The ions of magnesium, nickel and cobalt have almost the same size (iron ions are slightly larger, and manganese ions even larger).

That is why nickel and cobalt may easily substitute magnesium in magnesium minerals, being more enriched there in comparison with nearby minerals containing iron oxides or manganese oxides.

However, even the bigger ions of iron and manganese do not differ much in size of the magnesium ions, which is why in olivine some part of the magnesium ions are substituted with iron or manganese ions, besides the nickel and cobalt ions, because iron and manganese are very abundant everywhere, even if they do not fit as well the crystal structure as nickel and cobalt.

I didn't know that, but that makes sense. And magnesium is resolutely divalent, while iron, manganese, and cobalt are happy enough to be divalent, and nickel, while not as stuck-up as magnesium, strongly prefers it.

BTW, if you haven't seen it, you'll probably be interested in https://news.ycombinator.com/item?id=44698409.