Penetration twins

Aoki, Y. and Nakamuto, Y., 1984. Penetration twins of potassium chloride. Journal of Crystal Growth, 67, 579-586. 

Penetration twins have an irregular composition plane, and the twin plane and the composition plane are not the same. As a representative example, the 111 twin of fluorite, CaF, is shown in Fig. 7.8. 

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. 

If two individuals conjugate on an r or a z face, a nucleus of a Japanese twin with 1122 as the composition plane is formed. This indicates that a Japanese twin is not a penetration twin (i.e. not the upper half of an X-shape), but a contact twin (i.e. the upper half of a Y-shape). Horizontal banding in geode agate (see Section 10.9) appears through grain size variation due to gravitational sedimentation, and con-... 

CHRYSOBERYL. The mineral chrysoberyl. an aluminaie of hery Ilium corresponds to the formula BeALO. , crystallizes in the orthorhombic system with both contact and penetration twins common, often repeated resulting in ro.setted structures. Hardness. 8.5 specific gravity. 3.75 luster vitreous color various shades of green sometimes yellow. A variety which is red by transmitted light is known as alexandrite. Streak colorless transparent to translucent, occasionally opalescent. Chrysoberyl also is known as cymopbane and golden beryl. 

FLUORITE. Fluorite is a calcium fluoride mineral CaFi crystallizing in the isometric system, often in superb cubic crystals. Twinned crystals are common, usually as cubic penetration twins. It is found in many diverse... 

The <111> penetration twins on both 100 and 111 faces of the CVD diamond crystals have been identified and they have been shown to play a major role in determining the film morphology. °°... 

The T(ioo twin structure was also studied in Ref [99]. The pyramidal structures, often seen on the (100) faces of HOD films, were identified as penetrating twins, as shown in Figure 6.4 (a). The (111) face of the pyramidal structure tilted by 15.8° with respect to the basal (100) face, and the [221] direction of the twin is parallel to the [100] direction of the basal (100) face. It was inferred that the atomic arrangement of carbon at the beginning of the penetrating twin formation looks like the one shown in Figure 6.4 (b), which is seen from the [110] direction of the basal (100) face. 

Formation of hillocks and penetration twins on the (100) surface of single crystal diamond was studied by Tsuno et al. [101]. The misorientation of the (100) surface was less than 3° from the exact (100) lattice plane. A NIRIM-type... 

Figure 6.4. (a) Schematic view of a penetrating twin (b) cross section of the nucleus of a penetrating twin viewed along the [110] direction. The dashed lines are dimer bonds. Surface-bonded hydrogen atoms are not depicted [99],... 

Figure 6.8. (a) Surface structure with a growth hillock and a penetration twin, and (b) its atomic configuration [101]. 

Figure 108 Several twinning forms (a) a Japan law twin of quartz (b) a penetration twin of pyrite (c) a Carlsbad twin of orthoclase.

---