(100) Deformation twins in naturally deformed amphiboles

Abstract

The occurrence of growth twins with (100) as a composition plane has been well established in clinoamphiboles, but no evidence has been presented that (100) twins may form by natural deformation. We now report that in naturally deformed hornblende crystals, selected from an amphibolite mylonite in the Central Scandinavian Caledonides, lamellar (100) deformation twins have been identified using optical and transmission electron microscopy (TEM). The lamellar twins are present within highly bent and kinked parts of the hornblende crystals. Selected area diffraction patterns show that twin and matrix domains are related by a 180° rotation about a^*. Additional defects associated with the (100) planes are unit dislocations with b̄ = [001] and stacking faults bounded by partial dislocations. The identification of (100) mechanical twins in clinoamphiboles illustrates the similarity in mechanical twinning behaviour of clinoamphibole and the related clinopyroxene structure.