Rhodonite and Pyroxmangite

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

The manganese pyrozenoids have kinked rather than straight chains and thus don't share the cleavage of pyrozenes. Blame it on calcium. Manganese is a well-behaved mid-sized cation and, if it were abundant, would probably form a pyrozene similar to enstatite. There is, in fact, an uncommon magnesium-manganese pyroxene, kanoite.

However, rhodonite typically contains calcium. If there were enough to form a diopside, again we'd have a straightforward manganese pyroxene. But the calcium only amounts to a few per cent, so it tries to fit into the chains of manganese octahedra, distorting them and creating sinuous channels between the chains.

The silica chains wind down the channels, linking them. The apices of the tetrahedra link to the centers of sheets in the next layer. Note that the tertrahedra, somewhat unusually, share edges with the Ca-O octahedra. The repeat interval on the silica chains is five tetrahedra.

Pyroxmangite

Pyroxmangite is the high pressure polymorph of rhodonite. The chains are pushed together so the Ca-O octahedra link across the open channels and share an oxygen atom.

The silica chains wind down the channels, linking them. The apices link to the centers of chains in the next layer. Unlike rhodonite, the repeat interval is seven tetrahedra rather than five.

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Created 10 Oct 1997, Last Update 16 December 1999

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