MINERALOGICAL CLASSIFICATION OF IGNEOUS ROCKS 91 

 AUXILIARY CONSTITUENTS 



Auxiliary constituents are constituents, mostly pneumatolytic 

 or metamorphic, which may be used in the nomenclature as min- 

 eral modifiers in the formation of sub-families. Rocks containing 

 these minerals may have independent names if desired. The 

 auxiliary minerals are seldom of importance. 



Topaz Primary scapolite 



Tourmaline Muscovite 



Cordierite Lepidolite 



Corundum Zinnwaldite 



Fluorite Apatite, etc. 

 Andalusite 



It will be observed that most of the auxiliary constituents are 

 light in color; they are, consequently, computed among the leuco- 

 crates. It is true that if this is done, tourmaline-granite will fall 

 among the leucocratic rocks, but since this rock is aplitic and the 

 mineral pneumatolytic, this is not undesirable. 



Glass must be computed from an analysis. One can usually 

 surmise its composition from the character of the phenocrysts and 

 the appearance of the rock. When undetermined, the rock must 

 be given a tentative name, such as hyaline-rhyolite, etc. 



RULES EOR COMPUTING ROCKS FROM THEIR MODES 



i. The sum of the minerals in the mode should be 100=1=0.5. 

 If less, recalculate 1 to 100. The sum of the leucocrates (quarfe- 

 loids plus auxiliary minerals) so obtained determines the class. 



Class 1. Leucocrates form less than 95 per cent of the total rock. 



Class 2. Leucocrates between 95 and 50 per cent. 2 



Class 3. Leucocrates between 50 and 5 per cent. 



Class 4. Leucocrates less than 5 per cent. 



2. Determine the orders in Classes 1, 2, and 3 directly from the 

 Ab-An ratio, the division lines being 0-5-50-95-100. In rocks 



1 All of the necessary computations may be performed in an instant of time by 

 means of a slide-rule. 



■ These classes are tentative. If thought desirable (see question 1, below), the 

 rocks will be divided into five classes. 



