Pyroxenoids

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Single-chain silicates like pyroxene, but the tetrahedra composing chains are rotated and twisted. Octahedrally coordinated cations occur between chains as in pyroxenes. The interval of repetition is different for each pyroxenoid (below). The twisting results in lower symmetry than pyroxenes (all pyroxenoids are triclinic) and a splintery cleavage and sometimes fibrous habit.

The most common pyroxenoid is wollastonite (CaSiO3), which has Ca2+ in irregular octahedral coordination linking the chains. It is common in metamorphosed limestones. Other pyroxenoids include bustamite, (Mn,Ca,Fe)SiO3, with a repeat of ~14 Å, rhodonite, MnSiO3, with a repeat of 12.5 Å, and pyroxmangite, (Mn,Fe)SiO3, with a repeat of ~17 Å. There is significant chemical variation and solid solution in the pyroxenoids, except for wollastonite, which is usually very close to pure in composition.


Some or all content above used with permission from J. H. Wittke.