Goethite and Diaspore Structure

Steven Dutch, Professor Emeritus, Natural and Applied Sciences, Universityof Wisconsin - Green Bay

Goethite, (FeO(OH)) and diaspore (AlO(OH)) are among the many hydroxides ofiron and aluminum. Usually these minerals are so intermingled that we applycollective names to the mixtures: limonite for iron and bauxite for aluminum. Inboth cases, the oxygen-hydroxyl sheets form a hexagonal close-packed array. Theiron 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 andyellow) join the close-packed oxygen sheets (in shades of blue). There are no"tunnels" - there appear to be in the polyhedral representation onlybecause vacant octahedra are not shown.

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Created 6 March, 2002, Last Update 31 May 2020