Ferrosilite III: A Triclinic Pyroxenoid-Type Polymorph of Ferrous Metasilicate

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Abstract
The relationships between the triclinic unit cell of ferrosilite III and those of pyroxmangite, rhodonite, and wollastonite lead to the hypothesis that this polymorph of ferrous metasilicate has a pyroxenoid-type crystal structure with single silicate chains that repeat after every nine silicon tetrahedra. Vector relations between the triclinic cell and an apparent pseudomonoclinic cell support this hypothesis. Although the proposed silicate chain has a longer repeat length than any now known, it represents a logical extension of those found in other pyroxenoids and suggests that even longer repeat lengths may yet be found among phases with pyroxene compositions.