THE NORM IN IGNEOUS ROCKS 



63 



In Analysis B we have the same condition, except that FeO and 

 Fe2 03 are both present. After the allotment for the feldspars has 

 been made, and AI2O3 remaining over has been given to corundum 

 (C), Fe2 03 and FeO, in the proportion 1:1, are allotted to magnetite 

 (mt), and silica remaining over is quartz (Q). 



ANALYSIS B 



Tehamose (Quartz Porphyry). Professional Paper i/j, p. 132, No. 10 



Tamaya, Chile 



The calculation of Analysis C is like the preceding one in its 

 allotments for orthoclase (or), albite (ab), anorthite (an), corundum 

 (C), and magnetite (mt). Then there remain over, besides SiO^, 

 2 molecular-proportion units of MgO, and 15 of FeO. These are 

 allotted to hypersthene (hy), (MgFe)0.Si02, in the proportion 

 (MgFe)0:Si02 as 1:1. MgO and FeO are used in hypersthene 

 in the ratio in which they happen' to stand when this mineral comes 

 to be made. Here the ratio is 2:15. In later analyses MgO and 

 FeO will be introduced into the calculations in the minerals diopside 

 (di), and olivine (ol), as well as in hypersthene. When all three or 

 any two of them are to be made MgO and FeO are to stand in all of 

 them in the same proportion in which they were used in the first of 

 these minerals calculated at the time. In Analysis C it will be noted 

 that hypersthene is the sum of two parts, MgO.SiO^ and FeO.SiOa, 

 each of which is to be found separately. FeO.SiOg may be looked 

 up in the table on p. 254. MgO.SiOa is equal to 100 times the 

 amount of MgO. These findings are added together for hypersthene. 



