176 JV. L. JBowen — Crystallization of Haplobasaltic, 



and the liquid changes along the straight line AXK. At 1218°, 

 when the liquid has the composition K, plagioclase of composi- 

 tion L(Ab,An 2 ) begins to crystallize, KL being a three-phase- 

 boundary. On further lowering of temperature the composi- 

 tion of the liquid moves along the boundary curve towards S, 

 plagioclase increases in amount and changes continually in com- 

 position, diopside also increases in amount, until at 1176° the 

 liquid is finally used up. The last of the liquid has the com- 

 position S and the feldspar has changed in composition to T, 

 ST being a three-phase-boundary. 



These examples make clear the necessity of determining 

 three-phase-boundaries in order that the composition of plagio- 

 clase at any temperature may be known. 



Crystallization of Mixtures in the Plagioclase Field. 



For all mixtures in the diopside field the change of composi- 

 tion of the liquid until the boundary curve is attained is repre- 

 sented by a straight line. (Note AFGr and AXK.) 



For all mixtures in the plagioclase field, however, the liquid 

 follows a curved course in reaching the boundary curve. The 

 crystallization of any of these mixtures can not, therefore, be 

 quantitatively described unless these crystallization curves in 

 the plagioclase field are determined, and for this reason these 

 curves were determined for two representative mixtures. 

 Though applied to only two mixtures the method may perhaps 

 prove useful in other more or less similar investigations and 

 Avill therefore be described in full. 



Determination of the Composition of Liquid and of Mix- 

 crystals in a Two-Phase Mixture. — In order" to find the com- 

 position of liquid in equilibrium with crystals at any tempera- 

 ture in a binary mixture, it is necessary only to hold a mixture 

 at the desired temperature, quench it and determine the refrac- 

 tive index of the glass.* In a ternary mixture, however, the 

 measurement of the refractive index of the glass is not suffi- 

 cient to fix its composition. The composition can, neverthe- 

 less, be located as lying on the curve joining the composition 

 of all glasses having that measured refractive index. Such 

 curves will be referred to as isofracts.f But it is known also 

 that the composition of the liquid must lie on the isotherm of 

 the temperature at which the liquid was held. It must, there- 

 fore, lie at the point of intersection of the isofract and the 



* N. L. Bowen : The Melting Phenomena of the Plagioclase Feldspars, this 

 Journal (4), xxxv, p. 585, 1913. 



f Objection to this term, based on its mixed derivation, seems to me to be 

 outweighed by the fact that the prefix iso is that commonly accepted in this 

 sense and therefore preferable to, say, equi while fract is mnemonic of refrac- 

 tive index. 



