648 CPOSS, IDDINGS, PIRSSON, WASHINGTON 



b\ Of the feldspathoids (lenads), the metasilicates — leuclte 

 and analcite — are rarer than the orthosilicates, nephelite and the 

 sodalites. Analcite is so rare as a primary mineral that it may 

 be omitted from the discussion, and may be regarded as com- 

 posed of albite and nephelite -j- 2 water. Nephelite is more 

 frequently associated with orthoclase than leucite with albite, 

 from which it appears that potassic feldspathic molecules become 

 polysilicate when sodic ones form orthosilicate. If there is not 

 enough available silica to form orthoclase, leucite forms. 



c) The method is also based on the infrequent occurrence of 

 orthorhombic pyroxene with the feldspathoids, and the frequent 

 occurrence of olivine with these minerals. From which may be 

 deduced the rule that the development of metasilicate or 

 orthosilicate of (Mg, Fe)0 is controlled in most cases by the 

 available silica after satisfying the feldspathic molecules. 



d) Finally, the occurrence of melilite (akermanite) in rocks 

 free from the polysilicate feldspars and the metasilicate, hyper- 

 sthene, indicates that this subsilicate mineral is produced because 

 of insufficient silica to form the lowest normal silicates com- 

 monly developed in igneous rocks. 



The order of affinity of the common rock-forming oxides for 

 silica, which is well established by the foregoing and other facts, 

 as well as by such investigations as those of Lagorio 1 and 

 Morozewicz, 2 is as follows, beginning with that which has most 

 affinity, K 3 0, Na 2 0, CaO, MgO, FeO. The oxides A1 2 3 and 

 Fe 2 O s to some extent are analogous to Si0 3 and play in certain 

 respects the role of acidic oxides. 



The validity of this order is illustrated and confirmed by the 

 following facts : K s O and Na 3 alone form polysilicates (ortho- 

 clase and albite) with the ratios (K, Na) 3 : Si0 3 : : I : 6. 

 They also form the metasilicates (leucite, analcite and acmite) 

 with the ratio (K,Na) 3 : Si0 3 : : 1 : 4. Na 2 also forms the 

 orthosilicate nephelite, as well as the minerals of the sodalite 

 group, with the ratio Na 3 : Si0 3 : : I : 2. The corresponding 



1 Tschenmak's Min. Petr. Mitth., Vol. VIII, pp. 421 ff., 1887. 

 2 Ibid., Vol. XVIII, pp. 221 ff., 1899. 



