Cadmium, and Mercury. 



389 



value for e of 2"44. Hexagonal crystals of greenockite from 

 sublimed cadmium sulphide have a slightly higher refractive 

 index, e = 2"456. The lower refractivity of the crystals formed 

 in contact with salts is attributed to dissolved impurities, for 

 no other optical differences were observed. 



The sublimate produced by heating cadmium in hydrogen 

 sulphide contains crystals of greenockite of four very different 

 habits : filaments and needles, stubby prisms, twins after 



Fig. 7. 



Fig. 7. A, B, Greenockite twin. C, etch figures on sphalerite. D, E, 

 doubly refracting areas on sphalerite. 



1011, and twins after 2023. The drawings (fig. 7), made from 

 microscopical studies show the twins. JBy the elongation of 

 twins like A, feather-like crystals are produced. The angles 

 measured are as follows: aj_(1011) : c(0001)=43°; m (1010) : 

 x (1011) =47 ; m (1010) : z (2021) = 28°; wi(10ll): «(40il)= 15°; 

 p (2023) : v (404l)=43°; <u(4041) : s(202l)=ll°. 



The extinction angles measured from the trace of the twin- 

 ning (and composition) planes are 43° and 32°. The latter 

 angle was measured accurately more than forty times on several 

 twins, with variations less than 1°. The value 32° ± 10' was 

 obtained. For greenockite the calculated angle is 31° 58'. 

 Poorly developed twins of this type may represent the sup- 

 j)osed monoclinic modification of cadmium sulphide. 



Optical properties of pure Greenockite. 



The chief optical properties of pure greenockite may be in- 

 ferred from the upper curves of fig. 8. It is uniaxial — positive 



Am. Jour. Sci.— Fourth Series, Vol. XXXIV, No. 202.— October, 1912. 

 26 



