Diqpside — Forsterite — Silica. 



235 



Fig. 19. 



mixtures the 82 per cent mixture is completely molten at a 

 temperature slightly lower than the 77 per cent mixture. 

 These observations show that there is a minimum in the neigh- 

 borhood of the 82 per cent mixture, though it is not more than 

 2°-3° lower than the melting-point of diopside. The occur- 

 rence of the minimum is entirely consistent with all the rela- 

 tions found in the system. 



The Precipitation, Complete Solution and Reprecipitation 

 of Forsterite. — There is a small area, close to the point of 

 intersection of the boundary curve and conjugation line, every 

 point of which lies at the intersection of two three-phase- 

 boundaries. The point P of fig. 19 represents one of these. 

 The crystallization of a liquid of com- 

 position P takes place in the following 

 manner: Forsterite first crystallizes out 

 and the liquid changes in composition 

 towards S. At the temperature of the 

 point S pyroxene of composition L begins 

 to crystallize out. When the tempera- 

 ture is lowered the composition of the 

 liquid follows the boundary curve, 

 forsterite begins to dissolve and the 

 pyroxene to change in composition. 

 When the temperature of the point P is 

 reached, the liquid has the composition 

 P, forsterite has completely redissolved 

 and the pyroxene has the composition M, 

 EM being the three-phase-boundary for 

 the temperature of the point P. The 

 composition of the liquid now leaves the 

 boundary curve and follows the general 

 course PIH lying entirely in the pyrox- 

 ene field. During the temperature in- 

 terval between P and H, the mixture 

 consists entirely of pyroxene crystals and 

 liquid, without crystals of forsterite. At 

 the temperature of the point H the 

 pyroxene has. the composition N and 

 reprecipitation of forsterite begins, HN 

 being the three-phase-boundary for the 

 temperature of the point H. With 

 further lowering of temperature crystal- 

 lization of both pyroxene and forsterite 

 continues, the liquid changing in composition along the bound- 

 ary-curve towards K, and the composition of the pyroxene 

 changing towards O. When the temperature of the point K is 

 reached the liquid finally disappears, the last vanishing quantity 



