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So, the general idea is this:

A humanoid species' blood uses a chemical to carry oxygen throughout their system. When iron is introduced into their bloodstream by say, stabbing them, it results in the chemical reacting with it and creating an acidic substance, causing pain and thus creating the (in a way true) legend that these creatures are vulnerable to iron.

Now, this is for a fantasy setting, so the material doesn't have to be a real chemical. I just want to know if this idea seems possible. Could be that the iron reacts with the compound containing the chemical, which then results in an acidic residue due to the leftover elements.

As for blood coloration, I'm thinking the chemical would give it an orange-ish or peach-like color when in the body, but then take a golden, metallic hue when clotting outside it. A bit out there but still.

Update: Pyrite! Apparently Fool's gold is more dangerous than it seems.

Pyrite Disease is caused by the mineral being exposed to humid air, breaking it down to iron sulfide, corrosive sulfuric acid and sulfur dioxide gas when it reacts with oxygen and water! Now, this isn't exactly what I want, but a good base to work off of.

Let's say there's a mineral similar to pyrite but with a different composition (no need to go into extreme detail). It still has that golden color, but is able to be incorporated into structures similar to hemoglobin in the human blood. Additionally, due to its exotic composition, it only produces those harmful chemicals when exposed to two things in excess: Iron, and Oxygen.

In short:

Pyrite + Oxygen + Water= Iron Sulfide, Sulfuric Acid and Sulfur Dioxide Gas

Unique Metal + Oxygen + Iron = Iron Sulfide and Sulfuric Acid. Certainly not a reaction you'd want to have in your veins.

Sure, someone trying to tie it down with precise chemistry may need to make some acrobatics with its formula, but at the end of the day the result is ideal: A replacement for blood that has a metallic gold color, and results into a corrosive substance when in contact with iron.

I'd love to hear some feedback for this idea!

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    $\begingroup$ sulfur is one of the chemicals that can substitute oxygen, and it's highly reactive to iron ...when heated, so maybe fire blades now make sense? $\endgroup$
    – user88550
    Aug 18 '21 at 18:20
  • $\begingroup$ also sulfur can be any color from dark brown to yellow, orange, red and green $\endgroup$
    – user88550
    Aug 18 '21 at 18:21
  • $\begingroup$ Huh, I wonder if I could make an imaginary metal based on iron sulfide (fool's gold FeS). Could be that it forms something like hemoglobin, but when extra iron is added to the mix it ends up reacting and creating something like sulfuric acid? Plus it'd have that golden shine! $\endgroup$
    – Magbread
    Aug 18 '21 at 22:19
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Hydrogen peroxide.

Toxicity of iron and hydrogen peroxide: the Fenton reaction

Iron is an essential constituent of a number of proteins involved in oxygen transport or metabolism. It must also be transported around the body, stored and made available for synthesis of iron proteins. The ability of iron to undergo cyclic oxidation and reduction is an important aspect of its function. However, such redox activity can generate free radicals and other strongly oxidizing species capable of causing a wide range of biological injury. This can occur through a variety of mechanisms. Iron can promote radical formation from physiological or xenobiotic compounds, e.g. by catalysing autoxidation, it can initiate lipid peroxidation, and react with hydrogen peroxide to produce more highly reactive and toxic species.

The blood of your creatures is strongly oxidizing - circulating peroxisomes full of hydrogen peroxide are ready to burn up any intruders. Peroxide is also a way to carry oxygen around; a risky way but a way. Oxidation is a good defense against pathogens; we use that too. Your creatures have much stronger oxidative defenses that we do. They never have free iron in their system because iron can trigger the oxidative defenses.

In your scenario of a wound by an iron weapon, the oxidative defenses suffer a meltdown. Catalyzed by the iron, oxidation in the blood stream of your creatures catalyzes auto-oxidation until all of the reactive oxygen species are used up. The creature itself is dead long before that and possibly on fire from the heat produced by the uncontrolled feed-forward loop.

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  • $\begingroup$ Hey that's an interesting idea! I'm not quite sure if I want to go with that, the bursting on fire thing is certainly interesting though! To be clear, I was planning for the blood to be used by a subrace of humans, as well as some wild animals. The humans are supposed to somehow get their normal blood changed into this (still figuring out how, magically or through some sort of risky transfusion?) Still, I could reign back the dosage needed for a "meltdown" to create more tension but have iron weapons have that deadly effect on the more monstrous animals of the world! $\endgroup$
    – Magbread
    Aug 18 '21 at 22:13
  • $\begingroup$ I did not get that these were humans! The thing about humans is we have iron in our blood all the time. A reaction vigorous enough to make you sick and specific to small amounts of metallic iron and not to Fe ion is pretty tricky. $\endgroup$
    – Willk
    Aug 18 '21 at 22:50
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    $\begingroup$ I was thinking the change would happen through exposure to excess of the special material in the bloodstream or transfusion of already changed blood, which would either result in death or to the body adapting and starting to use that method for oxygen transfer instead. It's pretty out there, which is why I'm just trying to get the gist of how it'd work chemically. So far I'm thinking it could be a chemical similar to Pyrite? I don't think I'll dive into chemistry in the story and explain exactly how it works. Handwaving it as an exotic/magical metal seems like the best option for now. $\endgroup$
    – Magbread
    Aug 18 '21 at 23:11

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