Dialog Box: Sections and Growth Zones [Input1 Menu]
In the Section/Zoning mode, a section is drawn through the current crystal or crystals, parallel to the screen or paper (observer x = constant). Sections and growth zones are shown only when the Display mode, in the Modes menu, is set to Section/Zoning. This dialog and the sub-dialogs it controls set the parameters for sections and growth zones.
The radio buttons at the top give the choice between the two main modes: Single section outline, which is useful for identifying the orientation of a crystal in a section of rock or other material; and Growth zones, which plots repeated sections at intervals according to various growth models. The Growth Zone Settings button brings up another dialog for additional growth zone settings.
A section, whether single or repeated as growth zones, is taken parallel to the y and z observer axes (see Coordinate Systems), at the x coordinate specified in the Section height box. This coordinate should not be larger than the dimension of the crystal in the x direction, or nothing will be shown.
In the non-fixed scaling modes (see Scaling), you can scale the Section only to occupy the viewing area, or you can include the entire crystal (All corners ). Typically the latter mode is used if you are displaying a drawing of the crystal along with the section.
The coordinates of section corners can be listed to a file and/or to a Text window.
The colors, b/w patterns and pens can be specified for the different types of edges shown in a section; these colors are different from the the standard colors for edges in the Line Colors and Line Patterns and Pens dialogs in the Input2 menu.
With the Add button you can add edges representing the current section to the drawing of the crystal as a whole. The crystal can then be rotated to better show the orientation of the section. These edges disappear the next time the crystal is recalculated. You must replot after exercising this option.
Some basic facts and terminology must be understood in order to be able to use the suboptions for growth zoning.
The lines which appear in a section plot, or section edges, are the intersections of crystal faces with the section plane. If such a line of intersection is not also an exterior crystal edge, the resulting line will be called a one-face edge. If it is a crystal edge, then the result is a two-face edge, since the two faces intersect the section plane along this line.
A sector is ideally a pyramidal polyhedron which has one external face at its base, and the center of the crystal at its apex. If growth rates are not constant with time, faces and therefore sectors may be created or annihilated, and the geometry may be more complex, but a sector is always defined by a single crystal face. If the growth is linear, a section through the crystal center will show a sector as a triangular area, and if growth zones are drawn, there will be lines parallel to the crystal face. However, if a triangular area passing through the center is bounded on the outside of the crystal by a two-face edge, the area observed is not a sector, but the boundary between two sectors - those defined by the two faces which intersect at the edge.
In idealized drawings such as those made by SHAPE, special relations are very common because of symmetry and default setting geometry. In particular, the section often passes through the exact center, and two-face edges may often be seen. In real sections of crystals in rocks or synthetic materials, which are usually random with respect to height of the section and orientation of the crystal, relationships are in general less special and more complex; two-face edges are less common, and sectors and sector boundaries are very seldom seen in their entirety.