Ordering of the stacking sequence in cookeite with increasing pressure; an HRTEM study

Abstract

To explore the effect of pressure on the stacking-sequence of the chlorite cookeite, a systematic TEM study of stacking microstructures has been conducted on natural samples from pegmatitic and metamorphic environments representing a large pressure field over a relatively narrow temperature range. The low-P and low-T conditions (1-5 kbar, 280350°C) are represented by late-crystallized cookeite from pegmatites and by cookeite from low-temperature metapelites. For the high-P conditions (5-16 kbar, 280-430 0C), cookeite samples from metapelites and metabauxites were selected. Combining selected-area electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM) imaging shows that the polytypes evolve from a fully semirandom type to fully ordered types under increasing total pressure. For both po1ytypes the repeat distance (from one to five layers) of the basic structures and the diversity of repeats increase with pressure. Above 10 kbar, long-period polytypes (LPP), up to at least a 14-layer repeat, coexist with perfectly ordered basic structures. Neither the unique type of intra layer stacking (Ia) nor the singlelayer subcell volume measurements improve understanding of the role of pressure on the ordering process. Clearly, the regular po1ytypes of cookeite behave as po1ymorphs that are remarkably sensitive to total pressure.