Cross-hatched twinning

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.)...

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. 

Figure 8.21. (a) BF micrograph (g = 20l) showing a cross-hatched pattern in microclLne. The plane of the specimen is (001). (b) Symmetrical electron diffraction pattern of area shown in (a). Note the streaking of the albite twin spots. (From McLaren 1978.)... 

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