10.6 Twins 213

Figure 10.12. Change in form and surface microtopographs observed in regrowth experiments on a Japanese twinned sample used as a seed [15]. When the Japanese twin grows freely, the form changes from a V-shape to a Y-shape.

sists of idiomorphic crystals of quartz less than 1 m. On quartz crystals of this size, {1010} faces do not appear, and the crystals take on a ditrigonal dipyramidal form bounded by r and z faces. A Japanese twin is considered to have been formed if two individuals conjugate on well developed r and z faces. The introduction of {1122} as the composition plane is simply due to the geometrical relation between the two individuals.

Figure 10.12 shows a series of sketches showing the variation in morphology and surface microtopography as growth proceeds, starting from a natural Japanese twin as seed grown in an industrial autoclave for quartz synthesis [15]. It is clearly seen that, starting from a partly broken Japanese twin with a V-shape, the form changes to a Y-shape when growth occurs in an open space. This experiment proves that the V-shape does not represent the upper half of an X-shape but that of a Y-shape, and therefore that the Japanese twin is not a penetration twin but a contact twin. It was later observed that natural Japanese twins occasionally exhibit a Y-shape when they grow in open spaces. Figure 10.12 also vividly demonstrates the sorts of changes that occur in the process of transformation from a rough to a smooth interface, and it may also be seen that two individuals are Dauphiné twinned.

10.6.3Brazil twins

The Brazil law is used to describe an intergrowth of rightand left-handed structures with the c-axis as the twin axis. Since this is a relation between different structures, strictly speaking this does not belong to the twin category, but it has always been treated as an important twinning phenomenon. In Brazil twins, two

214 Rock-crystal (quartz)

Figure 10.13. Brewster fringes frequently seen in amethyst crystals. L and R indicate

left-handed and right-handed structures, respectively.

individuals are in a reflection relation on {1120}, and occur as twin sectors in lamellar form bounded by {1010}, {1011}, and {0111}. Brazil twins occur universally in quartz crystals formed at lower temperatures, as in amethyst occurring in geode, and are only exceptionally found in quartz crystals formed at higher temperatures, such as quartz crystals occurring in pegmatites. One report [16] claims that Brazil twin lamellae are found in quartz particles constituting agate in geode exceeding 10 nm, and the widths of the lamellae range from nanometer to micrometer order. The so-called Brewster fringes often observed in amethyst (Fig. 10.13) are the boundaries of Brazil twin lamellae formed by the coagulation of numerous Brazil twin lamellae.

Considering the fact that the lamellae widths are of nanometer to micrometer order and that a Brazil twin is formed by combining right-handed and left-handed structures, we have to assume the presence of clusters with right-handed and lefthanded structures in the ambient phase, with the Brazil twin being formed by the joining together of these clusters. A Brazil twin is not formed by transformation from left-handed to right-handed structures while a crystal is growing by incorporation of ionic entities or SiO4 tetrahedra as the growth unit. This is in agreement with the observation that Brazil twins are universally observed in quartz crystals formed at lower temperature rather than at higher temperature, since the probability of cluster formation is higher at lower temperatures.

In amethyst crystals showing Brewster fringes, a pattern such as that shown in Fig. 10.14 is observed in the growth sectors of or on the surface of r faces, and not in

10.6 Twins 215

(a)

(b)

(c)

Figure 10.14. (a), (b) Reflection photomicrographs of weakly etched surface, and (c) the

structure of Brewster fringes on an r face (model by Lu and Sunagawa [17]). Zigzag (A in

(a) and (c)) and closed lamellae (B in (a) and (c)) appear alternately. L left-handed

quartz; R right-handed quartz.