THE NORM IN IGNEOUS ROCKS 65 



In Analysis D after the allotment for magnetite (mt) , 3 molecular- 

 proportion units of Fe2 03 are left over. These go in as hematite 

 (hm). Then MgO, 4, and FeO, o, are left over. They are used to 

 make a hypersthene free from FeO. In like manner in other analyses 

 hypersthene might be made of FeO.SiOj, the other component 

 MgO.SiOa not being available. 



In analysis E the allotments are made for orthoclase (or), albite 

 (ab), anorthite (an), and magnetite (mt). CaO remaining is allotted, 

 with MgO, FeO, and SiO„ to diopside (di), CaO.(MgFe)0.2SiO„ 

 the proportions of the constituents by the formula being 1:1:2. MgO 

 and FeO together are equal to CaO, and are used in the proportion in 

 which they are found when the mineral comes to be made. Here 

 the ratio is 36:5, or nearly 7:1. The silica is twice the lime. MgO 

 and FeO remaining are used for hypersthene (hy), still in the same 

 ratio 7:1 (seep. 63, line 11). SiO^ is allotted by hypersthene in 

 amount equal to MgO + FeO. 



It is to be noted that we cannot have diopside and corundum 

 together in the norm. 



Analysis F presents the case where K2O is allotted with AI2O3 and 

 SiO^ to orthoclase (or), and Na^O is allotted to AI2O3 and SiO^ for 

 albite (ab). AI2O3 remaining, 9 units, can satisfy only 9 units of 

 CaO for anorthite (an). Then CaO remaining is allotted to diopside 

 (di), as far as MgO and FeO are available. Of CaO 27 molecular- 

 proportion units are still left. These take an equal amount of SiO^ 

 for wollastonite (wo), CaO.SiOj, 1:1. It is clear that since MgO 

 and FeO were not present in sufficient amounts to make diopside with 

 all the lime, there remain none of these constituents for hypersthene 

 or olivine. So then with wollastonite there will be no hypersthene 

 or ohvine. 



We now come to Analysis G, in which with abundant SiOa, the 

 AI2O3 covers KjO and partly covers NaaO. Orthoclase (or), is 

 made, and albite (ab), as far as the AI2O3 admits of it. Soda, Na2 0, 

 5 units, left over, there being no AI2O3 available for it, takes Fe2 03 

 and SiO^ for acmite (ac), Na20.Fe2 03.4Si02, in the proportions, 

 1:1:4. It is clear that with acmite there will be no anorthite. After 

 the allotment is made for magnetite (mt), the CaO takes MgO, FeO, 

 and Si02 for diopside (di). The remaining MgO and FeO are used 



