CHEMICOMINERA L RE LA TI ON SHI PS IN RO CKS 227 



only 35 per cent of Si0 3 when the alkali-silica ratio is less than 

 .045. For a combination of albite and 2MgO.Si0 2 the range 

 is shown by the line AFo, and for a combination of albite and 

 olivine, 2(Mg,Fe)O.Si0 2 , in which Mg: Fe : : 3:1, the range 

 is AOL From this it is seen that if orthosilicate of magnesium 

 accompanies that of iron the nephelite limit will advance toward 

 the left, that is, in the direction of higher silica percentages, and 

 the quartz limit will recede in the same direction. 



In case iron and calcium or magnesium occur as metasilicates 

 the range of possible combinations of these with albite is shown 

 by the lines AF, AC, AM. If magnesium and iron occur 

 together in equal proportions, as in some hypersthene, the line 

 nearly coincides with AC. So also if the metasilicate has the 

 composition CaMgFeSi 3 O g , or Ca 2 MgFeSi 4 1 2 (a diopside) 

 or Ca 2 Fe 3 Mg 3 Si 8 24 (actinolite molecule in amphibole) the 

 range would coincide with the same line. An increase of mag- 

 nesium would move the line to the left toward AM. The 

 range of positions for diopside is indicated by the bracket in the 

 diagram. The range of positions for enstatite would lie to the 

 left of this for the most part. From this it appears that rocks 

 having soda and alumina in equal amounts together with meta- 

 silicates of iron, magnesium and calcium, if they occur to the 

 right of the limits just described would necessarily contain 

 nephelite, and if they occur to the left of these limits they must 

 contain quartz. Since metasilicates are the highest silicate of 

 these elements occurring in igneous rocks it follows that no 

 other ferromagnesian molecule may be postulated which will 

 appropriate more silica from the magma. Hence the most 

 favorable case, namely that involving the highest silicate of 

 alkali and alumina and the highest of calcium, iron, and mag- 

 nesium, requires the presence of quartz in all holocrystalline 

 rocks occurring to the left of these lines. In actual fact the 

 three elements just named usually occur in somewhat similar 

 proportions, so that the position of the line AC represents the 

 limit of quartz in the ordinary case most favorable to its 

 scarcity. Any combination of orthosilicate will tend to move 



