434 Foote and Bradley — Chemical Composition of Analcite. 



rocks. An analysis by Hillebrand,* No. 18 in the table, and 

 another by Clarke & Steiger, f not quoted by Dana, appear to 

 be among the best that have been made. Their results, and 

 the ratios derived from them, are given below : 



Analysis by Hillebrand. 



Per cent Ratio 



Si0 o 55-81 4'27 



A1 2 3 22-43 1-01 



Na a O 33-47 1*00 



H 2 G 8-37 2-15 



Analysis by Clarke and Steiger. 



SiO„ 57-06 4-82 



A1A-- 21-48) 



Fe 2 3 -13 f 107 



CaO -16) 



Na 2 12-20 \ iUU 



H 2 8-96 2-53 



A glance at all the ratios given above will show that the 

 ratio between soda and alumina is nearly 1 : 1, but that silica 

 and water vary largely from the ratios 4 and 2 demanded by 

 the formula. In Table I the ratio for silica is greater than 4*2 

 in ten cases, it is less than 3*9 in six cases, and it is between 

 4 and 4-2 in only seven cases. Similar irregularities are true 

 as regards the ratio for water. Clarke and Steiger,;}; in com- 

 menting on their analysis given above, state : "It is at once 

 evident that our sample of the mineral varies nota- 

 bly in composition from the requirements of theory. The 

 silica is 2 J per cent too high while alumina and soda are cor- 

 respondingly low. No probable impurity and no presumable 

 errors of manipulation can account for so great a divergence. 

 The variations are large enough, common enough and regular 

 enough to command attention/' These investigators have sug- 

 gested that the mineral is an isomorphous mixture of ortho- 

 and trisilicate, the formula of which can be reduced to the 

 simple expression NaAl X. H 2 in which X represents 

 7iSi0 4 -I- mSi 3 8 . This modification, however, does not take 

 account of the water, w T hich appears to be as variable as silica. 



The evidence to be gathered from the data which have been 

 given points on the whole to a case of variable composition, 

 such as was previously found in the case of nephelite§ and 

 which can be explained by assuming solid solution. Consider- 



* Bull. U. S. Geol. Surv., xx, 27, 1885. 

 f This Journal [4], viii, 251, 1899. 

 X Loc. cit. 

 § This Journal [4], xxxi, 25, 1911. 



