10.6 Twins 209

variation in the separation of growth banding. This is utilized in the analysis of Earth science problems, such as evaluating environmental conditions, analyzing the effect of solution flow upon growth rates, and estimating the flow direction of hydrothermal solution, based on the analysis of changes in the separation of growth banding in crystallographically equivalent growth sectors and their fluctuations in the same direction. Based on this viewpoint, a difference is noted between hydrothermal veins, in which one-directional flow of hydrothermal solution is assumed, and pegmatite formed in a void in solidifying magma. In the former, one-directional flow of solution is clearly seen, whereas in the latter the flow directions change during growth.

To demonstrate the extent to which the flow of solution affects the crystal morphology, we describe an experiment performed by Balitzky (ref. [14], Chapter 4). Balitsky investigated the effect of solution flow on the growth rates of the r and z faces of quartz crystal in an autoclave, by reversing the position of the seed. As the supply of solute component changes depending on whether we have up-flow or down-flow, the growth rates and impurity partitioning can change according to the positioning of the seed. Although the general tendency is for larger development of the r face, since Rr Rz when crystals grow in an isotropic environment, the r and z faces develop nearly equally because Rr is promoted and Rz Rr, if the z face is faced up-current.

This experiment demonstrates that a distinction between the r and z faces based on the size difference becomes impossible. Also, the growth sector of the z face takes on a violet color. In an isotropic environment, the violet coloring appears selectively in the r growth sectors, and the z growth sectors are colorless. This experiment also demonstrates that the partitioning of impurity Fe is affected by the growth rate.

10.6Twins

10.6.1Types of twins

Several different types of twinning relations have been observed in quartz, such as: the Dauphiné law, a twin relation between two right-handed or two lefthanded structures with the c-axis as the twin axis; the Brazil law, a twin relation between a right-handed and a left-handed structure; the Japan law, with the c-axes forming an angle of 84°34! with the composition plane (1122); and others. (It should be noted that all these types might be described differently in different localities.) Dauphiné twins are formed in growth, in transformation, and by external factors such as mechanical, thermal, and electrical forces, and do not differ greatly from the morphology of single crystals. Compared with the positions of the s {1121} and x {5161} faces, those appearing on an m face will differ according to whether they are leftor right-handed; the s {1121} and x {5161} faces appear on

210 Rock-crystal (quartz)

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.

right and left corners of an m face in a Dauphiné twin. Therefore, we shall pay particular attention in this section to the explanation of why a characteristic morphology appears in growth twins according to the Japan law and the Brazil law. It usually happens that two individuals twinned according to the Japan law also contain Dauphiné twins and Brazil twins.

10.6.2Japanese twins

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.

10.6 Twins 211

Figure 10.10. (a) X-ray topograph and (b) the corresponding sketch of a fan-shaped

Japanese twin. X-ray topograph by T. Yasuda [13].

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.

Figure 10.10 shows an X-ray topograph of a fan-shaped Japanese twin. It is observed that (i) dislocations concentrate in the composition plane of two individuals (A), and (ii) the spacing of the growth banding in the r or z growth sectors adjacent to the composition plane (B) is approximately twice as wide as that in the growth sectors of crystallographically equivalent r and z growth sectors a distance away from the composition plane (C, D). Growth banding in the former sectors is wavy, whereas in the latter it is straight. This observation demonstrates that the growth is promoted due to preferential growth by concentrated dislocations in the composition plane, and platy or fan-like morphology is due to the pseudo re-entrant corner effect [13], [14]. The morphology of a Japanese twin should

212 Rock-crystal (quartz)

Figure 10.11. X-ray topograph of a V-shaped Japanese twin. X-ray topograph by T. Yasuda

[14].

transform from a V-shape to a fan-shape as growth proceeds, irrespective of the effects of the original or the pseudo re-entrant corner effect.

Figure 10.11 is an X-ray topograph of a V-shaped sample. A contrast image of a fanshape is discernible in the V-shape. Contrary to expectations, the morphology of the Japanese twin transforms from a fan-shape to a V-shape as growth proceeds. Whether a V-shape or a fan-shape results is dependent on whether {1010} faces appear at the contact point of two individuals (V-shape), or whether {1011} faces appear (fan-shape). The {1011} faces are nearly perpendicular to the dislocations concentrated in the composition plane, whereas {1010} faces are inclined at an angle. This creates the difference in the step heights of the growth layers that have originated from the dislocations, and affects the growth rates. This observation demonstrates that if {1011} faces are present in the composition plane, the pseudo re-entrant corner effect is at maximum effect, but once {1010} appears, the effect diminishes sharply [14]. Furthermore, we may expect that at the nucleation stage of a Japanese twin, the quartz crystals should take ditrigonal dipyramidal form, with no {1010} faces present.

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-