Crystal systems characteristic symmetry element

Crystal system Characteristic symmetry element or combination of symmetry elements... 

The thirty-two crystal classes (crystallographic point groups) described in Section 9.1.4 can also be classified into the same seven crystal systems, depending on the most convenient coordinate system used to indicate the location and orientation of their characteristic symmetry elements, as shown in Table 9.2.1. 

Table 1.7. Seven crystal systems and the corresponding characteristic symmetry elements.

Within each of the six crystal systems, there arc specific crystal classes. Each class displays distinctive symmetry elements. There are 32 possible classes distributed among the six crystal systems. One of the crystal classes within each system possesses all of the symmetry elements that re characteristics of its space lattice cell. These are ealled the holohedral class of that system. Other classes within each system possess somewhat fewer symmetry elements and are called tncmhedral classes. 

The symmetry of the crystal structure is a direct consequence of dense packing. The densest packing is when each building element makes the maximum number of contacts in the structure. First, the packing of equal spheres in atomic and ionic systems will be discussed. Then molecular packing will be considered. Only characteristic features and examples will be dealt with here, following Kitaigorodskii [38]. ... 

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