CLASSIFICATION OF IGNEOUS ROCKS 6 J J 



MgO 



and the ratio ,-, ~ must be maintained according - to the kind 

 FeO 6 



of mica. 



Si0 2 and Ti0 2 are to be transferred according to their 

 respective ratios 



Si0 2 Ti0 2 



K^) = D and K^ = D - 

 After which Si0 2 must be adjusted among the molecules 

 affected. In general some Si0 2 will be liberated and the amount 

 of quartz in quartz-bearing rocks will be increased. In other 

 cases lower silicates will be raised. 



The calculation of muscovite is simple, for the reason that it 

 occurs mainly in rocks in which there is an excess of A1 9 3 , 

 estimated as corundum. 



Since in common muscovite H=2K the following ratios 

 obtain : 



H 2 C> A1 2 Q 3 SiO 



K 2 ' K 2 ' 6 ' K 2 ' 



Such a muscovite equals orthoclase plus corundum plus water. 

 2 H 2 + K 2 + 3A1 2 3 + 6Si0 2 = K 2 + A1 2 3 + 6Si0 2 + 

 2 A1 2 3 + 2H 2 . 



Hence to the molecules of hypothetical corundum add half as 

 many of the orthoclase and an equal number of H 3 . The distri- 

 bution of Si0 2 remains as before, the amount of quartz has not 

 been affected. 



From the foregoing it is seen that the development of augite 

 in rocks reduces the amount of anorthite molecules calculated 

 among the salic minerals and also reduces the amount of hyper- 

 sthene or olivine that appear as femic. It affects the distribution 

 of silica by liberating a part of that allotted to anorthite when 

 the (Mg,Fe)0 is derived from hypersthene, or when Na s O is 

 transferred from albite molecules. Its development would 

 reduce the amount of olivine rather than hypersthene when both 

 are otherwise normatively present. 



The crystallization of hornblende reduces the amount of 

 anorthite and hypersthene or olivine reckoned as standard min- 



