Re-entrant corner effect

Figure 7.4. The re-entrant corner effect, (a) One re-entrant corner (b) two re-entrant corners, one at each end.

Figure 7.5. (a) Twin with re-entrant corners in three directions (shown by arrows) and (b) its expected re-entrant corner effect, (c) Morphological change is expected for repeated twinning. 

M. Kitamura, S. Hosoya, and 1. Sunagawa, Re-investigation of the re-entrant corner effect in twinned crystals. J. Crystal Growth, 47.1979,93-9... 

Figure 10.9. Various forms of rock-crystal twinned according to the Japan law from Narushima Island. Nagasaki Prefecture. Japan. The evolution of crystal forms in the order a->b->c is expected as growth proceeds, due to the re-entrant corner or pseudo re-entrant corner effect.

Japanese twins of quartz have attracted interest since ancient times because they exhibit a remarkably platy V-shape, in contrast to the hexagonal prismatic morphology of coexisting single crystals. Since they grow on substrate, the V-shape was assumed to represent the upper half of an X-shape [12], which implies that the Japanese twins are penetration twins. The platy form of Japanese twins has been explained as being due to preferential growth at a re-entrant corner formed by two individuals. If the platy form is indeed simply due to the re-entrant corner effect, we should expect a variation of forms, from V-shape to fan-shape, as the effect proceeds, as shown in Fig. 10.9. If, however, it represents the upper half of an X-shape, Japanese twins should be penetration twins, not contact twins. 

As already explained in Section 7.2, the re-entrant corner effect in its original sense can be expected only when two individuals are perfect crystals, containing no dislocations either in the individual crystals or in the composition plane. In real crystals, the pseudo re-entrant corner effect, by the mediation of dislocations, creates changes in the morphology of the twinned crystal. 

Sunagawa and T. Yasuda, Apparent re-entrant corner effect upon the morphologies of twinned crystals A case study of quartz twinned according to Japanese twin law,... 

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