Steven Dutch, Professor Emeritus, Natural and Applied Sciences, University of Wisconsin - Green Bay
The manganese pyroxenoids have kinked rather than straight chains and thus don't share the cleavage of pyroxenes. 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 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.
Created 15 October 2009, Last Update