Goethite unit cell edge

Fig. 3.6 Relationship between the unit cell edge length b of synthetic goethites and the level of structurally incorporated Al ", Mn ", Cr ", Co " and V " (data from Schulze Schwertmann, 1984 Stiers Schwertmann, 1985 Schwertmann et al., 1989 Gerth, 1990 and Schwertmann Pfab, 1994, respectively).

Fig. 3.7 Top Relationship between the unit cell edge length a of synthetic goethites and various structurally incorporated metals. Bottom Rate of change of a per mol of substituted metal (= slope of the upper curves) vs. ionic radius of the respective metal cations (Gerth, 1990, with permission).

Lepidocrocite (Greek lepidos = scale, flake and krokoeis = saffron-coloured) is iso-structural with boehmite (Tab. 2.1). Unlike goethite and akaganeite which have a turmel structure, lepidocrocite is a layered compound. The orthorhombic unit cell contains four formula units and has the edge lengths, a = 1.2520(6) nm h = 0.3873(2) nm and c = 0.3071(6) nm (Ewing, 1935 Oles et al., 1970 Christensen Norlund-Christensen, 1978). 

Fig. 3.3 Relationship between the edge length b of the unit cell and the AI-for-Fe substitution of synthetic goethites.

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