Hematite rhombohedral crystals

Al-hematites formed slowly from Al-ferrihydrite at 25 °C over 20 years, varied between rhombohedra at low substitution and multidomainic ellipsoids ca. 100 nm across with a grainy interior at higher substitution (Al/(Al-rFe) = 0.15) (Fig. 4.20e f) (Schwertmann et al. 2000). Allophane as a source of A1 had the same effect (Schwert-mann et al. 2000a). Mn substituted hematites grown from ferrihydrite were ellipsoidal in the presence of oxalate and platy in the presence of NaHCOa buffer (Cornell Gio-vanoli, 1987 Cornell et al., 1990). Cu substituted (0.09 mol mol" ) hematite grows as large (0.2 pm) rhombohedral crystals the crystal faces are most probably 102 or 104 (Fig. 4.20d) (Cornell Giovanoli, 1988). 

Hematites grown hy forced hydrolysis of acidic Fe " solutions at elevated temperatures also show a range of crystal morphologies. The type of anion, the acidity and the presence of additives appear to be main factors that influence the shape (Matijevic Scheiner, 1978 Kandori et al., 1991 Bailey et al., 1993). Additives appear to act by adsorbing on specific planes of the growing crystal. At close to 100 °C, rhombohedral crystals, 50-100 nm in size, sometimes showing intergrowths (stepped appearance)... 

Fe203, known as ferric oxide or hematite, possesses rhombohedral crystal lattice... 

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. 

Fig. 4.19 Upper Crystal forms of platy and rhombohedral hematite (Courtesy H. Stanjek). Lower Micaceous hematite from Western Australia (Courtesy R. Giovanoli, magnification 250x).

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