Ferguson and Merwin — The Ternary System. 123 



of the pure compound are stable in contact with a suitable 

 liquid. 



5. Certain members of the monticellite solid solutions. 

 Monticellite takes up forsterite in solid solution to the 

 extent of about ten per cent and the decomposition tem- 

 perature of the solutions is thereby raised. Monticellite 

 itself probably decomposes at too low a temperature to 

 ever occur at a primary phase. 



The temperature-concentration relations of the liquids 

 which may be in equilibrium with each of these phases 

 have been thoroughly investigated where necessary by 

 means of the quenching method, and the results obtained 

 have been correlated with the existing data on the remain- 

 der of the ternarv system. 



The compounds 5Ca0.2Mg0.6Si0 2 and 2CaO.MgO. 

 2Si0 2 have not been prepared previously. Attempts to 

 prepare a compound of the formula 8CaG.4Mg0.9Si0 2 

 (Schaller's akermanite) gave negative results. 



The monticellite solid solutions and the compound 

 akermanite are discussed at length but the wollastonite 

 and the 5Ca0.2Mg0.6Si0 2 solid solutions are only briefly 

 mentioned as they will be made the subject of a sub- 

 sequent paper. 



Experiments were made on the tridymite-cristobalite 

 inversion temperature, which was found, for this system, 

 to be below 1500 °C, in approximate agreement with Fen- 

 ner's original value of 1470°; the great sluggishness of 

 the inversion precluded a more exact determination on 

 our part. 



Geophysical Laboratory, Carnegie Institution of Washington, 

 Washington, D. C, April, 1919. 



