232 



JV. L. Bowen — The Ternary Syst 



em 



the point D. The composition of the liquid now leaves, the 

 boundary curve and crosses the pyroxene field on the curve FE, 

 pyroxene continuing to crystallize and changing in composition 

 towards S. When the temperature of the point E is reached 

 cristobalite begins to crystallize. At this temperature the 

 liquid has the composition E and pyroxene the composition S, 

 SE being the three-phase-boundary through the point D for 

 the three phases, pyroxene, cristobalite and liquid. With 



Fig. 17. 



further lowering of temperature the composition of the liquid 

 changes along the boundary curve from E towards Z, cristo- 

 balite and pyroxene continue to crystallize and the pyroxene 

 changes in composition towards T. At the temperature of the 

 point Z cristobalite changes to tridymite, if perfect equilibrium 

 is attained, and with further lowering of temperature tridymite 

 and pyroxene continue to crystallize. When the temperature 

 of the point G- is reached the liquid finally disappears ; the last 

 minute quantity has the composition G, and the pyroxene has 



