Kinetic control of ordering and exsolution in omphacite

Omphacites with a range of compositions from the blueschist rocks of Syros, Greece and from included blocks in serpentinite from the Motagua fault zone, Guatemala, have been examined by transmission electron microscopy. The complex microstructures observed are all on a fine scale and are composed of exsolution and antiphase features. At least three different assemblages have been identified: (a) a single ordered phase with antiphase domains, (b) two exsolved phases, one ordered and the other disordered and (c) two exsolved phases, both apparently ordered. Selected area electron diffraction patterns provide evidence for P2/n, P2/c, and P2 space groups in different parts of the ordered crystals.The microstructures cannot all be readily explained in terms of a simple equilibrium phase diagram and some are attributed to alternative and metastable transformations. It is suggested that under the low temperature conditions of blueschist metamorphism, omphacite crystallised metastably below its ordering temperature with imperfect cation order. Subsequent development by cation ordering and exsolution was controlled by kinetics, the fastest available transformation being used to give reductions in free energy. Ordering preceded exsolution and may have occurred in a series of steps as predicted by the Ostwald step rule. Slight differences in composition and initial state of order locally influenced which reaction pathway was embarked upon and many of the products may be metastable. Such behaviour is apparently sensitive to thermal history.