Abstract
High-resolution and conventional transmission electron microscopy has been used to study exsolved augites from the Laramie Anorthosite Complex, and the hrtem images have been interpreted through extensive computer-generated image simulations. The results illustrate the complexity of interface structures and morphologies in exsolved pyroxenes and confirm that defects such as dislocations and stacking faults play an important role in pyroxene exsolution processes.
Pigeonite in the Laramie augite occurs as both “100” and “001” lamellae, and the degree of coherency of the interfaces is inversely related to the lamellar size. Both sets of lamellae grew by the propagation of growth ledges, and hrtem images of the dislocations on “100” growth ledges are consistent with unit Burgers vectors of [001]. The interfaces of “001” pigeonite lamellae have a stepped morphology where they intersect (100) stacking faults in the pigeonite; the faults have the stacking sequence +- -+ and therefore can be described as single blocks of orthopyroxene structure. The 10/12[001] partial dislocations at the terminations of the stacking faults are dissociated into two equal partial is approximately 5 nm apart, and the stacking sequence of the faults between the two partials is + - + Where “100” and “001” pigeonite lamellae approach each other, the “100” lamellae typically have bulges on both sides, and the “001” lamellae are hooked, probably at least partially because of interaction between the strain fields of the growing lamellae. In an additional unrelated observation, narrow orthopyroxene lamellae grew from their tips as entire lamellae, rather than by a ledge mechanism. Each growing orthopyroxene unit cell is led by two partial dislocations, the strain fields of which cancel each other.
Most of the stacking faults observed in orthopyroxene involved skew reversal of an even number of (100) octahedral layers, and most of the faults observed in pigeonite also consisted of even numbers of octahedral layers and were separated by slabs of ordinary pigeonite containing an even number of octahedral layers. However, a number of odd-layered faults also were observed in both orthopyroxene and pigeonite, indicating that a full description of the stacking phenomena in real pyroxenes requires the use of 0.45-nm (100) slabs of structure, rather than the 0.9-nm layers used by some authors, HRTEM images indicate that the stacking faults in pigeonite nucleate symmetrically about the B chains of the pigeonite structure. The structures and distribution of the stacking faults show that their positions are structurally controlled and confirm that they formed below the C2/c → P21/c transition temperature.
