178 



with the geometrical form exhibited by these mimetic crystals, 

 they are said to manifest optical anomalies. To the discussion of 



these anomalies we shall return later on. 

 4. Some instances may elucidate this 

 occurrence of mimetic forms. 



Thus, chrysoberyll is a beryllium-alumi- 

 nate of the composition: BeO, ALO Z . 

 The mineral is orthorhombic (a: b: c = 

 0,4.701:1 : ofaoo); but its space-lattice 

 is obviously pseudo-hexagonal, as is 

 proved i. a. by the fact, that the angle 

 (001) : (Oil) is 6013'. Now three indivi- 

 duals will combine in such a way that 

 they form a trilling after the plane 

 Fi 136 (031), so that the apparently hexagonal 



Chrysoberyll. complex of fig. 136 is formed. By 



suitable development of the component 



individuals the re-entrant angles will recede gradually more and 

 more into the background; finally they become imperceptible, the 



d. e. ./. g. 



Fig. 137. 



Basal sections of: a, b, c: Aragonite; d: Bromlite; e: Cerussite; /: Chlorite; 

 g: Potassiumsulphate. 



aggregate is flattened towards the face a = (100), and in this way an 

 individual is produced which in its external aspect no longer deviates 

 from a true hexagonal crystal. The facets of (100] commonly show 



