Close packing of anion layers

There are twice as many interstices between the sheets of anions as there are anions in a layer. The cations fit into the octahedral (sixfold coordination) or the smaller, tetrahedral interstices. For compounds with Fe ions, charge balance require- 

1) Lehrstiihlfur Bodenkunde,Technische Universitat Miinchen, 85350 Freising-Weihenstephan, Germany 

Compound Crystallographic system Space group Stacking of close packed anions Unit cell dimensions nm a b c Z 

Goethite and lepidocrocite have recently been moved from Pbnm to Pnma and from Cmcm to Bbmm, respectively. As a result of this change, the crystal planes and directions in this book are different from those in the 1 edition. 

The cations in these compounds are Fe and/or Fe . In iron oxides, Fe is always in the high spin (unpaired d electrons) state. As Fe with five d electrons has no crystal field stabilization energy (CFSE see Chap. 6), regardless of whether it is octa-hedrally or tetrahedrally coordinated, there is little preference for one or the other type of site. For Fe , on the other hand, CFSE is higher for octahedral than for tetrahedral coordination, so the octahedral coordination is favoured. 

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Both ionic and covalent natures of the Si-0 bond contribute to the preference for (Si04)4- tetrahedron formation in both crystalline and glassy silicas. In addition, each O anion is coordinated by two Si cations, corresponding to comer sharing of the oxide tetrahedra, preventing the close-packing of anion layers and resulting in relatively open structures [5],... 

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