Q. J. Brush— Chemical Composition of Durangite, 465 



weighed as sulphates and then converted into chlorides and 

 separated by ether and alcohol. 

 The results of the analysis are as follows : 



I. II. 



Arsenic acid, 53-11 



Alumina, 



' 17-19 



Ferric oiide,.::.-.".::. 



...'.] 2-08 

 13-06 



Lithia, 





Fluorine, 



7-67 7- 



102-99 



esponds to 3-23 per cent 

 hich, being subtracted, the analysis foots up 99-76, 

 ng the percentages of the elements we have the 



-462 -477 -465 2*298 



Multiplying this ratio by the quantivalence of the elements 

 we find that the ratio of E : ft : As is 1 : 3 : 5 with 9(0, Fl.) 



The formula may therefore be written (Na, Uy (Al, Fe, Mn) 

 As^O, FI^)^ 



The ratio of oxygen to fluorine in the mineral is as 9'4: 1. 

 This is a confirmation of the conclusion drawn by me from 

 the analysis of the lighter colored crystals described in the 

 original 'paper.* 



The mean of my two analyses gave : 



Arsenic acid, 54-16 



Alumina, 20'35 



Ferric oxide, 4-92 



Manganic oxide, 1"43 



Soda, 11-76 



Fluorine, .undetermined 



The variety examined by Mr. Hawes contains less alumina, 

 and considerably more iron, which accounts for its dariier color 

 and slightly higher specific gravity. His results prove the 

 mineral to be an arseniate analogous in chemical composition 

 to amblygonite as suggested in my previous paper. 



