644 s - R OZU 



the separation of gelatinous silica in slightly hot hydrochloric acid 

 of the strength of 22 per cent. 



The chemical analysis was made by W. Yasuda in the chemical 

 laboratory of the Imperial Geological Survey of Japan. The 

 sample for the analysis was taken from the clear and fresh part of 

 a crystal, and powdered to grains one millimeter or smaller in 

 diameter. To remove the impure parts, which contained inclusions 

 of olivine, magnetite, and glass, the grains were separated into 

 three portions by Thoulet's solution, having specific gravities of 

 2.747 and 2.760. The analysis was made of the sample with the 

 specific gravity lying between the two values. The result is as 

 follows : 



Si0 2 43-5i 



AI2O3 35-75 



FeO trace 



MgO 1 . 1 1 



CaO , 19.48 



Na 2 0.61 



K 2 0.05 



100.53 



Subtracting silica and magnesia corresponding to the olivine 

 molecule, and potash as impurity, and calculating the remainder 

 as parts in 100, we have: 



W (percentage) Mol. prop. 



Si0 2 43-3° O.73 



A1 2 3 36.31 0.36 



CaO 19.77 0.35 



Na 2 0.62 0.01 



From which it is found that the composition of the anorthite may 

 be represented as a mixture of 2Ab and 3 5 An, or Ab s . 4 An 04 . s , which 

 corresponds closely to the value determined optically as Ab s An 9S 

 — Ab 4 An 96 . 



