SULPHIDES. 



9.7 p.. The latter is not very well defined, so that, from the fact that it 

 coincides with a band found in apatite, one can hardly infer that it is 

 due to the PO 4 radical. 



SULPHIDES. 



STIBNITE (Sb 2 S 8 ). 



(Large fresh cleavage piece parallel to b, perfectly plane without striations. Area 



i by 1.5 cm. Curve a, fig. So.) 



The reflecting power of the sulphides of Zn, Pb, Fe, and Sb are 

 worthy of notice. They are known for their metallic luster, especially 

 stibnite (Sb 2 S 3 ). Their reflecting power in the infra-red is equally 

 conspicuous for its high value, which is uniform throughout the region 

 examined to 12 p.. Stibnite is the most marked in this respect. By 

 transmission it was found to be unusually transparent, the absorption of 

 a thin film, 0.45 mm., being about 45 per cent throughout the whole 

 region to 12 p., beyond which there appears to be another absorption 

 band. The reflecting power was found to be about 35 per cent to 1 1 p, 



soy, 





3 4 s 6 7 a 9 10 n n a 

 FIG. 80. Stilbite(a); Pyrite (b) ; Galena (c); Sphalerite (rf). 



is/t 



so that after eliminating the reflection the mineral is almost transparent. 

 This is well illustrated in curve b, fig. 55, which is for a section 10 times 

 that for curve a. It will be noticed the true absorption is only about 5 

 per cent. It is difficult to decide from the present date whether the 

 band beyond 14 p. is due to absorption or to selective reflection. The 

 reflection curve drops in the region of 13 ju,, which would indicate a 

 selective reflection just beyond this point. 



The observations were repeated on different specimens, and no differ- 

 ence greater than experimental errors was found in the reflecting 

 power, so that it can be definitely stated that the reflecting power is 

 higher in the visible spectrum, and drops to a lower value beyond 12 p.. 

 This, as well as the other sulphides, was found to be opaque to Roentgen 

 rays. 



