Table 1.

Ferric iron content in Prussian blue in relation to other iron-oxide minerals and iron-oxyhydroxides found in tissues, as calculated from chemical formulae and volumetric mass density.

Compound	Chemical formula	Density [g/cm3]	FeIII conc [mol/cm3]
Prussian blue (soluble)a Prussian blue (insoluble)b	KFeIII[FeII(CN)6] FeIII4[FeII(CN)6]3 x H2O (x = 14–16)	1.45 1.72–1.78	4.71 6.35
Magnetitec	Fe3O4 FeIII2O3 FeIIO	5.21	45.0 (total Fe: 67.5)
Maghemited	γ-FeIII2O3 FeIII2O3 FeIII2/3O	4.80	61.5
Hematitee	α-FeIII2O3	5.27	66.0
Ferrihydritef,g	FeIII8.2O8.5 (OH)7.4 FeIII10O14(OH)2	4.3 4.9	47.0 60.0

^aKeggin & Miles 1936⁴⁹, ^bBuser et al. 1977⁵⁰;

^c,dMagnetite and its fully oxidized form, maghemite, are strongly ferrimagnetic minerals. The oxidation of magnetite to maghemite produces one vacancy at every third of the former iron(II) sites in the lattice, hence the lower density compared to magnetite. To emphasize the relationship between maghemite and magnetite, the chemical formula of maghemite can also be written as Fe^III₂O₃ Fe^III_2/3O or in sum, Fe_8/3O₄.

^e,f,gHematite and ferrihydrite (for structural details see Michel et al. 2007, 2010^54,55) are weakly ferrimagnetic minerals. In the biological iron storage protein ferritin, iron is typically stored in the form of a nanocrystalline core of ferrihydrite or hematite, sometimes of a magnetite-like phase (magnetite or maghemite), at least in human brains⁵⁶. Magnetic data of horse spleen ferritin can be explained by a combination of ferrihydrite and a variable but small amount of strongly magnetic phase like magnetite or maghemite⁵⁷.