226 JT. L. Bowen — The Ternary System: 



Methods of Determination of the Three-Phase-Boundaries. 



The Composition Method. — After discussing theoretical sys- 

 tems of various types, we shall now return to the system under 

 investigation and point out the methods used in determining 

 the three-phase-boundaries. 



Figure 12 indicates the relations at 1400°. The phases pre- 

 sent for various compositions at equilibrium are as follows : 



area ECFHG all liquid 



EGA liquid E to G and forsterite 



GKA liquid G, pyroxene K and forsterite 



KAD forsterite and pyroxene K to D 



LDB tridymite and pyroxene L to D 



LHB liquid H, pyroxene L and tridymite 



HBF liquid H to F and tridymite 



KLHG liquid G to H and pyroxene K to L. 



The lines GK and HL are evidently three-phase-boundaries. 



In order to determine this figure experimentally it is neces- 

 sary to know the position of the isotherm EGHF for 1400° 

 especially of the points G and H which lie on the two boundary 

 curves of fig. 6. The temperature of all points along the 

 boundary curves has already been experimentally determined 

 (fig. 6). It is now necessary only to determine the points K 

 and L. The point L can be fixed by taking any mixture in 

 the triangle CBD, that is, any mixture of pyroxene and silica 

 and holding it at 1400°. If it is all liquid, or if it consists of 

 liquid and only one solid phase, another composition must be 

 tried until one is found which at 1400° consists of liquid and 

 the solid phases pyroxene and tridymite. If the composition 

 of the pyroxene can now be determined, the point L is thereby 

 determined. The pyroxene cannot be separated and analyzed, 

 but its composition can be determined fairly closely by optical 

 methods and the point L approximately fixed. In the actual 

 procedure it is never necessary to try a number of mixtures as 

 it might appear to be from the above discussion. If a mix- 

 ture corresponding to a point (x) on (or very close to) the 

 boundary curve at a temperature slightly higher than 1400° is 

 used, the desired result, viz., two solid phases and liquid, will 

 necessarily be obtained. 



In a similar manner the point K can be determined. A 

 mixture which at 1400° will give forsterite and pyroxene is 

 held at 1400° and the composition of the pyroxene (point K) 

 determined optically. By working at other temperatures the 

 three-phase-boundaries (lines corresponding with GK and HL) 

 for various temperatures can be approximately fixed by this 

 method of determining the composition of the pyroxene in a 

 three-phase mixture (the composition method). 



