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Space bodies like planets, asteroids and comets are formed by the remaining materials of stars explosions. Therefore they are usually made up of several elements.
There is the possibility that a particular element is prevalent. In my story I want to have a body made of >90% Fe, that will then be used as a orbiting mine.
If a body like this can exist, can it be big enough to be used as a mine?
You might be able to hand-wave this, but it's really really far-fetched.
Imagine a "smallish" supergiant blue star, ten times the mass of the Sun, merrily fusing hydrogen to helium, helium to carbon, neon, oxygen and so on. And at the center of it all, a growing core of unfusionable iron.
Normally this setup will lead to a supernova, and the force of the explosion would throw the now gaseous iron around.
But here, a freak event happens: a dense, compact degenerate star passes close enough and slowly enough to briefly create a contact binary pair. The Roche lobes are matched in such a way that most of neon, carbon and helium shells are stripped away from the large star, falling sharply against the smaller star and triggering a series of catastrophic explosions (a supercataclysmic variable). The end result of the process sees an asymmetric planetary nebula being thrown away, similar to what happens in some common envelope scenarios, a large part of the supergiant's core disrupted and the rogue star being flung away, momentarily rejuvenated by the light elements influx.
The remains of the large star collapse, but there is no longer sufficient energy to trigger a supernova. About half a solar mass of iron core is left. After the shock settles down, the star remnant is unable to sustain fusion and starts fizzling out.
A couple of million years later, you have a slowly cooling large object made of >90% iron, with a thin atmosphere of oxygen and neon, and a crust of carbon and iron compounds, much larger than Jupiter, with a surface gravity above 70 G (I made the calculations without considering that iron density at the center of such an object would be much greater than at normal temperature and pressure. So chances are that the radius is much less and surface gravity proportionally higher).
Or, possibly, the aborted supernova explosion might have lead to the birth of several large planetoids or "droplets" of iron, and some of them might have been slingshotted away from the primary. This makes for much smaller, but still enormous (think planet-sized) "blobs" of much purer iron, with a much lower surface gravity. These would be proportionally more exploitable.
For a naturally occurring body, the limit is probably a few hundred atoms, which is unlikely to be large enough for your purposes. Only 92% of iron is Fe-56, and it's hard to envisage a natural process that might preferentially select Fe-56 and not the other stable (or extremely long-lived) isotopes.
The exception to this is if you're willing to go into the very far future, when other isotopes have had time to tunnel into Fe-56. But we're talking further into the future than any SF has ever gone. https://en.wikipedia.org/wiki/Iron_star
Long ago, I was a tour guide in Ford's (Detroit) Rouge campus. We toured bus loads of tourists through the glass, rolling-steel, and assembly plants (the Foundry was too dangerous). In the rolling steel mill, they'd reheat a billet of steel and then roll it into sheets. I've forgotten how much one weighed, but it had to be more than 3 tons, 7 or maybe 15 comes to mind, but its been too long. The tour passed directly above the glowing red-hot ingots as they were rolled thinner and thinner. The heat felt like you were too close to a campfire. Ford had to install a barrier above the rolls because there were a couple of wise-guys who tossed pennies onto the sheets to watch them melt and get smushed into the steel, making a long streak. Only problem with this was that that small amount of copper totally ruined the billet. So little that you'd think it would be negligible, but it wasn't. My point is that 90% iron may or may not be useful; it depends on what else is present (even in trace amounts). To answer your question, I don't think we understand solar system formation well enough to answer your question. My best guess is that the degree of separation you want must occur inside a planetary body, and so finding a chunk of it would require the break-up of said body. I see no difficulty with that - the asteroid belt was at one time believed to be the residue of a collision, and while current theories don't use that idea, a collision like the one between Earth and Theia (hypothetical) certainly could lead to small chunks of debris in orbit. I'd say the "hand-waving" would have to be why such bodies are rare (in such a scenario). My second suggestion would be that the main bodies (both) escaped the solar system, leaving only random fragments...although now that I think about it perhaps them being swept up into some gas giant might make more sense, IDK.
Apparently it can get rather big.
Recently published on Science
We report the detection and characterization of the USP planet GJ 367b using high-precision photometry and radial velocity observations. GJ 367b orbits a bright (V-band magnitude of 10.2), nearby, and red (M-type) dwarf star every 7.7 hours. GJ 367b has a radius of 0.718 ± 0.054 Earth-radii and a mass of 0.546 ± 0.078 Earth-masses, making it a sub-Earth planet. The corresponding bulk density is 8.106 ± 2.165 grams per cubic centimeter—close to that of iron. An interior structure model predicts that the planet has an iron core radius fraction of 86 ± 5%, similar to that of Mercury’s interior.
The planet probably formed as a consequence of some large impact which stripped the original body from most of its mantle, leaving only the core orbiting the star.
