Albite twin

Figure 8.12(a) is a two-dimensional representation showing the origin of a fault vector R associated with the presence of a slab of twin one unit cell thick. In the albite twin law, the two parts of the twin are related by a rotation of 180° about b. From Figure 8.12(b), we see that Rj is defined as... 

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 8.20. DF micrographs showing the boundary between a domain of fine-scale albite twinning (right) and an untwiimed domain (left) in microcline. In (a), g=20T and the albite twins are in contrast. In (b) g = 040 and the albite twins are out-of-contrast however, fine-scale lamellae approximately parallel to b are now visible. The plane of the specimen is (001). (From McLaren 1978.)...

Figure 8.22. BF micrograph of microdine showing peridine twins partially converted to fine albite twins. Other twins have been totally converted. Remnant portions of the peridine compontion planes are slightly inclined. Beam direction near [001]. (From Fitz Gerald and McLaren 1982.)...

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 8.25. 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.)...

Figure 8.40. (a) BF micrograph showing albite-twinned albite exsolution lamellae in a cryptoperthite (Or72) and (b) its associated SAD pattern. (From McLaren 1974.)... 

A somewhat more complex exsolution microstructure involving albite twin lamellae in a peristerite of bulk composition Ab9i.3An4.oOr4.T was studied by Gjonnes and Olsen (1974) using a TEM fitted with an energy dispersive x-ray spectrometer (Chapter 7). In this specimen, the twin... 

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