Haplodioritic and Related Magmas. 



171 



The line FG which bounds the three-phase field AFG- has 

 been called a three-phase-boundary.* AF and AG likewise 

 bound the three-phase field, but they are merely lines radi- 

 ating from A, their position being completely determined by the 

 points F and G. In order to be able to predict the phases 

 present at 1230° in a given mixture it is obviously necessary to 



Fig. 9. 



BEE' K K' G G 



Fig. 9. Phase relations at 1230° (full lines) and 1250° (broken lines). 



know the position of the line FG, the three-phase-boundary for 

 that temperature, or at 1250° the line F'G', and so oufor other 

 temperatures. 



Determination of Three-Phase-Boundaries. 



The determination of three-phase-boundaries can be accom- 

 plished in a number of ways the principles of which are read- 

 ily understood by reference to fig. 9. For a given position of 

 the isotherm DFE, i. e., a known temperature, it is only neces- 



* N. L. BoAven, The Ternary System : Diopside-Forsterite-Siliea, this Jour- 

 nal (4), xxxviii, p. 222, 1914. 



