90 ALBERT JOHANNSEN 



tourmaline-granite, etc. This applies whether the modifying 

 constituent is a mafite or an auxiliary. 



THE MINERAL GROUPS 



It is not sufficient to divide the constituents of the rock into 

 those that are light and those that are dark, but it is necessary to 

 make certain definite groupings. The primary division, of course, 

 is into quarfeloids and mantes. Under the former are included: 



QUARFELOIDS 



Quartz (Qu). 



Potash feldspar (Kf), including orthoclase and microcline, and the ortho- 



clase molecule in microperthite, anorthoclase, etc. 

 Plagioclase (Plag), including the albite molecule in anorthoclase as well as 



all plagioclases. 

 Feldspathoids (Foids), nephelite, leucite, sodalite, hauynite, noselite, 



melilite, primary analcite, primary cancrinite, eudialyte, etc. 



The rear angle of the double tetrahedron represents the mantes. 

 It is the position of the remainder after the quarfeloids and 

 auxiliary constituents have been deducted. 



MAEITES 



Dark micas (biotite, phlogopite, etc.). 



Amphiboles. 



Pyroxenes (including uralitized pyroxenes). 



Olivine. 



Iron ores (magnetite, ilmenite, chromite, pyrite, hematite, etc.). 



Cassiterite. 



Garnet. 



Primary epidote. 



Allanite, zircon, rutile, and other dark minor accessories. 



SECONDARY CONSTITUENTS 



Secondary constituents are calculated as the originals from 

 which they came. Thus ore replacements of the mafites are com- 

 puted as mantes, kaolin as feldspar, chlorite as a biopyribole, 

 cancrinite and analcite as feldspathoids, serpentine as a mafite, etc. 



