Crystal imperfections twinning

Preferential dissolution at naturally occurring crystal imperfections, such as dislocations, twinning planes, and other structural defects (Berner and Holdren, 1979 Berner and Schott, 1982 Lasaga, 1981b Brantley et al., 1986 Schott and Petit, 1987 Blum and Lasaga, 1987). 

The residual intercalant is retained at crystal imperfections such as crystallite boundaries or twin lines and x-ray diffraction may show graphite reflections only. Residue compounds with large interplanar distances have residual intercalant between the carbon planes . In bromine residue compounds ordered layers of intercalant are found by electron diffraction . Thus there is no distinct borderline between dilute lamellar and residue compounds. 

Another imperfection in crystals is called "twinning". This usualty happens when enantiomorplis are present, or possible. A good example is quartz, i.e.-... 

The powder patterns obtained by X-ray diffraction and selected area electron diffraction do represent averages over very large numbers of particles but the averaging over size, orientation and imperfection of crystals removes much of the important information, especially that on the correlations of properties,e.g, the orientational relationship of adjacent crystal regions or the dependence of twinning on size. 

Fig. 14. Twins produced by plastic deformation and broadened by annealing at 650°C in an imperfect single crystal of a - U grown by progressive phase-change from j3 to a. The bands with different shades correspond to the recovery of the subgrains of the initial crystal. Electroetching reveals small disorientations between the subgrains X150. (D. Calais, P. Lacombe and Mme Simenel, Rev, met., 56, 261 (1959)).

Figure 1. Complexity of silver bromide crystal growth without imperfections, with a single twin plane, and with a pair of twin planes.

Interfacial defects are boundaries that have two dimensions and normally separate regions of the materials that have different crystal structures and/or crystallographic orientations. These imperfections include external surfaces, grain boundaries, phase boundaries, twin boundaries, and stacking faults. 

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