I would argue that the pattern we've seen can be explained if we think about what ionic potential means in terms of interactions between ions. On the left above, let's start with caions of low ionic potential - K+ would be a good example. These ions are such weak fluffy concentrations of charge that they don't make strong bonds to O2-, and so they don't go into high-temperature minerals of igneous rocks but instead stay in the melt, and in weathering they quickly break free from bonds to O2- and go into solution.

        In the middle of this sketch, we can consider cations of intermediate ionic potential - Al3+ and Ti4+ would be good examples. These ions are smaller and more highly charged and thus provide a more focused charge that allows stronger bonds to O2- in solids. At the same time, the focus of positive charge is not so great that cations repel each other in those solids. As a result, these cations go into high-temperature minerals of igneous rocks, and in weathering they stay in solids and thus in soils.

        At the right, we can consider cations of highest ionic potential - N5+ and S6+ would be good examples. These ions are still smaller and even more highly charged, and thus provide an extremely focused beacon of positive charge. That allows very strong bonds to O2- to form radicals like nitrate and sulfate. However, the strength of positive charge is so great that O2-s are hard-pressed to shield cations from each other, and so we end up with no CO2 or SO3 minerals, and these ions instead tend to break free in weathering and go into solution.

        That's a model to account for what we're seeing on our table, and . . .  

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The first slide of this presentation
The last slide of this presentation
Railsback's main page about the Earth Scientist's Periodic Table of the Elements and Their Ions
e-mail to Bruce Railsback (rlsbk@gly.uga.edu)
Railsback's main web page
UGA Geology Department web page

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