| Answers to Questions |
| 9. In the shearing geometry here (shear plane parallel to the screen, shear direction horizontal) the basal slip planes in OCP will tend to become oriented parallel to the screen and prismatic subgrain boundaries will be normal to the screen and to the shear direction, hence making vertical traces on the screen. The grains themselves will tend to be drawn into ribbons oblique to the screen. Screen-parallel sections of these ribbons will give grain shapes which are ellptical, with a longer axis vertical on the screen and a shorter axis horizontal on the screen. |
| 11. Some of the dilated grain boundaries are the black grain boundary gaps at (32,9), (78,7), and (50,50). |
| 13. It may be a grain with its c-axis initially about perpendicular to the screen. Such a grain might kink by slip on its basal planes (in response to the horizontal shoertening), but even if it rotates its lattice by such kinkingm it will remain in extinction because the c-axis of any kinked regions will retain a trend parallel to the horizontal polarizer of the microscope. |
| 19. The sense of shear is evidently dextral, since the opening directions "leans" to the right. The shear strain is approximately 0.1. The can be estimated from the change in position of the large orange grain at (60,8) relative to the small orange grain at (60,58). |
| 20. They would bprobably be filled as rapidly as they formed, either by veins of another mineral, or by overgrowths of the same mineral in optical continuity with the original crystals. If veins of another mineral, the gap fillings would be obvious, tabular features. If overgrowths in optical continuity, they might be invisble petrographically, though still "veins" in the sense of a crack-filling. |
| 26. No. One cannot see the details of how this ribbon formed, but it is clearly not a simple, smeared-out original grain. |
| 28. Strain in grain 6 is small but perceptible. It has shortened some thing like 5% horizontally. |
| 29. The northern part of the 1/6 boundary has migrated west. The horizontal shortening of the sample is of the order of 20%, somewhat more or less depending on what pairs of particles you use. |
| 31. Grain 6 has its slip plane more favorably oriented for picking up a large shear stress than does grain 1. Grain 6 may accordingly have deformed mainly by single-slip while grain 1 was deforming by multiple-slip or by sunoptical kinking. In either case, grain 1 may have picked up more stored strain energy (higher defect density) than grain 6, so grain 6 tends to eat grain 1, maximizing the rate at which strain energy is dissipated. |
| 47. On the grain-scale, both brittle (cracking) and ductile (twinning) processes are active. So on this scale the behavior is mixed. On the scale of several of these images, enough to bridge the zone of tring deformation, the displacement of, say, a 2 cm grid of particles wold make a fairly regular pattern (without marked jumps anywhere in the displacement gradients) so on this scale the deformation would look approximately continuous, and the behavior would look ductile. |
| 49. There is a problem here! The pink grain is growing several times faster on its SE side than on its NW side. One could have explained this by saying the grains on its SE side happen to have lattice orientations making then easier to replace, but this argument is taken away by the apparent orientation-independence of growth velocity shown by both the brown grain and by the pink one. The top boundary of the green grain has migrated faster. |
| 50. Perhaps the brown grain contains a higher density of defects incorporated during the growth process than the green grain, or greater elastic strains acquired during temperature-change. (No answer to this queation is obviously correct.) |
| 51. One could look for some morphological asymmetry in the migrating boundary. For example, there is a downward-pointing white lobe in the brown/white boundary at (49,20). On either side of this lobe there are sharper reentrants in the white grain. This suggests pinning of the boundary at the sharp reentrants while it advances at the lobe. Hence the white grain may be replacing the brown grain locally. (This interpretation is hazardous. Nearby, on the same boundary, these relations look reversed(?). Amore convincing example is seen in image 52). |
| 52. Phase-change twinning is well-known, but these twins are deformation twins induced by the motor, because they appeared after the phase-change passed this point. |
| 57.
The brown and green grains are expected to grow again. The old pink grain probably will not reappear, since it was completely removed earlier, so there us nothing left for new pink crustal to grow upon as a substrate. a long-shot possibility is that it could appear again, by growth controlled by the lattice of a suitably oriented grain of the alpha phase. But there are now several grains and subgrains in the region where the pink grain used to be, so the chances of its reappearance in exactly its original lattice orientation are slight. |
| 60. Propagation of the tip of a shear band can be measured relative to the frame fixed with its origin pinned to a particle in either the hanging wall or the footwall. But the propagation distance and velocity are likely to be different in these two frames. |
| 61. Shear bands 1 and 2 have both been active, especially 1. |
| 62,63,64. Shear band 2 is not a simple shear band. Between images 62 and 64 there is about 10% shortening parallel to the band as shear displacement accumulates on it. Shear band 2 is an example of a (negative) stretching shear zone. |