9: Twinning
Crystal twinning occurs when two separate crystals share some of the same crystal lattice points in a symmetrical manner. The result is an intergrowth of two separate crystals in a variety of specific configurations.
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- 9.5: Mallard's Law
- The law of Mallard states that twin elements are always rational (i.e. direct lattice elements): therefore, a twin plane is a lattice plane, and a twin axis is a lattice row. These twin elements are pseudosymmetry elements for the lattice of the individual. The twin operations produce now slightly different orientations of the lattice of the individual, which are only quasi-equivalent, and no longer equivalent, as in the case of twinning by merohedry.
Thumbnail: Twinned pyrite crystal group. (Public Domain; Vassil ).