Hibonite

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

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Hibonite is a rather uncommon mineral on earth but occurs in meteorites. Its nominal formula is Ca₃Al₁₂O₃₉, although there is widespread substitution for both the Ca and the Al.

Hibonite is a fairly ugly mineral on earth but tends to be a pretty blue in meteorites. As a refractory oxide, it is probably among the earliest mnerals to have condensed from the solar nebula, and is of interest because it has been studied for clues about early solar activity, particularly for content of noble gases emitted by the solar wind.

Because of the high amounts of aluminum and oxygen, one might expect similarities with corundum, but the calcium intoduces major complexities. Ths structure consists of stacks of sheets of aluminum octahedra, interspersed with layers of aluminum and calcium.

The aluminum octahedra layers consist of corundum-like sheets of AlO₆ octahedra (light blue) with intermediate layers of AlO₆ octahedra and AlO₄ tetrahedra (purple). Between these "sandwiches" are layers with CaO₁₂ polyhedra (orange), pairs of AlO₆ octahedra (dark blue) and AlO₅ bipyramids (green).

Above is one of the corundum-like aluminum oxide layers

This is an intermediate layer. Octahedra are purple, tetrahedra dark blue.

Above is a corundum-like layer with an octahedral-tetrahedral layer atop it. There would be a third octahedral layer on top of these layers.

Above is a view looking down onto a calcium-aluminum layer. Octahedra are blue, bipyramids are green (looking down their threefold axes) and calcium polyhedra are orange.

Above are views of the calcium coordination polyhedra. They're cuboctahedra, cut along a central plane and with one side rotated. The geometric term is triangular orthobicupola.

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Created 16 April 2019, Last Update