426 Andersen — System Anorthite-For sterile-Silica. 



IV. Discussion of Some Types of Ternary Systems with- 

 out Solid Solution. 



General Remarks. 



Before entering into the discussion of the ternary part of the 

 anorthite-forsterite-silica system a synopsis of the general 

 features of some ternary systems without solid solution will be 

 given. The purpose is only to elucidate problems dealt with 

 in this paper, in order to make them intelligible without any 

 elaborate physico-chemical representation. No attempt is, 

 therefore, made to render the strict thermo-dynamic proofs of 

 the principles involved, all statements being simply referred 

 to the graphical representations.* 



The ordinary ternary diagrams are, as is well known, simply 

 topographic maps of the temperature-concentration space 

 model where the shape of the surface is indicated by tempera- 

 ture contours, isotherms. In the following figures, which only 

 represent hypothetical diagrams, the isotherms are left out, but 

 the direction of falling temperature is indicated by arrows on 

 all the principal curves and this gives a general idea of the con- 

 figuration of the surface. 



Only systems without solid solution are considered and 

 among them a few simple types are selected as illustrations. 



Type 1. The System without Compounds. 



The simplest ternary system is the one with the three com- 

 ponents as the only solid phases. The relations of this ordinary 

 eutectic type of three component systems are well known and 

 should need no detailed explanation. However, to form a base 

 for the description of more complex types the general features 

 of this simple system will be briefly repeated. 



Synopsis of terms. — The system has three quadruple points, 

 A, i, and h (fig. 4), which all are binary eutectics of the three 

 two-component systems A-B, B-C and C-A. From the 

 quadruple points, three boundary curves {saturation curves), 

 hm, im and km, slope down towards the quintuple point ?;?, 

 the ternary eutectic. The relations of this system (and, in fact, 

 of all ternary systems without solid solution) are, in general, 

 known when the liquidus of the system is determined or in 

 other words, when the temperature at which the first solid 

 appears on cooling has been fixed for all compositions. The 



*More elaborate discussions of ternary systems without solid solutions aie 

 to be found, e. g. in the following works : W. D. Bancroft, The Phase Eule, 

 146, 1897; W. C. Geer, J. Phys. Chem., viii, 257, 1904; F. A. H. Schreine- 

 makers, Roozeboom : Heterogene Gleichgewichte III, 1911 ; H. Brand, 

 Neues Jahrb. Min., Beil. Bd., xxxii, 640, 1911 : H. E. Boeke, Centralbl. Min. 

 1912, 257 ; G. A. Rankin and F. E. Wright, this Journal (4), xxxix, 17 and 

 51, 1915. 



