CLASSIFICATION OF IGNEOUS ROCKS 673 



CaO d , in diopside, plus CaO an , derived from anorthite, in conse- 

 quence of the transfer of A1 3 3 insufficient amount to satisfy 

 the ratio 



A1 2 3 



CaO d + CaO an 



Moreover, the ratio 



(Mg,Fe)0 



D 



CaO d + CaO a 



= D, 



approximately 1, must be maintained by transferring (Mg,Fe)0 

 from femic hypersthene or olivine. If the amount of these min- 

 eral is comparatively small a limit is set to the amount of alu- 

 minous pyroxene that can be produced. 



b) If there are no acmite molecules estimated among the 

 femic minerals, sufficient Na 3 must be transferred from soda- 

 feldspathic molecules to satisfy the ratio 



Na 2 _ D _ 



CaO, ± CaO, 



where ± CaO an is the lime transferred from, or to, anorthite. If 

 transferred from anorthite to diopside the sign is + , if to 

 anorthite from diopside the sign is — . 



This follows from the fact that the transfer of Na 3 from 

 soda-feldspathic molecules liberates equal molecules of A1 2 3 , 

 .and if all of these are not needed to satisfy the ratio 



ALO, 



CaO d ± CaO a 



= D, 



as is the case in a number of pyroxenes given in Table XII, the 



A1 3 3 not required withdraws CaO from diopside molecules to 



maintain the stoichiometric proportions obtaining in the salic 



minerals by making additional anorthite. 



Fe 3 3 is to be transferred from extra Fe 3 3 (hematite) or 



from FeO . Fe 3 3 (magnetite) in sufficient amount to satisfy the 



ratio. 



Fe 2°3 =D 

 CaO ri ± CaO aR 



