Diojpside — Forsterite — Silica. 



233 



the composition T, GT being a three-phase-boundary. The 

 whole now consists of pyroxene of composition T and tridy- 

 mite in the proportion, pyroxene : tridymite = DX : DT. 



It is apparent from the foregoing discussion that all mix- 

 tures of forsterite with the pyroxene P become completely 

 crystalline at the temperature of the point 1ST, or, vice versa, 

 begin to melt at the temperature of the point N. The three- 

 phase-boundary PN can, therefore, be determined by taking 

 any mixture of forsterite with pyroxene of composition P, or 

 pyroxene P itself, and determining the temperature of begin- 

 ning of melting. It will be recalled that this was the method 

 used in determining the three-phase-boundaries. 



Fig. 18. 



1390° 



^ 



1380° 



N P 



Carkjsl^ 20 



40 60 



ricjSi-Oj 



Similarly any mixture of tridymite with the pyroxene T, but 

 not pyroxene T itself, begins to melt at the temperature of the 

 point G and the first minute quantity of liquid formed has the 

 composition G. 



In tig. 18 the behavior is represented of mixtures whose 

 composition lies closer to the point at which the boundary- 

 curve, forsterite-pyroxene, cuts the pyroxene conjugation line. 

 FE is a portion of the conjugation line and GKH a portion of 

 the boundary curve. If a liquid of composition M is crystal- 

 lized, forsterite first separates and the composition of the liquid 

 changes toward O. At the temperature of the point O, 

 pyroxene of composition L begins to crystallize, OL being the 

 three-phase-boundary for the temperature of the point O. The 



