I am by no means a material scientist or biologist, but I have studied a lot of them and have some curiosities.

It would be interesting to see how calcium doping modified the properties of the alloy. AFAIK the temperatures that iron smelts at is to high for the carbonate or phosphate bonds to remain stable, so most of it should have ended up as free calcium or phosphorus.

I also imagine that the type of bones have a lot to do with it, since avian bones have a different composition and density than say, a moose bone. Different kinds of animals also have evolved different metal doping concentrations.