THE NORM IN IGNEOUS ROCKS 73 



available AI2O3, 93, and SiO^. The K^O remaining over is allotted 

 to potassium metasilicate (ks), K^O.SiOj, with SiOj, in the ratio 

 1:1. After all the KjO has been used NajO takes Fe203, as far as 

 Fe2 03 is available (there being no AI2O3 left to unite with it), and 

 SiOj, for acmite. Na^O still remaining over is sodium metasilicate. 

 CaO remaining after the foregoing assignments takes MgO and FeO 

 in the requisite amounts, and in the ratio in which they stand, 8:0, 

 for diopside (di). MgO remaining, there being no FeO, is allotted 

 to hypersthene, and the SiOa which has not been used is quartz. 



All the analyses thus far presented, from A to I inclusive, have 

 been those in which SiOj is abundant. The calculation of analyses in 

 which SiO, is relatively low is usually more difficult. Orthoclase 

 (or), K20.Al203.6Si02, requires more sihca than leucite (Ic), 

 K20.Al203.4Si02, and leucite requires more silica than kaliophilite 

 (kp), K20.Al203.2Si02. In the same way albite (ab), Na20.Al2 

 03.68102 requires more silica than nephehte (ne), Na20.Al203. 

 2Si02. It should be noted that the ratio between K2O and AI2O3 

 is always as 1:1 in orthoclase, leucite, and kaliophilite, and that the 

 ratio between NajO and AI2O3 is always i : i in albite and nephelite. 

 Hypersthene (hy), (MgFe)0.Si02 recjuires more silica than olivine 

 (ol), 2(MgFe)0.Si02, for the same amount of (MgFe)O. With low 

 silica, therefore, a substitution is made of one or more minerals which 

 require less silica than the minerals employed in the straightforward 

 calculations thus far considered. 



Where silica is low the simplest adjustment is that illustrated by 

 Analysis J. Here the minor inflexible molecules, ilmenite (il), and 

 apatite (ap), are first calculated. In this analysis apatite cannot get 

 its quota of CI or F, for these elements have not been determined. 

 The molecular weight is however taken as 336, and the percentage 

 weight of the mineral obtained by multiplying the amount of P2O5 by 

 336. The table on p. 258 for the percentage weights of apatite is 

 based on a molecular weight of 336, F or CI having been available. 

 AI2O3 is present in sufficient amount to allow with silica for the forma- 

 tion of orthoclase (or), albite (ab), and anorthite (an). After the 

 allotments for magnetite (mt), and diopside (di), there remain of 

 MgO 67, and of FeO 20 units. The silica available at this point is 

 62. This is not enough to make hypersthene with the MgO and 



