast rotation, with respect to a geographically fixed reference frame, whereas the other argues for no such porphyroblast rotation, but instead rotation of the matrix foliation around the porphyroblast. Thus, porphyroblasts with SSITs cannot be used as shear-sense indicators until it is conclusively determined which model best explains them.
Any successful model must explain features associated with SSITs, including: (1) foliation truncation zones, (2) smoothly curving SSITs, (3) millipede microstructure, (4) total inclusion-trail curvature in median sections, (5) porphyroblasts with SSITs that have grown together, (6) evidence for relative porphyroblast displacements, (7) shear-sense indicators inside and outside porphyroblasts; (8) crenulations associated with porphyroblasts and (9) geometries in sections subparallel to spiral axes (axes of rotation). A detailed study of these features suggests that most, if not all, can be explained by both the rotational and non-rotational models, in spite of these models involving diametrically opposed movement senses. Therefore, geometrical analysis of individual porphyroblast microstructures may not determine which model best explains SSITs until the kinematics required to form these microstructures are better understood, in particular the sense of shear along a developing crenulation cleavage. Specific tests for determining the shear sense along crenulation cleavages are proposed, and results of such tests may conclusively resolve the debate over how SSITs form.