Layer structures with corner-sharing octahedral units

This compound is isostructural with brucite (Mg(OH)2) and Cdl2. The unit cell is hexagonal with a = 0.3258 nm and c = 0.4605 nm. The structure consists of sheets of corner-sharing, trigonally distorted Fe(OH)6 octahedra stacked along the [001] direction. The Fe" ions occupy only half the available octahedral interstices and this results in a structure in which each filled layer of sites alternates with an empty layer of sites. The OH radical behaves as a single entity. Amakinite is a rare mineral of the composition (Fe,Mg,Mn)(OH)2, also with brucite structure. Fe(OH)2 is readily oxidized by air and even by water, upon which the colour changes from white to brownish. The structure can be maintained up to a replacement of one tenth Fe" by Fe " (Bernal et al., 1959). 

Biissem (B12) determined the approximate crystal structure of C4AF. Subsequent determinations or refinements were reported for preparations with. Y = 0 (C F) (B13,C4,B11),. v = 0.285 and 0..36 (C2) and, v = 0.5 (C3). Fig. 1.8 shows the structure for the compositional range with 0.33 <. Y < 0.7. It is built from layers of corner-sharing octahedra similar to those in perovskite (CaTiOj), alternating with layers composed of chains of tetrahedra, together with Ca " ions. The layers are perpendicular to the h axis and the chains are parallel to c. The composition of an individual octahedral layer in the ac cross-section of the unit cell is MiO. and that of an individual chain is where M and T denote octahedral and... 

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