Twinning and composition variation

Twin planes are most frequently internal boundaries across which the crystal matrix is reflected (Section 3.11). Some twin planes do not change the composition of the crystal while at others atoms are lost and a composition change can result. If faults that alter the composition are introduced in considerable numbers, the crystal will take on the aspect of a modular material and show a variable composition. 

This feature can be illustrated with respect to frequently observed twinning that occurs on 101 planes in the rutile form of titanium dioxide. Large numbers of 

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