478 Wells, Wheeler and Penfield — Rubidium and 



Crystallography. 



The rubidium trihalides crystallize in the ortho-rhombic 

 system and are isomorphous with the corresponding cresium 

 compounds, showing a close similarity both in crystalline habit 

 and in axial ratios. 



The forms which have been observed are 



a, 100, i-l 



b, 010, i-i 



c, 001, o 

 m. 110, I 



a, on, 1-5; 



/, 021, 2-2 

 e, 102, \-l 



V, in, i. 



With the exception of the pyramid p, which was observed 

 as a small face only on Rbl . I„, these are the same as were 

 observed on the caesium trihalides, while the brachydome g, 

 012,-J4 which was found only on Csl . I 2 , was not observed on 

 any of the rubidium compounds. 



Of the three potassium trihalides, which were examined, 

 only one, KBr . BrI, was orthorhombic, like the ctesium and 

 rubidium compounds. The others, KI . I 2 and KC1 . C1I, are 

 monoclinic, but they can be referred to axes which are similar 

 to those of the orthorhombic series. 



The cleavage of the rubidium trihalides is perfect parallel to 

 c, less perfect parallel to a ; neither is easily produced. The 

 crystals are very brittle and usually break with a conchoidal 

 fracture. The potassium trihalides are exceedingly brittle and 

 no cleavage was observed. The optical properties were not 

 studied, owing to the difficulty of preparing orientated sections. 



In the following table the axial ratios of all of the alkali- 

 metal trihalides are given, arranged as in the caesium paper. 

 In the table of angles, those which were chosen as fundamen- 

 tal are marked by an asterisk. 







I. 



Series with ioc 



ine 



ir. 







r 



a : 



b 



: c 



a : b : 







CsI.I 2 



0-6824: 



1: 



1-1051 



1 



1-4655: 



1-6196 



Rbl . I„ 



0-6858: 



1: 



1-1234 



1 



: 1-4582 



1-6381 



KI . I„ 



j 0-7.065 

 { Monoc 



: 1 





1 



: 1-4154 







linic, a = 86° 47|' 





a— 86 



°47i' 



CsBr . I„ 



0-6916 



: 1 



: 1-1419 



1 



1-4460 



1-6511 



CsBr . BrI 



0-7203 



: 1 



: 1-1667 



1 



1-3882: 



1-6196 



RbBr.BrI 



0-7130 



: 1 



: 1-1640 



1: 



1-4025 



1-6325 



KBr . BrI 



0-7158 



1 



1-1691 



1 : 



1-3970 



1-6333 



CsCl . BrI 



0-7230 



:1 



: 1-1760 



1 



1-3831 : 



1-6268 



RbCl . BrI 



0-7271 



1 



. 1-1745 



1 



1-3753: 



1-6153 



CsCl . C1I 



0-7373. 



1 



1-1920 



1 



1-3563: 



1-6167 



RbCl . C1I 



0-7341 



1 



1-1963 



1 



1-3622: 



1-6296 



KC1 . C1I 



( 0-7335: 



1 : 



1-2204 



1: 



1-3633: 



1-6638 



Monoclinic, a = 83° 20' 



a =83° 20' 



