Diaspore structures

Certain iron oxides are isostructural with other metal oxides. Goethite, for example, is isostructural with diaspore (a-AlOOH) and hence, is sometimes referred to as having the diaspore structure. Iron oxides and their isostructural metal oxides are listed in Table 2.1. 

Fig. 3.1 Relationship between the unit cell volume of various MOOH minerals or synthetic compounds with the diaspore structure and the ionic radius of M ".

In our survey of octahedral structures in Chapter 5 we saw that double chains of the rutile type could be further linked by sharing vertices to form either 3-dimensional framework structures, of which the simplest is the diaspore structure, or a puckered layer as in lepidocrocite. These two structures are shown in perspective in Fig. 14.12. In Fig. 14.13(a) the double chains are seen end-on, as also are the c.p. 

Laubengayer and Engle state that crystalline Ga(OH) 3 can be prepared as follows a GaClg solution is precipitated at 90°C with NH3 solution, and the hydroxide is washed thoroughly and heated with 6 ml. of water in a 15- ml. autoclave for about 89 hours at about 167°C. This results in a very fine powder with a moisture content corresponding roughly to that for the trihydroxide and a characteristic, metastable structure, which slowly converts on heating (e.g., a total of 166 hours at 170°C) to the diaspore structure of the metahydroxide GaO(OH) (see below). 

The compound GaO(OH), with a diaspore structure, is obtained in slow precipitation of a sodium gallate solution by atmospheric COg, according to Bbhm and Kahan. 

Formula weight 102.73. The crystalline form dissolves in dilute mineral acids. Diaspore structure (EOg structure type). Heating in an autoclave above 305°C converts it to -Gsl O. ... 

Diaryl pyrrolopyrroles (DPPs), 79 441-443 commercial, 79 443t Diarylthallium(III) derivatives, 24 634 Diaspore, 2 345t, 347, 426 classification, 2 422 decomposition sequence, 2 392, 421 mineralogical and structural properties of, 2 423t... 

Francombe, M.H. Rooksby, H.P. (1959) Structure transformation effected by the dehydration of diaspore, goethite and -ferric oxide. Clay Min. Bull. 4 1-14... 

Lastly, the positions of hydrogen atoms have been located in a series of low-surface-area aluminium oxide-hydroxides. Hence, the long known solid-state structures of diaspore (a-AlOOH) and boehmite (y-AlOOH) have been recently probed using H CRAMPS [45] along with X-ray and neutron diffraction techniques [46]. For diaspore results point to the presence of Alaf/Xa-OH) groups with 6-fold and 4-fold coordination at aluminium and oxygen, respectively, while Al2(/x-OH) moieties with 6-coordinate aluminium and 3-coordinate oxygen are revealed in boehmite [46]. 

The structure of boehmite contains double sheets of octahedra with A1 ions at their centers, and the sheets themselves are composed of chains of octahedra. In diaspor the oxygens are in a hexagonal close packed layer those within the double octahedral layers in boehmite are in a cubic pacldng relationship. Luminescence center Cr characterizes steady-state spectra (Solomonov et al. 1994 Shoval et al. 1999). The natural boehmite and diaspor in our study consisted of twelve samples. The laser-induced time-resolved technique enables us to detect Cr emission centers (Figs. 4.62-4.63). 

Fiq. 12-12.—The structure of the diaspore crystal, A1H02. Oxygen atoms are at the corners of the octahedra and aluminum atoms at their centers. Hydrogen bonds are indicated by tubes. 

The reactions in Eqs. 5.1 and 5.2, taken in the broadest geochemical sense, refer only to the replacement of one ion by another existing in a solid structure. Thus, for example, Eq. 5.1 could be applied to provide a description of solid solution composition alternate to that in Section 3.3. Instead of basing the composition of Al-goethiteon the solid components diaspore and goethite, as in Eq. 3.41, one could describe a continuum of possible compositions by combining Eqs. 3.39a and 3.39b into the cation exchange reaction... 

There are two well-known oxide-hydroxides (AlOOH) with closely related structures diaspore and boehmite. Diaspore occurs in some types of clay and bauxite. It has been produced by the hydrothermal treatment of corundum, a-Al203. Whereas boehmite is characterized by cubic close-packing of the anions, diaspore has a hexagonal close-packed structure. This difference probably accounts for the direct thermal transformation of diaspore to corundum at relatively low temperatures (450-600°C). 

Figure 3 Structure of (a) diaspore and (b) boehmite. The aluminum atoms are octahedrally surroimded hy five oxide and one hydroxide anions. The hydrogen bonds are shown as double hues...

Figure 2.41. Comparison of the core structure of (a) siloxy- substituted alumoxane gels with (b) diaspore, and (c) boehmite minerals. The aluminum and oxygen atoms are shown in blue and red, respectively. Reproduced with permission from Chem. Mater. 1992, 4, 167. Copyright 1992 American Chemical Society.

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