THE NORM IN IGNEOUS ROCKS 6i 



Of the minor inflexible molecules in the femic group magnetite, 

 ilmenite, and apatite are very common. The others occur occasion- 

 ally. 



Among the femic minerals diopside, hypersthene, and olivine 

 appear very often in the norm, acmite and wollastonite are not 

 unusual, while ackermanite, potash metasilicate, and soda meta- 

 silicate are rare. Among the minor inflexible mineral molecules 

 magnetite, ilmenite, and apatite commonly appear; hematite, titanite, 

 perofskite, fluorite, and pyrite are not infrequently met with; and 

 chromite, rutile, and calcite are rare. 



The minor inflexible molecules, with the exception of magnetite 

 and hematite, will not be considered at the outset. They are not 

 present in the first eight of the series of calculated analyses, but were 

 they present they would claim attention in the first place. Their 

 calculation is simple, but the form of presentation gains in clearness 

 by bringing them in only after the main features of the calculation 

 have been dealt with. The key to the disposal to be made of the 

 important oxides SiO„ Al.Oj, Y&,0^, FeO, MgO, CaO, Na,0, and 

 K^O lies in the relative affinities of K^O, Na,0, CaO, MgO, FeO, 

 and Fe203, for SiO^ and Al^Oj. The point of prime importance is 

 the amount of the two oxides SiO^ and AI3O3. In the simpler cases 

 K2O, Na^O, and CaO are to be allotted to ALOj and SiO^ in the 

 right proportions for the formation of the feldspars. K^O has the 

 strongest affinity for AI2O3 and SiO^. It therefore has the first 

 claim, and, after it has taken its quota of these oxides, Na2 0, with 

 the next strongest affinity for them, receives its quota. Lastly CaO 

 with an affinity less than the others is to be satisfied. The oxides 

 MgO and FeO do not unite with both AI2O3 and SiOa at the same 

 time in the normative minerals. They combine with SiOa alone to 

 form hypersthene and olivine, or with CaO and SiO^ to form diopside. 

 In Analysis A the simplest possible case is given. SiO^ and 

 AI2O3 are abundant, occurring in sufficient amounts to answer every 

 claim of K^O, Na2 0, and CaO upon them. Al^O, remaining over is 

 corundum, and SiOa remaining over is quartz. Ti02 and P^O^ are 

 not present, and no other elements occur which should be calculated 

 as the minor inflexible mineral molecules. Fe2 03, FeO, and MgO 

 not being present, there is nothing to be allotted for femic minerals. 



