Mechanical twinning in diopside Ca(Mg,Fe)Si2O6: Structural mechanism and associated crystal defects

Abstract

iopside twins mechanically on two planes, (100) and (001), and the associated macroscopic twinning strains are identical (Raleigh and Talbot, 1967). An analysis based on crystal structural arguments predicts that both twin mechanisms involve shearing of the (100) octahedral layers (containing Ca 2+ , Mg 2+ and Fe 2+ ions) by a magnitude of c /2. Small adjustments or shuffles occur in the adjacent layers containing the [SiO 4 ] 4− tetrahedral chains. While the (100) twins are conventional with shear parallel to the composition plane, this analysis predicts that (001) twins form by a mechanism closely related to kinking. A polycrystalline diopside specimen was compressed 8% at a temperature of 400° C, a pressure of 16 kilobars, and a compressive strain rate of about 10 −4 /s. Transmission electron microscopy on this specimen has revealed four basic lamellar features: 1) (100) mechanical twin lamellae; 2) (100) glide bands containing unit dislocations; 3) (001) twin lamellae; 4) (101) lamellar features, not as yet identified. The (001) twins often contain remnant (100) lamellae of untwinned host. Twinning dislocations occur in these (100) lamellae and in the (001) twin boundaries with very high densities. Diffraction contrast experiments indicate that the twinning dislocations associated with both twin laws glide on (100) with Burgers vector b = X [001] where X is probably equal to 1/2 on the basis of the structural analysis. Parallels are drawn between mechanical twinning in clinopyroxenes and clinoamphiboles. The exclusive natural occurrence of basal twins in shock-loaded clinopyroxenes and of analogous ( 1 ¯ "> 1 ¯ 1¯ 01) twins in clinoamphiboles is given a simple explanation in terms of the relative difficulty of the “kinking” mechanism as compared to direct glide parallel to the composition plane.