Disordered twin planes

This is a structure-building component in which two constituent parts of the structure are twin-related across the interface. The twin plane changes the composition of the host crystal by a definite amount (which may be zero). Ordered, closely spaced arrays of twin planes will lead to homologous series of phases. Disordered twin planes will lead to non-stoichiometric phases in which the twin planes serve as the defects. There is a close parallel between chemical twinning and crystallographic shear (see Section IR-11.6.3). 

B) The model of De Wolff disorder gives no explanation for the line broadening of reflections which are not affected by this type of lattice disorder. Chabre and Pannetier ascribed this effect to a micro twinning of the ramsdel-lite/rutile lattice on the planes [0 2 1] and [0 6 1], These faces are believed to be growth planes of EMD [45, 46],... 

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. 

Seven of the ten known forms of ice have their protons disordered, as in ice Ih, or the cubic form Ic. Ice n is one of the ordered forms. Its crystals are always twinned in such a way as to correspond closely to the symmetry of space group Ric. By careful X-ra.y work, and more easily by ND," this disguise has been penetrated. The untwinned structure, in space group jR3, is shown stereoscopically in Figure 3. There are two crystallographically distinct D O molecules, which would be identical were the c-glide plane... 

Plastic deformation of metals and alloys occurs by slip and/or twinning on certain crystallographic planes and along certain crystallographic directions and is accomplished by the movement of dislocations (Volume 2, Chapter 7). Plastic deformation results in considerable disorder, leading to phase transformations in some cases. The de-... 

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