Hematite structural

The structure of ferrihydrite has been the object of numerous studies in the past and several different structures have been proposed. The main difficulty affecting elucidation of the structure is the low degree of order. The original models of Towe and Bradley (1967) and Chukhrov et al. (1976) are based on XRD data and involve a defective hematite structure based on an hep array of anions with vacant Fe sites and a considerable amount of water. The Fe ions are distributed randomly over the interstices and there is more OH and H2O and less Fe in ferrihydrite than in hematite, i. e. there is a lower Fe/O ratio (< 2/3). 

Structural relationships exist between certain planes in the hematite structure and those in other iron oxides, namely magnetite and goethite (Tab. 2.6). There is, for example, a relationship between the (111) plane of magnetite and (001) plane of hema-... 

Galvez et al. (1999) demonstrated that phosphorus up to a P/Fe mol ratio of 0.03 mol mol , can be incorporated into the hematite structure by heating P-con-taining 2-line ferrihydrite. Support for structural incorporation comes from a higher unit cell c (1.3776 => 1.3824 nm), IR-stretching bands of P-OH, a lowered intensity ratio of the XRD 104/113 lines and congruent release of Fe and P upon dissolution. 

The transformation of ferrihydrite to hematite by dry heating involves a combination of dehydration/dehydroxylation and rearrangement processes leading to a gradual structural ordering within the ferrihydrite particles in the direction of the hematite structure. This transformation may or may not be facilitated by the postulated structural relationship between the two phases. EXAFS studies have shown, for example, that some face sharing between FeOg octahedra, characteristic of hematite, also exists in 6-line ferrihydrite (see chap. 2). 

The only stable oxide lattices that have so far been of value as mixed metal oxide pigments are those with spinel, rutile, and hematite structures. These lattices possess not only good thermal and chemical stability, but also have a high refractive index which is important for good optical pigment properties. 

Transparent yellow iron oxide has the a-FeO(OH) (goethite) structure on heating it is converted into transparent red iron oxide with the a-Fe203 (hematite) structure. Differential thermogravimetric analysis shows a weight loss at 275 °C. Orange hues develop after brief thermal treatment of yellow iron oxide and can also be obtained by blending directly the yellow and red iron oxide powders. 

The hematite structure appears to violate Pauling s third rule. Explain how this is done without excessively destabilizing the structure. 

Category III. Corundum-Hematite Structure, / (AhOs-FeiOs)... 

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