ORDERING OF THE STACKING-SEQUENCE IN COOKEITE WITH INCREASING PRESSURE - AN HRTEM STUDY

To explore the effect of pressure on the stacking-sequence of the chlo rite cookeite, a systematic TEM study of stacking microstructures has been conducted on natural samples from pegmatitic and metamorphic envi ronments representing a large pressure field over a relatively narrow temperature range. The low-P and low-T conditions (1-5 kbar, 280-350 d egrees C) are represented by late-crystallized cookeite from pegmatite s and by cookeite from low-temperature metapelites. For the high-P con ditions (5-16 kbar, 280-430 degrees C), cookeite samples from metapeli tes and metabauxites were selected. Combining selected-area electron d iffraction (SAED) and high-resolution transmission electron microscopy (HRTEM) imaging shows that the polytypes evolve from a fully semirand om type to fully ordered types under increasing total pressure. For bo th polytypes the repeat distance (from one to five layers) of the basi c structures and the diversity of repeats increase with pressure. Abov e 10 kbar, long-period polytypes (LPP), up to at least a 14-layer repe at, coexist with perfectly ordered basic structures. Neither the uniqu e type of intralayer stacking (Ia) nor the single-layer subcell volume measurements improve understanding of the role of pressure on the ord ering process. Clearly, the regular polytypes of cookeite behave as po lymorphs that are remarkably sensitive to total pressure.