Lepidocrocite and Boehmite 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 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 cations form octahedra arranged in corrugated layers.

In the oblique view at left, octahedra range in color from yellow in front to dark blue at rear. Hydroxyl ions hold the sheets together through hydrogen bonding, shown schematically by red lines. This weak bonding gives these minerals perfect cleavage parallel to the sheets, except that the individual crystals are usually microscopic.
The diagram at left is a top view of the structure. The top layer of octahedra is in green, the bottom in blue. Octahedra at the tops of corrugations are lighter. Hydrogen bonds are shown in red.
The diagram at left is a side view of the structure. Octahedra are in green and blue. Hydrogen bonds are shown in red.

  The view at left looks at the close-packed layers.

The oxygens within each zigzag sheet (in this view they are steps) are cubic close-packed, but the packing does not continue from sheet to sheet.

Hydrogen bonds are shown schematically in red.

The view at left shows two sheets. Oxygens are in shades of blue, cations in red and green, and hydrogen bonds in red.

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Created 6 March, 2002, Last Update