Magnetite Fe

Maghemite (magnetite - hematite) has a structure similar to that of magnetite. It differs from magnetite in that all or most Fe is in the trivalent state. Cation vacancies 

Maghemite has a cubic unit cell the a value of which was first determined by Hagg (1935). The currently accepted value of a is 0.834 nm. Each cell of maghemite contains 32 0 ions, 21 /s Fe ions and 2 73 vacancies. Eight cations occupy tetrahedral sites and the remaining cations are randomly distributed over the octahedral 

1) In a normal spinel structure, the formula is XY2O4 with 8 ions occupying the tetrahedral sites and 16 ions occupying the octa- 

During dehydroxylation of lepidocrocite, the water molecules leave the lepidocrocite crystals causing the crystals to break up, so that only tiny crystals of maghemite form (see Chap. 14). Because the crystals are so small, the vacancies are not ordered, hence the XRD pattern shows no superstructure lines. The [001] and [110] directions of the maghemite crystals parallel the [001], and [100] directions, respectively of the lepidocrocite crystals. 

Although this compound has the nominal formula FeO, stoichiometric material cannot exist as a stable phase at low pressures or at pressures in excess of 10 MPa. The non-stoichiometry is accommodated by oxidation of a proportion of the metal ions and the creation of cation vacancies (Lindsley, 1976). 

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Caustic corrosion (gouging) occurs when caustic is concentrated and dissolves the protective magnetite (Fe O layer. Iron, in contact with the boiler water, forms magnetite and the protective layer is continuously restored. However, as long as a high caustic concentration exists, the magnetite is constantiy dissolved, causing a loss of base metal and eventual failure (Fig. 4). 

Conductive films such as magnetite (Fe,0 ) or mill scale on steel, and conductive nonmetals such as carbon can function as cathodes when in contact with anodes... 

Self-Test 7.23A What is the minimum temperature at which magnetite, Fe,04, can be reduced to iron by using carbon (to produce C02) ... 

Sugimoto and Matijevic (3) prepared uniform spherical particles of magnetite (Fe,04) as well by partial oxidation of ferrous hydroxide gel with nitrate. The uniform magnetite particles were obtained at a slight excess of Fe2+, and the mean size critically depended on the excess concentration of Fe2+ or pH. 

Haber-Bosch NH3 synthesis magnetite (Fe) h2, n2 nh3 fertilizer, gunpowder, explosives... 

The Ores of Iron. The chief ores of iron are its oxides hematite. Fe- Og, and magnetite, Fe O, and its carbonate side rite, FeCO.,. The hydrated ferric oxides such as limonite are also important. The sulfide pyrite, FeSo, is used as a source of sulfur dioxide, but the impure iron oxide left from its roasting is not satisfactory for smelting iron, because the remaining sulfur is a troublesome impurity. 

CR2 Olivine, serpentine, saponite, minor magnetite, FeS, pentlandite, pyrrhotite, calcite 2... 

Iron Hematite [Fe203], goethite [FeOOH], magnetite [Fe Fei+04] Iron and steel... 

Vanadium Magnetite [Fe +(Fe +,V " )204] in mafic-ultramafic intrusions Steel additive... 

FIGURE 20.1 Metals and their best-known minerals. Lithium is faund in spadumene (LiAISi20, and beryllium in beryl (see Table 20.1). The rest of the alkaline earth metals are faund in minerals that are carbanates and sulfates. The minerals for Sc, Y, and La are the phasphates. Same metals have more than one type of important mineral. For example, in addition to the sulfide, iron is found as the oxides hematite (Fe202) and magnetite (Fe O ) and aluminum, in addition to the oxide, is found in beryl (Be2Al2Si Ots). Technetium (Tcj is a synthetic element. 

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