;;.m; 



Allen timl Crenshaw — Sulphides of Zinc, 



the change for sphalerite. The same values apply to wtirtzite 

 also, within the limits of error of the determinations of refrac- 

 tive index — probably about ±"005, due to lack of homogeneity 

 of the blende and to observational errors. The analyses ancl 

 densities given in the table are by Allen and Crenshaw. 





% 



FeS 

 (MnS) 



d 



n 



Li 



n 



Na 



n ■ 



-1 



n-1 



Locality 



d 



d 

 Calculated for FeS. 





Li 



Na 



Li 



Na 



Sonora 

 Spain 

 Australia 

 Saxony 



0-2 

 8-6 



17- 



28-2 



4-090 

 4-023 

 3-98 

 3'935 



2-34 

 2-36 

 2-38 

 2-395 



2-37 

 2-40 

 2-43 

 2-47 



•328 

 •338 

 •346 

 •355 



•335 

 •348 

 •358 

 •373 



•442 

 •436 



•427 



■488 

 •471 

 •470 



Assuming that the FeS is dissolved troilite having a density 



of 4-78, the average refractive index of troilite calculated on 



the assumption of an additive relation is 3*08 for Li-light and 



n 2 — 1 1 



3*25 for Na-light. However, if the formula — j— — = X ~r is 



° ft + 2 a 



used in making a similar calculation, the refractive index for 

 Li-light is 4/7. So great a discrepancy indicates that the 

 additive relation of the refraction constants does not hold in 

 this case, or that it is some form of FeS other than troilite 

 that is present in the blende. The latter alternative appears 

 more likely, for mix-crystals of troilite and sphalerite should 

 have intermediate densities, whereas ferriferous blende has a 

 lower density than either of these minerals. A close agree- 

 ment between the formulas, the observed densities and refrac- 

 tive indices is obtained by assuming that the FeS in the blendes 

 has a density of 3 # 8 and an average refractive index of 2 8 for 

 Li-light. 



Etch-figures and anomalous double refraction. — Triangular 

 etch-figures on a cleavage surface of sphalerite heated in ZnCl 2 

 are sketched in fig. 7, C (p. 389). 



Pressure at a point develops double refraction which may 

 become permanent. Fig. 7, D and E, show doubly refracting 

 areas on a cleavage surface around a point at which pressure 

 was applied. The positions of the plate are 45° apart, the 

 lines CI representing a cleavage direction. 



The tooth-like irregularities on fracture surfaces of sphalerite 

 may be distinctly doubly refracting. 



