Goethite, (FeO(OH)) and diaspore (AlO(OH)) are among the many hydroxides of iron and aluminum. Usually these minerals are so intermingled that we apply collective names to the mixtures: limonite for iron and bauxite for aluminum. In both cases, the oxygen-hydroxyl sheets form a hexagonal close-packed array. The iron and aluminum fill two thirds of the octahedral interstices, but in strips.
|An oblique view. In a polyhedral representation, strips of octahedra enclose "tunnels."|
|Top view. Note that the octahedra at the two different levels face opposite directions.|
In the ball model below, we see that alternating strips of cations (red and yellow) join the close-packed oxygen sheets (in shades of blue). There are no "tunnels" - there appear to be in the polyhedral representation only because vacant octahedra are not shown.
Created 6 March, 2002, Last Update 14 December 2009
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