172 JV. L. Bowen — Crystallization of Hajolobasaltic, 



sary to note that G represents the composition of the plagioclase 

 which at the temperature considered is in equilibrium with 

 both diopside and liquid. If a mixture which gives all three 

 phases is held at the desired temperature and the composition 

 of the plagioclase is determined by optical methods, then the 

 point G is determined and the figure for that temperature can 

 be drawn. This may be called a composition method since it 

 depends on the determination of the composition of the mix- 

 crystal. 



The three-phase-boundai v ips may be located by starting with 

 a mixture of known composition and determining the tempera- 

 ture at which the three-phase area is entered either from above 

 or below. Any point on the line AG, such as P, lies on the 

 border of the three-phase area for 1230°, and at this tempera- 

 ture, as at all lower temperatures, it consists entirely of plagio- 

 clase and diopside, but if the temperature is raised very slightly 

 the point enters the three-phase area for this higher temperature 

 (note that the point P lies well within the three-phase area for 

 1250° AF'G') i. e., liquid is added to the phases already present. 

 A three-phase-boundary can, then, be located by determining 

 the temperature of beginning of melting for any mixture. 

 Thus if any mixture of diopside with plagioclase of composi- 

 tion G is taken and the temperature of beginning of melting 

 is determined it will be found at 1230°. If the isotherm DFE 

 for 1230° as previously determined is drawn then the join FG 

 is the three-phase-boundary for 1230°. 



We may determine tbree-pbase-boundaries by entering the 

 three-phase field from above. The point R lies within the field 

 of plagioclase and liquid, E'F'G' at 1250°, is on the border of 

 the three-phase area for 1230°, and if the temperature is 

 lowered a little it enters the three-phase area for this lower 

 temperature, i. e., the liquid and plagioclase are joined by 

 diopside. By determining the temperature (approached from 

 above) at which plagioclase and liquid are joined by diopside 

 in any mixture the three-phase-boundary passing through the 

 point representing the composition of the mixture is thereby 

 determined. Thus, if the mixture R is taken it is found that 

 diopside first appears at 1230° and if the isotherm of 1230° 

 DFE is drawn, FK. joined and produced to G, then FG is the 

 three-phase-boundary. 



All three of these methods were applied to the determina- 

 tion of three-phase-boundaries. 



Composition Method. — The composition method (first 

 method) depends on the ability to determine the composition 

 of the plagioclase crystals. These occur always as minute 

 tabular crystals embedded in glass and their properties are very 

 difficult to determine. In soda-rich mixtures the index of the 



