Goethite faces

Fig. 4.2 Large goethite crystal formed hydrothermally in a quartz geode terminated by (210) faces (Courtesy R. Giova-noli).

Fig. 4.3 Common acicular, polydo-mainic goethite c stals terminated by (210) faces (see Schwertmann etal. 1985).

Fig. 4.4 Large goethite twin bounded by (101) side faces and (210) terminal faces (Schwertmann Pfab, 1994, with permission Schwertmann Cornell, 2000, with permission).

Fig. 4.5 High resolution electron micrograph of synthetic goethite crystals cut perpendicular to the needle axis [010]. The crystals are bounded by 101 faces. The large crystal contains faults and some of the smaller, fault-free crystals show ca. 1 nm lattice fringes corresponding to the c -parameter of the unit cell (0.9956 nm) (Schwertmann, 1984, with permission, courtesy H. Vali, Schwertmann Cornell, 2000, with permission).

Fig. 4.6 High resolution electron micrograph of natural goethite a) Diamond-shaped cross sections of domains running along [010] and bounded by 101 faces. Lattice fringes correspond to the c -parameter. b) Higher magnification shows the a fringes (ca. 1 nm) and structural distortions. (Smith Eggleton, 1983 with permission courtesy R.A. Eggleton).

The commonest habits for hematite crystals are rhombohedral, platy and rounded (Fig. 4.19). The plates vary in thickness and can be round, hexagonal or of irregular shape. Under hydrothermal conditions, these three morphologies predominate successively as the temperature decreases (Rosier, 1983). The principal forms are given in Table 4.1. Hematite twins on the 001 and the 102 planes. The crystal structure of hematite has a less directional effect on crystal habit than does that of goethite and for this reason, the habit of hematite is readily modified. A variety of morphologies has been synthesized, but in most cases, the crystal faces that enclose the crystals have not been identified. 

Tab. 10.1 Hydroxyl density on difFerent goethite and hematite faces (Barron and Torrent, 1996, with permission).

Fig. 10.2 Surface hydroxyl configuration on the goethite 001, 101, 100 and 210 faces. Distances of O and Fe ions to the projection plane are indicated next to the corresponding row of ions. Rows of singly, doubly, and triply coordinated O ions are indicated as S, D, and T, respectively. Solid line rectangles represent the two-dimensional (surface) unit cell. Dotted-line rectangles show contiguous singly coordinated hydroxyls (Barron and Torrent, 1996, with permission).

Fig.n.4 S urface complex oftetrahedrally coordinated phosphate (hatched) through two neighbouring, singly coordinated Fe-OH groups on the (101) face of goethite (Stanjek, unpubl.). 

It is obvious that the standard deviation for hematites is greater than that for goethites, mainly due to the greater variety of crystal faces. Al-for-Fe substitution did not directly influence the P adsorption for either synthetic goethites or hematites the surface area tends to increase with rising A1 incorporation and this, in turn, increases adsorption/unit weight (Ainsworth et al., 1985). 

Fig. 12.17 Dissolution morphology of synthetic 210 faces, c) Monodomainic Al-goethite (Al/ goethite crystals after partial dissolution in 6 M (Fe-tAI) = 0.097 mol mol" ) with cavernous dis-HCI at 25 °C. a) Pure goethite with dissolution solution at crystal edges (Schwertmann, 1984 a, along domain boundaries, b) Pure goethite with with permission), diamond-shaped dissolution holes bounded by...

Barron,V. Torrent, J. (1996) Surface hydroxyl configuration of different crystal faces of hematite and goethite. J. Colloid Interface Sci. 177 407-411... 

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