schaller: composition of nephelite 109 



In this case we may write, with Pirani, 



a log A = log A' (4) 



where a = cc', since c, unlike S or A, is independent of all the 

 other quantities in equation (2). Equation (4) can be plotted 

 in several ways, and permits a ready comparison of the otherwise 

 somewhat contradictory results on the absorption coefficient of 

 the same substance. 



MINERALOGY. — The chemical composition of nephelite. Walde- 

 mar T. Schaller, Geological Survey. 



The chemical composition of nephelite has long been a much 

 discussed problem among mineralogists. One general theory 

 endeavors to harmonize the analytical figures with the calculated 

 values by the assumption of a number of complex formulas. The 

 very complexity of these formulas is sufficient to cast doubts on 

 their correctness. Another view is that the groups (Si0 4 ) and 

 (Si 3 8 ) can replace each other, in an isomorphous sense, so that by 

 the presence of sufficient (Si 3 8 ) replacing (Si0 4 ), the analytical 

 figures obtained can readily be accounted for. Foote and Brad- 

 ley have recently proposed 1 still another explanation, namely that 

 as a substance on crystallizing may form a " solid homogeneous 

 solution with foreign matter . . . ", the mineral nephelite 

 consists of a pure compound, probably NaAlSi0 4 , with a varying 

 amount of dissolved silica. Such an interpretation certainly is 

 suggestive but before it can be accepted for the case of nephelite 

 it must be proved that the compound NaAlSi0 4 can dissolve silica. 

 Furthermore, the fact that chemically the "dissolved silica" 

 acts in an entirely different way from any known modification of 

 silica must be satisfactorily accounted for. 



I believe that the composition of nephelite can be readily ex- 

 plained without assuming complex formulas and without calling 

 on the hypothesis of "dissolved silica." The mineral albite has 

 the formula AlNaSi 3 8 and the corresponding isomorphous min- 

 eral anorthite has the formula AlCa±Si0 4 . The soda anorthite, 



1 Foote, H. W., and Bradley, W. M.: On solid solution in minerals with special 

 reference to nephelite. Araer. Jour. Sci., 4th ser., 31: 25. 1911. 



