Thanks to all again! Find below my answers/comments to all your replies. Colin Nave, I'll certainly collect higher resolution dataset to look for more diagnostic rings. Apo-ferritin xtallizes in the same conditions with the same cell (I know it from literature), I'll measure it too, to look for differences.
Jacob Keller, we loaded ferritin with Fe and Co and oxidized them obtaining CoxFe(3-x)O4/Fe2O3. About the brillant spots, by eye I attributed them to eventual ammonium sulfate xtals (2M in xtallization conditions) but it could be something else, compare to apo-ferritin will help. James Holton, I didn't think about it! Actually it's the same as nuclear spin in NMR: the orientation is random until an external magnetic field is applied. It would be very very interesting to collect diffraction under this condition, do you know an equipped beamline? Allister Crow, thanks for encouragement! I knew that someone had already studied it... your paper is very usefull. Myabe my case is different because I have Fe3O4 and an external magnetic field could induce order, I hope I can do this kind of experiment. John Hellywell, I'll certainly measure at higer resolution. Ferritin should be fully loaded since there are about 3500 metal atoms per protein shell. I'll keep you updated on my future results!
