646 CROSS, IDDINGS, PIRSSON, WASHINGTON 



5. With excess of A1 3 3 , (A1 3 3 > K 2 + Na 2 0): 



a) to extra A1 3 3 allot CaO in proportion of A1 3 3 : CaO 

 : : 1 : 1 for anorthite molecules. 



b) If there is further excess of A1 2 3 it is to be considered 

 as corundum, A1 3 3 . 



It it is not advisable to calculate muscovite with excess of 

 A1 3 3 instead of corundum, since it requires a readjustment of 

 orthoclase molecules, and muscovite may not occur in the rock, 

 the extra A1 3 3 entering alferric minerals. Its calculation is 

 considered subsequently. 



6. With insufficient Al 3 O s , (A1 3 3 < K 3 + Na 3 0): 



a) Extra Na 3 is allotted to Fe 2 O s in proportion Fe 2 O s : 

 Na 2 : : 1 : 1 for acmite molecules. 



b) If there is still extra Na 2 it is set aside for a metasili- 

 cate molecule (Na 2 Si0 3 ). 



c) When there is an excess of K 3 over A1 3 3 it is treated 

 in the same manner. It is an extremely rare occurrence. 



7. In working with reliable analyses in which Fe 2 3 and 

 FeO have been correctly determined : 



a) To Fe 3 O s is allotted excess of Na 3 under conditions6,#). 



b) To remaining Fe 3 3 is allotted available FeO in equal 

 proportions for magnetite. 



c) If there is any excess of Fe 3 3 it is calculated as hema- 

 tite. 



Analyses in which all the iron has been determined in one 

 form of oxidation, when it occurs in two, are of little value when 

 considerable iron is present. When the amount of iron is very 

 small the analyses may still be used as a means of classifying 

 the rock. For this purpose all the iron, if given as ferric oxide, 

 is to be calculated as FeO, except that necessary to be allotted 

 to Na 2 for acmite, and then used as below. 



8. a) Extra CaO after the foregoing assignments is allotted 

 to (Mg, Fe)0 in proportion CaO : (Mg, Fe)0 : : 1 : 1 for diop- 

 side molecules. 



In all molecules where (Mg, Fe)0 is present, MgO and FeO 

 are to be used in the same proportions in which they are found 



