Double click on the SHAPE icon. From the File menu, select New. A dialog asks if you want to read default display variables. This is not necessary, as they are read in at start-up, so click on No. This brings up the Title/Axes dialog first. For a title "Calcite scalenohedron" will be sufficient, since other information will be supplied in the printout. From the Crystal System pull-down, select trigonal hexagonal. In the a and c boxes, enter 1.0 and 0.8543 respectively. This is for the morphological axes, which are most often used in designating faces (the true or X-ray axes are different).
Click on OK, and the next dialog, Symmetry, will come up. Click on the radio button to the left of B3m, and then on OK.
The Forms List dialog is next. Click on Add Forms, which will bring up the Add/Revise Forms dialog.
The indices of the common scalenohedron are {21-31}. The third index, -3, is superfluous, and you should enter only 2 , 1 and 1 in the boxes under h, k and l. Since there is only one form, the Central distance can be 1.0. The remainder of the items in the dialog do not need to be changed or filled in. Click on OK in this dialog, and on OK again in the Forms List dialog to complete data entry.
In the message box "New input completed - calculate now?", click on Yes. In a short time the computation will be complete, and the drawing will appear in the Graphics window.
Now select Twins from the Input1 menu. From the set of radio buttons labelled "Type..." select Contact with composition planes. Now click on the Add button, which brings up the Twin Operator dialog. For the choice of reflection or rotation, select Reflection. Next enter the indices of the twin plane, (001). Click on OK to return to the Twins dialog.
Now click on the Composition Planes button, bringing up the Composition Planes list, then on the Add button in that dialog. Select Indexed plane, with indices (001).
This completes data entry for the twin. Click on OK in each dialog to remove them all. In order to put the changes into effect, you must select Calculate from the File menu.
To calculate exactly the same thing using rhombohedral axes, select trigonal rhombohedral for the Crystal system in the Title/Axes dialog, and enter 1.0 for a and 101.9 for alpha - this is again for morphological indices. The rhombohedral indices of the scalenohedron are {20-1}, and the indices of the basal twin plane are (111) - otherwise the input is the same.