Diopside — Forsterite — Silica. 263 



slight effect on the freezing-point actually observed may there- 

 fore be due to the existence of an equilibrium in the liquid 

 such as that expressed in the above equation. The equation 

 for the actual equilibrium is probably much more complicated 

 than the above, but, if the reasoning is correct, should contain 

 the molecule Mg 2 SiO t as one of the reaction products. In 

 confirmation of this view that diopside is dissociated in the 

 liquid state is the fact that, whereas the crystals of the CaSi0 3 - 

 AigSi0 3 series show a sharp minimum on the curve of 

 specific volumes at the composition of diopside,* the glasses 

 of the same series (undercooled liquid) show no minimum. f 

 The smooth curve of specific volumes of the glasses does, how- 

 ever, lie distinctly below the straight line joining the volumes 

 of the end members and may be considered to indicate, there- 

 fore, the formation of some of the denser diopside molecules in 

 the liquid, but only in equilibrium proportions. 



It is probable that more or less dissociation in the liquid 

 state is common with silicates and, when mixed, they mutually 

 affect each other in this respect. It is, therefore, not to be 

 expected that, in the usual case, silicate mixtures should obey 

 any simple law of freezing-point lowering. Disagreement with 

 the law is, however, probably more apparent than real, for, in 

 order to apply the law, a knowledge of the molecules present 

 is necessary, whereas there is usually a complete lack of knowl- 

 edge of this point, as well as of the heat-effects accompanying 

 any molecular changes. 



General Summary. 



The results obtained in the investigation of the three binary 

 systems involved are first presented. The system diopside-silica 

 shows the simple eutectic relation, as does also the system 

 diopside-forsterite. The system forsterite-silica shows one 

 intermediate compound MgSi0 3 (clino-enstatite) unstable at its 

 melting-point. 



In the ternary system it is found that clino-enstatite and 

 diopside form a complete series of solid solutions (monoclinic 

 pyroxenes) and therefore have a common field. As a further 

 consequence of this unbroken series of solid solutions there is 

 no ternary eutectic, the lowest point of formation of liquid in 

 the system being the binary eutectic diopside-silica. 



A brief theoretical discussion of solid solution in ternary 

 systems is given in which the significance of certain lines, 

 termed three-phase-boundaries, is pointed out. The results of 



* Allen, White, Wright and Larsen, this Journal (4), xxvii, 27, 1909. 

 •j-Larsen, E. S., ibid. (4), xxviii, 263, 1909. 



