8-FeOOH colour

The crystallography of iron oxides is complex because there are two oxides, FeO and Fe203, as well as a-FeOOH, which can occur in mixed crystals. Kittel [34] has given a full account of the chemistry and Feitknecht [35] has discussed the theory of the colours of these pigments. 

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. 

Akaganeite, P-FeOOH, is named after the Akagane mine in Japan where it was first discovered (Mackay, 1962). It occurs rarely in nature and is found mainly in Cl-rich environments such as hot brines and in rust in marine environments. Unlike the other FeOOH polymorphs, it has a structure based on body centered cubic packing of anions (bcp) (hollandite structure) and contains a low level of either chloride or fluoride ions. It has a brown to bright yellow colour. 

The process involves seeding tanks containing FeS04 7 H2O solution, alkali and metallic iron with FeOOH, and aerating the stirred suspension. The seeds, which are very fine, are precipitated from Fe" solution at 20-50 °C (depending on the colour required), i. e. 

Watson, I. (1979) Iron oxide pigments. Colour-frd competitors between natural and synthetic. Minerals in Industry, Aug., 43—51 Watson, J.H.L., Cardell, R.R.Jr. Heller, W. (1962) The internal structure of colloidal crystals of P-FeOOH and remarks on their assemblies in Schiller layers. J. Phys. Chem. 66 1757-1763... 

The products are silicic acid (H4Si04 see below) and colloidal hydrated iron oxide (Fe(OH)3), a weak alkali, which dehydrates to yield a variety of iron oxides, for example Fe203 (haematite—dull red colour) and FeOOH (goethite and lepi-docrocite—yellow or orange-brown colour). The common occurrence of these iron oxides reflects their insolubility under oxidizing Earth surface conditions. 

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