Abstract

The single-chain silicate structures of clinopyroxene, bustamite, and pyroxenoids are compared. The possible types of transformations between these structures are then characterized in terms of the change in lattice type accompanying the structural transformation. Four types of transformation are shown to occur; these may be divided into two groups. Observations made by high-resolution transmission-electron microscopy on partially transformed samples show that the mechanism of transformation between clinopyroxene and the pyroxenoids is one of glide of line defects on {001} of pyroxenoid and on {111} of clinopyroxene. The structural reorganization associated with the passage of such a line defect shows that it is not truly a partial dislocation, but that various dilferent displacements of atoms occur in the core of the defect.

Comparison of the structure of bustamite with that of the pyroxenoids shows that with the cell setting in which the latter possess a C-face-centered lattice, bustamite has a lattice that is best described as an interleaved complex of two F lattices. This indicates that the relative positioning of the silicate chains in bustamite is different from that in the pyroxenoids or clinopyroxene. Consequently, inversions in the solid state between bustamite and either clinopyroxene or pyroxenoid must proceed by mechanisms that carry out the inversions in two steps. Observations made by high-resolution transmission-electron microscopy on partially inverted samples indicate that the inversions from bustamite proceed by the propagation through the structure of line defects with Burgers vectors of ½[010] on the {102} planes (A1 cell) which create a wollastonite-like intermediate structure. This intermediate structure subsequently inverts to either clinopyroxene or pyroxenoid. The initial stage of the inversions in the opposite direction is shown also to be the creation of this wollastonite-like structure, probably by the glide of line defects on the {001} planes of pyroxenoid or the {111} planes of clinopyroxene.

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