Hematite hexagonal plates

Hematite Hexagonal plates Spindles, rods, ellipsoids, cubes. 

Morphologies of synthetic hematite include plates and discs, rods, spindles, spheres, ellipsoids, double ellipsoids, rhombohedra, stars, cubes and peanuts. In the absence of additives, hexagonal plates, which are often rounded, and rhombohedra predominate. Each morphology can be obtained by more than one synthesis route. Two common ways of producing idiomorphic hematite crystals in aqueous systems... 

Fig. 4.20 Synthetic hematites grown from ferrihydrite at temperatures <100°C (Schwertmann, unpubl.) a) Hexagonal plates grown at pH 7 and RT acicular crystals are goethite b) Laths grown at pH 11 and 80 °C in the presence of 2.5 10 M citrate (see Schwertmann et al., 1968). The fine granular material is unreacted ferrihydrite c) Framboids grown at pH 6 and 70°C in the presence of 2 10 M oxalate (see Fischer, ...

Fe(OH)2 exists as hexagonal plates as do the green rusts (Feitknecht Keller, 1950 Bernal et ak, 1959). The basic morphology of wiistite is cubic, but this compound is frequently obtained as very irregular particles. It is formed as irregular rounded crystals 20-100 (xm across by reduction of hematite with H2/H2O at 800 °C (Moukassi et al., 1984). 

Hydrothermal transformation of various Fe oxides. Ferrihydrite (2-line), lepidocrocite, akaganeite and goethite (if poorly crystalline) can be converted to large (1-3 am) hexagonal plates of hematite if kept under water in a teflon bomb at 180 °C for 10 days. 

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. 

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