CLASSIFICATION OF IGNEOUS ROCKS 647 



after FeO has been allotted to the molecules previously men- 

 tioned. That is, they are to be introduced into diopside, 

 hypersthene and olivine with the same ratio between them. 



d) If there is still an excess of CaO it is to be set aside for 

 calcium metasilicate (CaSi0 3 ) or subsilicate (4CaO. 3 SiO s ), 

 equivalent to wollastonite or akermanite. Such extra CaO will 

 in most cases actually enter garnet, an alferric mineral. 



9. With insufficient CaO, (CaO < (Mg, Fe)0) : 



a) Extra (Mg, Fe)0 is to be set aside for metasilicate or 

 orthosilicate, hypersthene or olivine, according to the amount 

 of Si0 3 present. 



The allotment of ' SiO 3 to form silicates begins with the bases 

 which occur with silica in but one proportion, and is carried on 

 as follows : 



10. To Zr0 3 allot Si0 3 in proportion of I : I for zircon. 



1 1. To CaO and A1 2 3 in anorthite is allotted equal SiO s to 

 form CaO. A1 3 3 .2Si0 3 . 



12. To CaO and (Mg, Fe)0 in diopside is allotted equal 

 Si0 3 to form CaO . (Mg, Fe) 0.2Si0 3 . 



13. To Na 3 and Fe 3 O s in acmite is allotted Si0 3 to form 

 Na s O. Fe 3 3 .4Si0 3 . 



14. To Na s O (or K 3 0) set aside for metasilicate molecules 

 allot Si0 3 to form Na 3 O.Si0 3 or K 3 O.Si0 3 . 



15. To Na 3 and Al 3 O s in sufficient amount to form with 

 NaCl sodalite, or with Na 3 S0 4 noselite, is allotted Si0 3 to 

 satisfy the formulas : 3 (Na 3 0. A1 3 3 2Si0 3 ). 2NaCl, sodalite, 

 2(Na 2 0. A1 3 3 .2Si0 3 ) .Na 2 S0 4 noselite. 



The allotment of silica to bases which may form two or more 

 silicates, ortho-, meta- or polysilicate, is controlled in part by 

 the amount of available silica, in part by the affinities of the 

 bases for silica as explained below, in accordance with certain 

 commonly observed facts, as follows : 



a) Quartz does not occur with nephelite and leucite, that is, 

 the feldspathic molecules will be polysilicates, orthoclase and 

 albite, if there is sufficient available silica. 



