schaller: plancheite and shattuckite 



^53 



optical constants, of which the refractive indices are the easiest 

 determined. If the refractive indices show a distinct difference, 

 then the minerals are not the same. 



The refractive indices of both minerals were determined by 

 the writer before any mention of shattuckite was published and 

 it was found that the lowest refractive index («) of shattuckite 

 was considerably higher than the highest (7) refractive index 

 of plancheite. The actual determinations are shown in table i ; 

 there are also given the independent determinations kindly made 

 by E. S. Larsen, of the U. S. Geological Survey. Mr. Larsen's 

 values are more accurate than those of the writer and should 

 be taken as the correct values. The accurate determinations of 

 the refractive indices of such finely fibrous minerals as those under 

 discussion is an operation requiring very careful work and con- 

 siderable experience. 



As table i readily shows, there is sufficient difference in the 

 optical constants of shattuckite and plancheite to preclude their 

 being identical. 



TABLE I 



Refractive Indices of Plancheite and Shattuckite 



« Stated to be near 1.70 (7), Lacroix, A. Mineral. France 4: 758. 1910. 

 b Two sets of determinations, made at different times. 



Zambonini's analysis of- plancheite yields the same formula 

 as^has been derived for shattuckite and he naturally concludes 

 that the two minerals are the same. Two suggestions are offered : 

 (i) that, through inadvertence, the mineral furnished Zambonini 

 (obtained from Lacroix in Paris) really was shattuckite and not 

 plancheite. This suggestion could have been readily proved 

 or disproved by a determination of the refractive indices of the 



