This dialog lists all the 32 crystallographic point groups, or crystal classes. The orientation is as indicated by the Herman-Maughin or International symbol. Note that a capital "B" preceeding an axis denotes an inversion axis, such as "bar-3". The orientation is as given by the H-M symbol, and in some cases two orientations are possible - see below.
The Remove Symmetry button offers the option of converting all faces which have been generated by symmetry to forms, and reducing the symmetry to group 1. The crystal can then be distorted from the ideal habit by changing the central distances of individual faces (through the Forms List dialog). This option is not available unless the crystal has been calculated.
When called from the Input1 menu, this dialog can only modify the main or host crystal of an epitaxial intergrowth. Epitaxial guest crystals are modified from the Epitaxial Relations dialog.
Note that space-group symmetry is needed for the Donnay-Harker Morphology option - the point group is normally derived from the space group, and this may cause replacement of the point group specified in this dialog, if the two are not the same.
Alternate Orientations . There are two listings for each of five crystal classes, for which alternate orientations are possible. The pairs are 321 - 312, 3m1 - 31m, B3m1 - B31m, B42m - B4m2 and B6m2 - B62m. In these symbols, the first number or letter after the high-order axis (3, 4 or 6) indicates either a set of two-fold symmetry axes parallel to the a crystallographic axes, or a set of mirror planes perpendicular to the a crystallographic axes. The next letter or number indicates symmetry elements aligned on the directions bisecting the a crystallographic axes. If there is no symmetry in the a -axis directions, there is a number 1 as a placeholder. A final 1 is usually dropped, as in 321 = 32.
Versions of SHAPE prior to V6.0 allowed only the first orientation of each of these pairs. In V6.0, it is possible to import crystallographic data including the space group, and it is necessary to specify the space group for the Donnay-Harker Morphology option. Since the standard settings of space groups may have either of the orientations of each pair (for example P321 and P312 are different space groups), it is necessary to include both to correlate the morphology with the structure axes. It is possible to describe the morphology in terms of the orientation which does not correspond to that of the space group in the International Tables , but the unit cell in this orientation is non-primitive, the a axes are longer in relation to the c axis, and the indices of actual faces are different.
Since pre-V6.0 SHAPE only allowed one orientation, it is possible that forms entered may be incorrect. Most errors of this sort may be corrected simply by changing the symbol in this dialog, for example from P321 to P312. It is also possible that older morphological data may not correspond with the X-ray axes, which use the orientation as given in the International Tables . In this case, if the old morpological data are to be used with the X-ray axes, it is necessary to transform the indices.