Orthorhombic mmm

Orthorhombic mmm V. = D, Orthorhombic-dipyramidal (= holohedral) Baryte, BaSO, 8... 

C-centering). Its Laue class was determined to be mmm since the SAED patterns show mmm symmetry. Thus, the IM-5 structure is C-centered orthorhombic. 

Point symmetry mmm (shorthand for 2lm, 2Jm, 21m) comprises three perpendicular mirror planes, which automatically generates three perpendicular twofold axes at their lines of intersection, and a center of symmetry at the origin. This point group applies to all orthorhombic centrosymmetric crystals, such as those of space group Pbca. 

Anhydrite is a calcium sulfate, CaS04, with an orthorhombic structure (mmm) where sulfur atoms, which are at the centers of tetrahedra of oxygens, and calcium atoms he on the lines of intersection of mirror planes (100) and (010). In nature, REE and Mn can easily substitute for Ca, becoming luminescence... 

Fig. 34. Orthorhombic system. (See also Fig. 25.) a. Unit cell type.. (H.COO),Sr.2HsO. Cla8s222. Left-and right-handed crystals, c. 1-Brom, 2-hydroxy-naphthalene. Class 222. d. Picric acid, Class mm. e. Oxalic acid. Class mmm. /. C,Br. Class mmm.

The same type of unit cell is appropriate for classes 222 and mmm (= 2jm 2jm 2/m) the cell edges, lie along the twofold axes. Donnay (1943) recommends that the longest shall be called b and the shortest c. When axes are chosen on morphological grounds the axis of the principal prism zone is labelled c, while b is the longer of the other two. The three classes mm, 222, and mmm constitute the orthorhombic system (Fig. 34). 

Sodium nitrite, NaN02, forms orthorhombic crystals of the shape of Fig. 179. This shape has holohedral symmetry mmm the internal symmetry might, however, be lower than this (see p. 269), and therefore atomic arrangements in all three classes of the orthorhombic system (mmm, 222, and mm2) may be considered. 

Orthorhombic. This system has three mutually perpendicular unit translations, whose perpendicularity is rigorously required by symmetry. This can only be assured if there are three mutually perpendicular twofold axes, either all proper (222) or the combination 222 (= mm2). The lattice itself has mmm symmetry because when the centering condition is added to either of the above, it expands them from 222(Z)2) or 2wm(C2l.) to mmmiDy,). 

Orthorhombic Space Groups. There are 59 of these space groups divided among three crystal classes 222(D2), mm2(C2l.), and mmm(DVt). Within each class there is at least one group associated with each of the four types of orthorhombic Bravais lattice, / C (or A), F, /. We shall make no attempt to derive these systematically, but a few examples and some useful observations are warranted. The complete list of the 230 space groups given in Appendix VIII should be consulted at this time. 

---