Magnetite octahedra

Magnetite Octahedra Intergrown octahedra (twins), rhombic dodecahedra, cubes, spheres, bullets... 

Fig. 4.26 Left Rounded magnetite crystals obtained by slow oxidation (over 150 days) of a 0.05 M FeCl2 solution at pH 11.7 and room temperature (Schwertmann Murad, 1990 with permission) Right Magnetite octahedra produced by oxidation of a 0.5 M FeS04 solution with KNO3 in 1.43 M KOH at 90°C (Schwertmann. Cornell, 2000 with permission).

Fig. 4.24 Crystal forms of magnetite, a, c) octahedron b) rhom-bodecahedron d) twin. (Kostov, 1968 with permission).

Nonpromoted catalysts rapidly lose their activity, particularly under severe conditions of operation. This is in no way contradicted by reports stating that reduced magnetites are good catalysts without addition of promoters. The author had made a spectrographic analysis of an octahedron crystal of Ural magnetite, and the examination disclosed a content of calcium, not less than 1 %, of titanium between 0.5 and 1 % and smaller contents of magnesium and aluminum, which proves that well-known promoters were present in the magnetite matrix. 

Fig. 8. Idealized crystal morpliologies of bacterial magnetite, (a) cubo-oetahedron Aquaspirillum magnetotacticum)-, (b and c) hexagonal prisms (coccoid and vibrioid cells) (d) elongated cubo-octahedron (wild-type cells).

Octahedral Octahedron, eight-sided (two pyramids base to base) Magnetite... 

Magnesioferrite (syn., ceylonite) (ICSD 49551 and PDF 36-398) MgFe,0, M= 200.00 12.15 wt.% Mg 55.85 wt.% Fe 32.00 wt.% 0 (Oxides and hydroxides) Cubic a = 838.3 pm Hlj, cF56 (Z=8) S.G. Fd3m Spinel type (Magnetite series) Isotropic nj3= 2.380 7.5 4520 Habit octahedron or massive. Color black. Diaphaneity opaque. Luster submetallic to metallic luster. Streak black. Geav e none. Fracture uneven. Ferromagnetic. Occurrence metamorphic rocks. 

There may be two cases when the metal ions fill in apexes of octahedron and tetrahedron. The first case has normal cation arrangement as character, where all the bivalent ions are located in A nodes, e.g. ZnFe204 belongs to this type. In the second case, the apexes of the octahedron are occupied with different valence ions. Magnetite has a so-called metamorphic spinel structme, and also there is a complex cation replaced spinel structure (Table 3.2). 

Ion exchanges between bivalent and trivalent iron cations located in the apexes of octahedron are easily carried out, which is the reason for the higher conductivity of oxides. Magnetite is hardly dissolved in a-Fe20s, but the utmost solubility increases with temperature, and the formed sosoloid can be regarded as submerged sosoloid. 

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