The orthorhombic system also contains the classes 2mm and mmm (Figure 2.6). The former contains two mirror planes, which must be at right angles. Two mirror planes at right angles automatically show diad symmetry along the line of intersection (Figure 2.6). Since m ≡ , this group could be designated 2 , and this is why it appears in the orthorhombic system, which is defined as possessing three diad axes. This crystal class could simply be designated mm since the diad is automatically present. However, mm2 is usually used for the later development of space groups (see Section 2.12). A crystal containing three diad axes can also contain mirrors normal to all of these without an axis of higher symmetry. Such a point group is designated mmm, or could be designated 2/mm. As can be seen from Figure 2.6, the multiplicity of the general form is now eight. The special forms {hk0}, {h0l} and {0kl} now show a lower multiplicity than the general one. The point group mmm shows the highest symmetry in the orthorhombic system. The point group showing the highest symmetry in a particular crystal system is said to be the holosymmetric class.
To plot a stereogram of an orthorhombic crystal centred on 001 if we are given the lattice parameters a, b and c, we proceed as shown in Figure 2.8. In Figure 2.8a the poles of the (001), (010) and (100) planes are immediately inserted at the centre of the primitive and where the x‐ and y‐axes cut the primitive, respectively. A pole (hk0) can be inserted at an angle θ along the primitive to (100), as shown in Figure 2.8a, by noting from
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