Direct identification of the six polytypes of chlorite characterized by semi-random stacking

Abstract

This paper demonstrates that the six standard polytypes of chlorite, whose definitions are based on the orientation of the interlayer sheet and the position of the interlayer sheet on the 2:1 layer, can be discriminated by atomic-resolution images recorded down [010], using a transmission electron microscope with a ∼2 Å point resolution and digital image processing. Several specimens were investigated to reveal their local stacking structures. A Ibb chlorite is highly twinned and twin boundaries consist of a llb+la stacking sequence at the interlayer sheet. An interstratified chlorite/biotite formed by hydrothermal alteration from biotite in granite consists of a mixture of several chlorite polytypic sequences, including lab, Ibb, llab, and llbb. These polytypic details of chlorite and other sheet silicates provide important insights into mineral stability, origin, and reaction mechanisms.