Roaldite is a mineral phase (possibly) described in steel but (rarely) discovered in iron meteorites. The structure looks like fcc iron containing in the center an N atom.
Is there anything known about the formation is steel which is perhaps applicable on meteorites?
Don't know about the natural presence of Fe4N and its meteorite relation, but yes it has a fcc type of structure. I know about it from my nitriding experiments of Fe (and its alloys). During nitriding it is formed at a certain temperature and nitriding potential range ( can check 'Lehrer diagram' for better understanding) and also known as gamma prime phase. At lower nitriding potential Nitrogen introduced by the nitriding treatment remain in solid solution whereas at higher potential (above gamma prime's limit) another type of iron nitride form, Fe2-3N (epsilon, hcp).
If considering the same reactions to occur at meteorites i.e presence of Fe, N and sufficient temperature, it is possible to have this phase.
@ A. Singh Yadav: How big are your Fe4N precipitates, and do they show any orientation relationship to the surrounding ferrite?
Although they starts from nano size, but as the time progresses during the continuous treatment they form a layer at the surface (few to 10s of micron thick). Different orientation relationships were observed for Fe4N (formed in ferrite matrix) by different treatment and different conditions, like Fe4N formed at above 500C a N-W OR with ferrite matrix and for
https://link.springer.com/content/pdf/10.1007/BF03398106.pdf
Article Microstructural investigation on the precipitation of α″ nit...
http://articles.adsabs.harvard.edu/full/seri/LPSC./0012/0001343.000.html
Article Der kristallographische mechanismus von Phasenumwandlungen i...
Article A TEM study of α″Fe16N2 and γ′Fe4N precipitation in iron-nitrogen
You can search the work of Prof. (Retd.) E. Mittemeijer's group from MPI Stuttgart. They have done a lot in nitriding.
I once tried to calculate the thermodynamics for the much more common carlsbergite (CrN). My conclusion was that it forms at quite low temperature. I would think that applies to roaldite as well. The thermo and diffusion data should be available to calculate this. What may be a bit uncertain is whether the parent material has lost dissolved N (prior to or concurrent with nitride formation) during cooling.
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