Abstract

In pyroxenoids containing cationic hydrogen [pectolite, schizolite, serandite, unnamed Ca(Sc,Fe3+)H[Si3O9], synthetic Cd2NaH[Si3O9], nambulite, Li-hydrorhodonite, santaclaraite, and babingtonite] the identity periods of the silicate chains are approximately 0.25Å shorter than those of corresponding non-acid pyroxenoids with similar mean cation size (wollastonite, bustamite, rhodonite). This observation is interpreted as caused by the reduction of repulsive forces between [SiO4] tetrahedra of the silicate chains due to an increase of the average electronegativity of the cations when cationic hydrogen is present. This effect (which exists also in polyphosphates and probably polygermanates) is an extreme case of the general effect that anionic tetrahedral chains will be less stretched when the average electronegativity of the cations is higher, which compensates the valence of the chains. The theory explains why acid chain silicates are common in pyroxenoids but not observed in pyroxenes.

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