THE NORM IN IGNEOUS ROCKS 



11 



It is to be noted that with quartz we will not have olivine in the 

 norm and vice versa. 



By making some hypersthene and some olivine, therefore, we can 

 allow for a small shortage of SiOj. If we attempt to calculate 

 Analysis L in the same manner it is found that after making orthoclase 

 (or), albite (ab), anorthite (an), magnetite (mt), diopside (di), and 

 olivine (ol), 219 more units of silica have been called for than are 

 available. The silica deficit is too great to be treated as in the pre- 

 ceding example. 



ANALYSIS L. TENTATIVE 



Laurdalose (Syenite Pegmatite). Professional Paper 14, p. 296, No. 14 



Stoksund, Norway 



In the tentative distribution albite (ab) calls for 6X125, or 750, 

 SiOj. Nephelite would use up only 250, or 2X125, SiO^. If we 

 allot in the first place (holding out the soda, NaaO, 125, and equal 

 AI2O3, 125) the proper amounts of the various oxides for orthoclase 

 (or), anorthite (an), magnetite (mt), diopside (di), and olivine (ol), 

 we shall have 531 units of SiO^ left, to go with the 125 NajO and 

 125 AI2O3. What we do then is to make a certain amount of albite 

 and a certain amount of nephelite. It is to be remembered that in any 

 allotment of Na,0 and AI3O3 to albite (ab), Na2O.Al2O3.6SiO,, 

 and nephelite (ne), Na20.Al203.2Si02, the ratio of Na^O to Al,03 

 will be as I : I. Being silica poor we made olivine (ol), with (MgFe)O 

 and not hypersthene. The formulas for the distribution of Na^O, 

 AI2O3, and SiO, between albite and nephelite are as follows: 



