Goethite and Diaspore Structure

Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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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.

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Created 6 March, 2002, Last Update 14 December 2009

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