Quartz Brazil 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. 

Figure 8.2. a-fringe patterns due to Brazil twin boundaries in amethyst quartz. BF image g = 10Tl. Note the reversal of contrast at successive boundaries. (From McLaren and Phakey 1966.)... 

Figure 8.2 shows an array of Brazil twin boundaries parallel to one of the major rhombohedral planes, say (TiOl), in amethyst quartz, imaged... 

Figure 8.S. Projection onto a plane normal to 83 = [ 110] of the Si04 tetrahedra, showing the structure of a Brazil twin boundary parallel to r(IlOl) in which the right-handed quartz (lower-right) is displaced with respect to the left-handed quartz (upper-left) by a fault vector R= j[110].

Figure 8.6. Overlapping a-fringes due to thin Brazil twin lamellae in experimentally deformed natural quartz, (a) BF g= lOll. (b) BF g = lOTO. Note the asymmetrical fringe profile in (a) and the symmetrical fringe profile in (b). (From McLaren et al. 1967.)...

Dauphine twin boundaries are sometimes parallel to low-index planes, but very often there appears to be no crystallographic control. This was certainly the case for the Dauphine twin boundaries observed between the Brazil twin boundaries in amethyst quartz by McLaren and Riakey (1969). 

Electron microscope study of Brazil twin boundaries in amethyst quartz. phys. stat. sol., 13, 413-22. 

McLaren, A. C., Retchford, J. A., Griggs, D. T., Christie, J. M. (1967). Transmission electron microscope study of Brazil twins and dislocations experimentally produced in natural quartz, phys. stat. sol., 19, 631-44. 

Fig. 3. Natural quartz crystal (a) including Brazil-type twin of right and left hand crystals (b). m mirror plane.

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