Twins
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Dialog Box: Twins [ Input1 Menu]

There are basically two types of twins: contact and interpenetration. Interpenetration twins simply reproduce the main crystal according to the twin operator supplied. Normally, interpenetration twins should be drawn with intercrystal edges; these edges are located after the individuals are reproduced.

Clicking on the Add or Revise button calls up the Twin Operator dialog (Standard or Cartesian).

If you select the contact option, and supply one or more composition planes with the Composition Planes button, the composition planes are added to the crystal, truncating it as necessary, then the crystal is reproduced according to the twin operations supplied. No intercrystal edges are found for contact twins.

Note that all twin individuals are identical, and that composition planes always pass through the center of the crystal (central distance zero). Of course interpenetration twins may look different after the intercrystal edges are located. To draw twins in which the individuals are not identical, use Epitaxial Crystals.

Edges in faces. When different crystals in intergrowths have parallel and coincident faces, the edges lying in this common surface may or may not be realistic or meaningful, and you can elect to draw them or not. Usually the drawing is simpler and more satisfactory without them.

Always show composition-plane edges . Twins by reflection on the composition plane should always match exactly at the junction, and a certain short-cut in the drawing of such twins is often useful. When this box is not checked, if the composition plane is forward-facing, edges with this composition plane are not drawn. This often allows simple twins to be drawn without using the Remove Hidden Lines option in the Display menu. However, if the twin is by reflection, the two individuals often do not match exactly on the composition plane, and visible edges may be omitted. Generally, for correct drawing of mismatching rotation twins this box should be checked, and also the Edges in faces box . If the Always show composition-plane edges box is checked, the Remove Hidden Lines option in the Display menu must be selected.

Scale successive interpenetration twins, by factor (1-x), x = . Interpenetration twins often have coincident faces. In 3D Drawing Modes, if the two individuals are assigned different colors, this usually causes the coincident areas to have a mottled appearance, which may shift in pattern as the crystal is rotated. This option rescales each twin individual by a small amount, (1 - x) where x is the value entered, so that the faces of the second individual just underlie those of the first, and so on. Appropriate values are approximately 0.00001 (10^-5), so that the second crystal is 0.99999 times as large as the first and so on.

This option should be used only for 3D modes - in Standard or 2D mode it will usually disrupt the drawing of edges when the Remove Hidden Lines option in the Display menu is chosen. After changing values, you must click Calculate as the corner locations of twins are changed permanently.