CHEMICOMINERAL RELATIONSHIPS IN ROCKS 223 



amphiboles. Magnesium may occur without iron in metasilicate 

 molecules, in diopside with calcium, and in enstatite without 

 calcium. The alkali-alumina silicates may be polysilicates, 

 metasilicates or orthosilicates. The ferromagnesian silicates 

 may be metasilicates, orthosilicates or subsilicates. Iron may 

 occur uncombined with silica in the form of magnetite, and 

 silica may occur uncombined with other elements in the form 

 of quartz or tridymite. 



Preliminary correlation. — Some of the more evident relation- 

 ships between the pyrogenetic minerals in igneous rocks and the 

 chemical composition of the magma have been stated in a pre- 

 vious article in this volume. 1 They are : the usual occurrence of 

 quartz with the polysilicate feldspars, and its non-occurrence with 

 the meta- and orthosilicate feldspathic minerals : leucite, nephe- 

 lite and sodalite; also the relation between these and the per- 

 centage of silica in the rock — the highest of these silicates 

 always forming which is possible with the available silica in 

 the magma. Another law seems to be that the alkalis control 

 an equal amount of alumina, forming alkali-feldspathic molecules, 

 and that aluminium in excess of this may combine with calcium 

 to form anorthite molecules, or with magnesium and iron to enter 

 pyroxene and amphibole molecules. Soda does not seem to 

 unite with ferric oxide to enter pyroxenes or amphiboles in any 

 considerable amount unless it is in excess of alumina. The 

 metasilicate, orthorhombic pyroxenes, hypersthene and enstatite, 

 usually occur in the more siliceous rocks instead of the ortho- 

 silicate, olivine, which usually occurs in the less siliceous rocks. 

 But this is not an invariable rule, and olivine occasionally occurs 

 in the more siliceous rocks (dacite) and not unfrequently in the 

 presence of quartz. The orthosilicate, mica, commonly occurs 

 in highly siliceous rocks together with quartz ; pyrogenetic 

 muscovite exclusively so. Biotite also occurs in rocks low in 

 silica. Its range of occurrence bears no fixed relation to the 

 percentage of silica in the rock. Magnetite occurs in rocks of 

 all degrees of siliceousness and is not directly dependent on the 



x On Rock Classification, Jour. Geol., Vol. VI, pp. 96-08. 



