THE NORM IN IGNEOUS ROCKS 



91 



into ackermanite. With silica lower still it is necessary to expend 

 as little of it as possible, making nephelite and ackermanite and dis- 

 tributing the silica then remaining between leucite and kaliophilite. 



In Analysis S which is calculated like Analysis P, the first of the 

 minor inflexible molecules to be made is chromite (cm), FeO.Cr^Oj, 

 the ratio between FeO and Cr2 03 being as 1:1. After ilmenite, 

 which follows, we make perofskite (pf), CaO.Ti02, with CaOiTiO^ 

 as 1:1. If silica has been present in abundance we should have 

 made titanite instead of perofskite. (Cf. Analysis I.) 



With the lowest-known ranges of SiO^, in rocks in which alumi- 

 nous spinel may form, AI2O3 and (MgFe)O being in excess, AI2O3 

 left over after making the feldspars, nephelite, or leucite, is corundum. 

 MgO and FeO uncombined after their allotment to such minerals as 

 magnetite and ilmenite may have to be entered with the femic 

 minerals simply as MgO and FeO. Their percentage weights are 

 determined by multiplying them by the molecular weights of MgO 

 and FeO. Such a case is illustrated in Analysis T. 



ANALYSIS T 



(Magnetite Spinellite). Professional Paper 14, p. 368, last analysis 



Routivaara, Finland 



Note. — CrjOj, .001, has been added to FejOj. 



The student in calculating the analyses given in Professional 

 Paper 14 should not expect to agree exactly in all cases with the 



