Magnetite electron delocalization

In acid media (pH 2) magnetite crystals ca. 10 nm across transform topotactically to maghemite via an adsorption reaction which traps mobile electrons from the bulk material and reduces interfacial Fe the Fe ions that form are selectively leached into solution (Jolivet Tronc, 1988). Electron delocalization also induces ferrihydrite in contact with small magnetite particles to transform into a spinel layer (Belleville etal., 1992). 

Just as vivianite is regarded as the simplest example of a mineral with isolated clusters of Fe2+-Fe3+ octahedra showing IVCT transitions, so too is magnetite considered to be the classic example of a structure-type with infinite chains of Fe2+ -Fe3+ octahedra exhibiting electron delocalization. Magnetite, Fe304 or Fe2+Fe3+204, has the spinel structure illustrated in fig. 4.18 with an inverse cation distribution ( 6.4). Thus, half the Fe3+ ions occupy isolated tetrahedral sites,... 

Opacity of mixed-valence minerals. The opacities of many end-member Fe2+-Fe3+ oxide and silicate minerals result from electron hopping between neighbouring cations when they are located in infinite chains or bands of edge-shared octahedra in the crystal structures. Opaque minerals such as magnetite, ilvaite, deerite, cronstedtite, riebeckite and laihunite owe their relatively high electrical conductivities to thermally activated electron delocalization, contributing to intermediate valence states of iron cations which may be detected by Mossbauer spectroscopy. 

---