Lepidocrocite colour

The orange coloured lepidocrocite, y-FeOOH, is named after its platy crystal shape (lepidos scale) and its orange colour (krokus = saffron). It occurs in rocks, soils, biota and rust and is often an oxidation product of Fe ". It has the boehmite (y-AlOOH) structure which is based on cubic close packing (ccp) of anions. 

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. 6.10 Left Placement of various synthetic goethites (G), lepidocrocites (L) and hematites (H) in CIE L a b colour space. Right Development of a and b in the CIE L a b colour space during the transformation of ferrihydrite (common starting point) to goethite or hematite, respectively (Nagano et al., 1994, with permission).

Fig. 6.11 Munsell colours of synthetic hematite, ferrihydrite, lepidocrocite, and goethite samples (Courtesy A.C.Scheinost Scheffer Schachtschabel,...

YR with values >6 for lepidocrocite and <6 for ferrihydrite. These ranges are caused mainly by a variation in crystal size. Upon cementation into dense, hard masses all these colours darken (lower value) and reliable identification requires grinding. 

Plate 16.1 a) Soil profile coloured by goethite (Ochrept, France), b) Soil profile coloured by hematite (Ultisol, Brazil), c) Soil profile coloured by lepidocrocite (Aquept, South Africa), d) Ferrihydrite formation by oxidation of Fe " in water seeping out of a Cley. 

Lepidocrocite is found in both natural and synthetic iron oxide pigments. Colour changes with increasing particle size from yellow to orange. David et al. (2001) have identified lepidocrocite as a component of yellow ochres q.v.) used at Tell el Amarna. Wainwright et al. (2002) and Watchman et al. (1993) have detected it from rock paintings in Argentina and North Queensland, Australia, respectively. 

The name lepidocrocite is derived from the Greek, meaning saffron-coloured flake . 

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