Quartz twinning

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,... 

Tsinzerlink E.V. (1941) Quartz twinning control under alpha-beta-conver-sion, Compt. rend. Acad. Sci. USSR, 33, 365-367 Chem. Abstr. 1941, 38, 4848. 

Figure 3.14 (a) Basal plane (00.1) of a Brazilian twin ofa-quartz (twin plane 11.0 ) etched in 20% HF for 24h (Heimann, 1975). The direction of rotation of the etch beaks (Honess, 1927) indicates the orientation of the twin domains (R = right-handed,... 

Another imperfection in crystals is called "twinning". This usualty happens when enantiomorplis are present, or possible. A good example is quartz, i.e.-... 

The occurrence of twinned crystals is a widespread phenomenon. They may consist of individuals that can be depicted macroscopically as in the case of the dovetail twins of gypsum, where the two components are mirror-inverted (Fig. 18.8). There may also be numerous alternating components which sometimes cause a streaky appearance of the crystals (polysynthetic twin). One of the twin components is converted to the other by some symmetry operation (twinning operation), for example by a reflection in the case of the dovetail twins. Another example is the Dauphine twins of quartz which are intercon-verted by a twofold rotation axis (Fig. 18.8). Threefold or fourfold axes can also occur as symmetry elements between the components the domains then have three or four orientations. The twinning operation is not a symmetry operation of the space group of the structure, but it must be compatible with the given structural facts. 

The Dauphine twins of quartz are formed when quartz is transformed from its high-temperature form (p or high quartz, stable above of 573 °C) to the low-temperature form... 

Group-subgroup relations and the emergence of twins in the phase transition /3-quartz — a-quartz (Dauphine twins)... 

Figure 7.1. Various growth twins, (a) Contact twin (albite) (b) inclined twin (quartz) (c) elbow twin (rutile) (d) cyclic twin (chrysoberyl) (e) lamellar twin (albite) ...

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. 

Large short-prismatic quartz crystals develop toward the center of a void. In many cases, these crystals are amethyst, in which Brazil twins and Brewster fringes are universally observed. 

T. Yasuda and 1. Sunagatva, X-ray topographic study of quartz crystals twinned according to Japan twin law, Phys. Chem. Min., 8,1982,121-7... 

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