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As far as I know, most of the "hard things" in living creatures composed of calcium (for example, bones plated with calcium over the keratin structure, and eggshells and seashells with $\ce{CaCO3}$). The calcium ion has an oxidation state of +2, and so does iron.

Why don't we find in nature creatures with bones of iron or eggs with iron shells, given that living creatures do use ionic iron for other purposes like in the heme group of hemoglobin?

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    $\begingroup$ iron would get oxidized, whereas calcium wont get oxidized as calcium has a maximum oxidation state of 2+. Iron could get oxidized to its 3+ state, ie, it would rust reducing the strength of the bones. $\endgroup$
    – Prakhar
    Nov 20, 2016 at 14:14
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    $\begingroup$ Structures made form calcium carbonate can be really tough (bones are stronger than concrete) and are much less dense than iron. Plus calcium is more abundant than iron in food. $\endgroup$
    – vapid
    Nov 20, 2016 at 14:30
  • $\begingroup$ @vapid "Plus calcium is more redundant than iron in food" is a circular argument: The reason it is not abundant in food is the very same reason why it is not used in nature. $\endgroup$ Nov 20, 2016 at 16:13
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    $\begingroup$ Iron eggs? I guess little birdy would have a hard time trying to break out of an iron egg, just saying ... $\endgroup$ Nov 20, 2016 at 16:16
  • $\begingroup$ biology.stackexchange.com/questions/9419/… $\endgroup$
    – Mithoron
    Nov 20, 2016 at 23:21

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One disadvantage of iron carbonate shells is they are unlikely to last in the oxidizing environment of Earth. Even in the laboratory, preventing the iron from converting to rust is problematic. Wikipedia:

Care must be taken to exclude oxygen $\ce{O2}$ from the solutions [to prepare iron(II) carbonate], because the $\ce{Fe^{2+}}$ ion is easily oxidized to $\ce{Fe^{3+}}$, especially at pH above 6.0.[1].

"pH above 6.0" would include our oceans, which we must assume are also aerated. Thus proposed iron carbonate shells would rust out.

Cited Reference

  1. Philip C. Singer and Werner Stumm (1970): "The solubility of ferrous iron in carbonate-bearing waters". Journal of the American Water Works Association, volume 62, issue 3, pages 198-202. https://www.jstor.org/stable/41266171
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  • $\begingroup$ I don't think the original question was assuming the iron would be in iron carbonate: it was why not metallic iron? $\endgroup$
    – matt_black
    Feb 11 at 14:05
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    $\begingroup$ The question mentions ionic iron, and as a comment suggested baby birds might find hatching from metallic shells a bit challenging. $\endgroup$ Feb 11 at 15:29
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Nature works with what it has not with what it could have

Biological systems are ingenious and often find clever ways to create materials useful to survival. Humans are even more ingenious than other creatures and organised groups of humans have found ways to create new materials not known in nature for the benefit of human survival. But human intelligence can do many things nature cannot by way of collective, coordinated action driven by intelligence and tools not just biology.

The modern world is familiar with many human-invented materials that transcend the limitations of natural materials. Like iron and steel. This has led many to wonder whether it might be better to have things like bones or shells made from those materials. Crabs and molluscs have hard shells to protect them from predators, would't they have better protection if they could make their shells of steel? Humans do this by building tanks, after all. Wouldn't bones made of steel be more resistant to breaks?

But the limitation preventing such adaptation is the availability of the materials to biological systems and the lack of mechanisms to exploit them.

It is worth remembering that humans have only been exploiting iron for just over 3000 years. The iron age only started around 1,200 BC despite coordinated groups of humans having been around for at least 10,000 years before than. The benefits of iron were large and transformed civilisation, but didn't appear quickly. Two factors were critical. People had to find concentrated sources of the right raw materials (minable iron ores) and the technology to forge iron products from them (this is hard as getting iron requires, inter alia, far higher temperatures than earlier metals).

For biological systems to have evolved to be able to exploit iron as a metal (I assume that is the question's intent as there don't seem to be any structural benefits of using known iron minerals as structures) then biology would have to solve the same problems of availability and processing.

And there is no easy way to achieve either access to enough iron to make large structures from it or to create metallic iron from known sources via available biological mechanisms.

Small amounts of iron are used in many living creatures (eg in oxygen carrying proteins in blood). But the amounts are tiny compared the the amounts that would be needed to make metallic iron shells or bones. There just isn't enough iron generally available to make large structures. Humans had to discover concentrated iron ores to get enough for their weapons and tools.

But how could living creatures–even if they could get access to iron in sufficient quantities–turn that into metallic structures inside their bodies? And, even if they could, would iron structures be useful? Pure iron is tough but not that hard. It rusts, easily. Many of the iron-based products we are used to seeing are not pure iron, but complex alloys that do a better job of balancing stiffness, corrosion resistance and other features. Most require deployment of other trace elements in their structures increasing the problems of access to the elements in a living environment and compounding the problems of deriving the resulting material from any available biological process. Humans need to use a lot of high-tech processes to create the common structures made from iron and most of those involve high temperatures that biological systems can't use.

Contrast this with the materials living systems actually use. Many shells are complex structures built from carbonate and phosphate minerals (bone is mostly phosphate not carbonate). But Calcium is common as is the carbon dioxide use in carbonates and the phosphorous in phosphates. And there are plenty of biological mechanisms for acquiring and processing those components and building structures with them at biological conditions.

So nature uses what it has access to and has available mechanisms to process under biological conditions. It can't easily leap to use other harder to access materials and processing mechanisms that are–as far as we know–impossible under biological conditions. And it does a good job as many of the structural materials it manages to generate are, weight or volume-wise, better than the common materials produced by human technology.

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