Goethite twinned

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. 4.10 Coethite twinning. Upper left Twins grown at pH 4 and 25 °C consist of two (a) or three armed (b) twin pieces (Schwertmann. Murad, 1983,with permission). Upperright Multidomainic star-like twin grown at [OH] = 0.3 ML" and 70°C with stirring (courtesy P. Weidler).Lower/ejt Schematic drawing of a twin-zone in a singly-branched goethite twin showing the lattice planes and the coherent and incoherent boundary (Maeda. Hirono, 1981, with permission).

Fig. 5-16. Large goethite twin with well developed 101 side and 210 terminal faees produeed from 2-line ferrihydrite in 0.3 M KOH at 70 °C in the pre-senee of 6 mol% V . Bar = 1 p,m. (Sehwertmann and Pfab, 1994, with permission).

Goethite Adcular Stars (twins), hexagons, bipyramids, cubes, thin rods... 

Epitaxial twins (Fig. 4.11) consist of a hematite centre with outgrowths of acicular goethite. As the structures of both goethite and hematite are based on an hep anion array, some of the interplanar spacings in the two compounds are similar and this fa-... 

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. 

Langmnir D (1971) Particle size effect on the reaction goethite = hematite + water. Am J Sci 271 147-156 Lee WH, Shen PY (1999) On the coalescence and twinning of cube-octahedral Ce02 condensates. J Crystal Growth 205 169-176... 

Figure 71. Sniall particles of twinned hematite formed in the electron microscope as a decomposition product of goethite 1215]...

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