Many crystals, when observed under the optical microscope, display characteristic domain patterns. One of the most common microstructures is related to twinning, with dominant twin planes oriented approximately perpendicular to each other. Such twins have various origins and may arise from growth phenomena or transformation twinning. These twin structures are often seen to change when the crystals are squeezed or even only lightly touched with a preparation needle. This phenomena has, in particular, been the bane of structural crystallographers who found that single crystals spontaneously twinned during preparation for subsequent X-ray or neutron experiments. Mechanical twinning is not only observed in technologically important materials such as high Tc superconductors but is also commonly encountered in mineralogically and petrologically relevant systems, such as perovskites, boracites, palmierites, leucites, quartz and feldspars, to name but a few. Figure 1.1 shows a rather typical domain pattern in a crystal with a palmierite structure (Pb3 (PO4)2) which was induced by a gentle squeezing between the finger tips.

Observations of such effects have long been noted in the mineralogical community and their importance has certainly been recognized by eminent mineralogists, such as Laves (1969) who called these twins ‘Druckzwillinge’ (i.e. stress-twins).