There are several elements that exist both as hard cations and as intermediate cations. A while back, I pointed to molybdenum as an example. Another is chromium, and it's a good one. Cr6+, the hard cation, has a high ionic potential and so falls in the blue swath of cations that make oxysalts, in this case chromates, and it's relatively soluble. Cr3+, on the other hand, is less soluble, enters early-forming igneous minerals, and stays in soil in weathering - Cr3+ has a greater affinity for solids. Thinking along those lines, Cr3+ predictably makes both an oxide mineral and a sulfide mineral. All this has environmental implications, in that reducing groundwater will leave behind Cr as Cr3+ on aquifer solids, but more oxidizing groundwater pumped as drinking water may bring with it chromium as Cr6+, leading to a toxicity problem.

          We can look at another comparison across this divide between hard and intermediate-to-soft cations . . .  

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