Transformation-induced twinning and modulated structures

Whenever a crystal transforms (usually on cooling) to a lower symmetry structure in which a number of orientational variants are possible, transformation twins are an almost invariable consequence if the new phase is nucleated at several different places in the original structure. The formation of Dauphine twins at the jS-to-a transformation in quartz is a classic example, which has already been mentioned briefly in Section 8.6. We now consider the twins and modulated structures associated with the monoclinic-to-triclinic transformation in the K feldspars, KAlSisOg. 

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). 

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). 

Partially ordered K-feldspars (such as orthoclase, adularia, and intermediate microcline) characteristically exhibit a tweed pattern consisting 

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