Alnoitic Rocks at Isle Cadieux, Que. 23 



they are encountered by the lines indicating the changing 

 composition of any crystallizing liquid mixture may be 

 shown approximately by projecting upon the base the sur- 

 faces where thus encountered. The position of the point 

 on the nephelite-diopside join (A) serves as a reference 

 or indicating point. 



In the CaO-MgO-Si0 2 system itself as shown in fig. 1 

 all of the liquids in equilibrium with monticellite are more 

 siliceous than monticellite itself. On the other hand 

 diopside lies in the pyroxene field, that is, the liquids with 

 which it is in equilibrium surround it and are both more 

 siliceous and less siliceous. When nephelite is added the 

 pyroxene field is pulled over toward the silica corner and 

 with about 19 per cent nephelite the boundary surface 

 pyroxene-forsterite passes through the indicating point 

 (A) which thereafter lies in the forsterite field. At 20 

 per cent nephelite the section through the tetrahedron 

 shows the projected fields in the relative positions of 

 fig. 3, the arrows indicating falling temperature. This 

 shows that forsterite crystallizes first, pyroxene second, 

 and then melilite from a mixture given by the indicating 

 point. 



With 40 per cent nephelite the relative positions of the 

 projected fields are as shown in fig. 4. Thus forsterite 

 crystallizes first and is joined simultaneously by both 

 pyroxene and melilite from a mixture of diopside and 

 nephelite as given by the indicating point. With 55 per 

 cent nephelite the fields as projected are shown in fig. 5. 



The manner in which the fields of forsterite and melilite 

 are drawn farther over toward the silica corner as more 

 and more nephelite is added is well shown by this series 

 of figures (1, 3, 4 and 5). 17 A simple, if only qualitative 

 explanation of this is that equilibrium in the liquid is such 

 that the nephelite in the liquid takes silica to itself and 

 leaves relatively little for the lime and magnesia, thereby 

 increasing the concentration in the liquid of the low-silica, 

 lime and magnesia compounds and causing their pre- 

 cipitation at an early stage. Indeed the series of figures 

 is but a graphical and qualitative expression of the 

 equations given on page 21. This affords an example 

 of the manner in which both affinity and solubility combine 



17 In fig. 1 the point corresponding to (A) is the diopside point itself. 



