Pericline twin

Marshall and McLaren (1974) showed that the boundaries of deformation pericline twin lamellae in experimentally deformed anorthite (An95) involve a displacement. The specimens were cut normal to [010] so that the twin boundaries were viewed edge-on and the twins were out-of-contrast for all g = hOl, because these reflecting planes are unaffected by the twin. However, in DF images with b-reflections (h+k = ln- -, l = 2n- - ) and with c-reflections (h+k = 2n, l = 2n+l), the twin boundaries are seen... 

Sanidine is monoclinic (space group C2/m), and there is complete disorder in the occupation of the tetrahedral (T) sites by the A1 and Si atoms. Over geological time, ordering takes place. In low (or maximum) micro-cline, the ordering is complete (all A1 in TiO sites), and the symmetry is reduced to triclinic (CT). There are four main orientational variants in this structure two orientations related by the albite twin law (rotation of 180° about b ) and two orientations related by the pericline twin law (rotation of 180° about b). The composition planes of these two twins are, respectively, (010) and the rhombic section which is parallel to b and approximately normal to (001). Thus, the characteristic cross-hatched pattern observed in (001) sections between crossed-polarizers in the optical microscope has, for many years, been simply interpreted as intersecting sets of albite and pericline twin lamellae formed at the monoclinic-to-triclinic transformation. However, TEM observations indicate that this model is too simple. Because these observations, collectively, also constitute an excellent example of the application of the principal modes of operation of TEM to a specific mineralogical problem, we discuss them in some detail. 

In most specimens, albite twinning predominates over pericline twinning. Generally, the albite twinning is on a very fine scale and occurs in domains separated by untwinned domains, arranged to form several types of overall microstructure (McLaren 1984). 

Figure 8.20(a) is a DF micrograph (g = 20T) showing part of a domain of albite twins and its boundary with an untwinned domain. Pericline twins. 

Figure 8.23. BF micrograph of microcline oriented with the electron beam near [104). (a) Elomain of albite twins completely enclosed in a matrix of pericline twins, (b) The SAD pattern corresponding to the area shown in (a). The streaks arise from the complex twin intersections in the domain boundary.

The observation that single pericline boundaries are commonly replaced by a large number of albite twin boundaries, possibly involving a l(X)-fold increase in total area of twin boundary per unit volume, suggests that the energy of a pericline twin boundary is greater than that of an albite twin boundary by a similar factor at normal temperatures (see later). 

Figure 8.24. BF micrograph of microcline. The intersection of albite twins (A and B) with pericline twins (A and B ) has produced a chessboard pattern of finely twinned regions and untwinned window regions. The electron beam is near [104]. (From Fitz Gerald and McLaren 1982.)...

Figure 8.25. Diagram showing the idealized intersection of a set of albite-twin lamellae ABABA... with a set of pericline twin lamellae A B A B A. .. to form the chessboard pattern of cross-hatched twinning. (From McLaren 1978.)...

Marshall, D. B., McLaren, A. C. (1974). The structure of albite and pericline twin boundaries in anorthite. Electron Microscopy 1974, Eighth International Congress on Electron Microscopy, Vol. 1, edited by J. V. Sanders D. J. Goodchild, pp. 490-1. Canberra, Australia Australian Academy of Science. 

Tsatskis I, Salje EKH (1996) Time evolution of pericline twin domains in alkali feldspars A computer-simulation study. Am Mineral 81 800-810... 

Experimental studies of alkali feldspars in Westerley granite with ordered microcline [63,64] and disordered sanidine [65-68] show that (010) [001] shp is active in both. Other dislocations derive from (010)[101] [Fig. 15(a)], (001) [110], and (121) [101] shp. In naturaUy deformed alkali feldspars, subgrain formation was observed, indicative of climb [357-360]. It has been suggested that shear-induced mechanical Albite and Pericline twinning in potassium feldspar may facilitate ordering [361] but this has been disputed [362]. Dislocations have no effect on diffusion in alkah feldspar [363]. 

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