Diojys ide — Forsterite — Silica. 213 



of mixtures fixes another point on the boundary curve and so 

 the whole curve may be located. The same work fixes the 

 temperature of a number of points on the boundary curve. It 

 is apparent from the foregoing that all liquids lying on the 

 boundary curve between the fields of A and B are in equili- 

 brium with A and B. 



The results are best expressed graphically by means of a 

 solid figure in which composition is plotted horizontally, on 

 triangular coordinates, and temperature vertically. For repre- 

 sentation on a plane surface, composition is plotted on trian- 

 gular coordinates and temperature is indicated by means of 

 isotherms. 



In the three binary systems there are a total of five solid 

 phases, — diopside, cristobalite, tridymite, clino-enstatite and 

 forsterite. The expectation is, therefore, that in the ternary 

 diagram each would have its field, i. e. that there would probably 

 be five fields, six boundary curves, and three ternary invariant 

 points.* The equilibrium diagram would then have the 

 general form shown in fig. 4. 



An equilibrium diagram analogous to fig. 4 would be the 

 result if there were no solid solution or only limited solid 

 solution between any of the solid phases. In the actual case, 

 however, the equilibrium diagram is very different, for clino- 

 enstatite and diopside form an unbroken series of solid 

 solutions, monoclinic pj^roxenes. The result is that clino- 

 enstatite, diopside and all the intermediate pyroxenes have 

 but one field in common, and there are, therefore, only three 

 boundary curves and only one ternary invariant point ; viz. 

 that corresponding with the equilibrium, — cristobalite, tridy- 

 mite, pyroxene, and liquid (fig. 5). 



It is now necessary to tabulate the quenching experiments 

 which gave the results outlined above. The composition of 

 any point m the triangle can, of course, be expressed in terms 

 of the three components at its corners, but the composition 

 is more readily visualized if the triangle is considered 

 divided into two parts by the pyroxene conjugation-line and the 

 composition of a point in the one part expressed as pyroxene 

 of a certain composition + silica and, in the other part, as 

 pyroxene + forsterite. In the tables this latter method will be 

 followed. The abbreviation En will be used to indicate the 

 composition MgSi0 3 and Di to indicate CaMgSi 2 6 . The 

 abbreviations are not intended to imply anything concerning 

 the state of crystallization of the mixture. Thus an uncrystal- 

 lized mixture (glass) of composition CaMgSi 2 6 60 per cent, 

 MgSi0 3 40 per cent would be indicated Di 60 En 40. 



*Such points are, of course, invariant only when the system is considered 

 as a condensed system. 



