Originally Posted by bill smith
MIO - Micaceous Iron Oxide - Fe2
is grey and shares the same characteristics under the SEM. If you look closely at this picture on the left.
Which I just love showing.
You can clearly see a thin dark greyish material flaking off the surface of the steel. It's highly consistent with Jones' macrographs and would be the correct composition because Fe3
is known as black rust. This is why it's important to do x-ray powder diffraction or XRD tests in order to determine the crystallography of the sample which will then give you the exact compound.
Fe304 has an isometric - spinnel crystal structure - http://www.reade.com/Products/Minera...magnetite.html
explains properties and why it's magnetic.
Fe2O3 is usually rhombohedral (rhomboidal) and non-magnetic. There is another possibility because there is another form of Fe2O3 called Maghemite, y-Fe2O3. http://webmineral.com/data/Maghemite.shtml
and this has an Isometric - Tetartoidal crystal structure and is magnetic.
A great site for explaining crystal shapes.
EDS data will only give you the elements.
In light of this photo I'm adding to my theory that the "gray layer" is most likely this "black scale" (with the continuing possibility of it being MIO). The black layer is the result of either
12 FeO(OH) --> 4 Fe3O4 + 6 H2O + O2
6 Fe2O3 --> 4 Fe3O4 + O2
Fe3O4 is haematite and is magnetic where as Fe2O3 isn't. I've read more about Jones and how they separated out this "thermite". They used a magnet.
2Al + Fe2O3
there is no magnetic material there (unless using maghemite) so why are they using that method for separation?
What they will do is pick up anything with Fe3O4 in. The red paint with Fe3O4 attached will be separated. The red paint contains Fe2O3 (in rhomboidal crystalline form, bright white in the SEM pictures).
The more and more I run this through my brain the more and more the simple things come to the surface. Rather than looking at and getting carried away with the pretty pictures and spectra it's always worth gathering more information before launching into the analysis.
I'd also like to thank Bill Smith, because without reading his post I wouldn't have necessarily gone back to basics - thanks truther!