Arrangement of Atoms in Crystals. 



175 



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structure. In most compounds the reverse is the case, 

 as is illustrated by the following comparison of molecular 

 volumes : — 



KjSOj... 64-91 

 Rb 2 S0 4 . 73-34 

 Cs 2 S0 4 . 84-58 



K.,Se0 4 ... 71-67 

 Rb.,Se0 4 . 79-94 

 Cs.,S0 4 . 9109 



KC10 t ... 5491 

 RbC10 4 . 61-33 

 CsC10 4 . 69-84 



KMn0 4 ... 58-53 

 RbMnO, . 63-23 

 CsMn0 4 . 7001 



7. The structure of the isomorphous series of rhombo- 

 hedral carbonates typified by Calcite, OaC0 3 , is illustrated 

 by fig. 2. In Calcite the calcium atoms lie on a face-centred 



Fi«. 2. 



rhombohedral lattice, corresponding to a face-centred cubic 

 lattice compressed parallel to a trigonal axis so that the 

 angle of 90° between the edges of the cube becomes an angle 

 of 101° 55'. The carbon ntoms occupy the intermediate posi- 

 tions. Each carbon atom is surrounded by three oxygen 

 atoms, as shown in the figure. The precise position of the 

 oxygen atoms is not determined by the symmetry of the 

 structure, they lie at some point along the line joining neigh- 

 bouring carbon atoms in the planes parallel to the face (111) 

 of the crystal. The ratio x/d in the figure, which fixes their 



O 



