670 CROSS, IDDINGS, PIRSSON, WASHINGTON 



from salic to femic minerals to form alferric minerals. And 

 since the transfer of A1 3 3 involves the CaO, K g O, and Na 2 

 to various degrees, it is convenient to compare the amount of 

 A1 2 3 to be transferred with the principal one of these com- 

 ponents taking part in the aluminous ferromagnesian mineral. 

 This component becomes a unit of comparison for the other con- 

 stituents of the particular mineral. In the pyroxenes and 

 amphiboles CaO is the component next to A1 2 3 most involved 

 in the change. It becomes the unit of comparison in these min- 

 erals. In the micas K 2 plays this role. 



The molecular relations which must be taken into account 

 are expressed by ratios in the Tables XII, XIII, XIV at the end 

 of this Part, and may be summed up as follows : 



Aluminous pyroxenes [Table XII). — 1. The ratio of A1 2 3 

 to CaO ranges from almost nothing to 0.23. From these data 

 the maximum limit is 



ALO, 



-== 0.2"? . 



CaO J 



2. The nearly equal proportions between Fe 2 O g and Na 2 

 indicate that the soda is present in the acmite molecule. 



3. The generally small ratio between Na 2 and CaO. The 



Na„0 .... 

 ratio „ „ is less than 0.1 in most cases. Moreover, the pres- 

 CaO r 



ence of a notable amount of acmite molecule is indicated by the 



optical properties of the pyroxene. 



4. The nearly constant ratio between MgO+FeO and CaO, 

 which is approximately 



MgO + FeO __ 

 CaO ~~ * ' 



5. The Si0 2 +TiO s is approximately equal to the number of 

 molecules of MgO + FeO + CaO + 4Na 2 0, corresponding to 

 pyroxene molecules (MgFe)O . CaO . 2Si0 2 and Na 2 . Fe 2 3 . 

 4Si0 2 and (Mg, Fe)0. (Al, Fe) 2 3 -Si0 2 . 



Aluminous amp/iiboles {Table XIII). — The relations are less 

 definite. There is a wide range in chemical composition, and it 



