Ferguson and Merwin — The Ternary System. 115 



the composition 5Ca0.2Mg0.6Si0 2 is reached was inter- 

 preted as indicating the existence of this compound. Its 

 decomposition temperature is somewhat difficult to deter- 

 mine directly but from the liquidus relations must 

 be 1365 ± 5°C, the temperature which corresponds to 

 point 13. 



The diopside (CaO.Mg0.28i0 2 ) solid, solution. — Diop- 

 side has previously been shown by several investigators 

 to form no solid solution with silica, f orsterite or pseudo- 

 wollastonite. Similarly it does not form solid solutions 

 with the compound 2CaO.Mg0.2Si0 2 to any great extent. 

 A charge of the composition CaO 26-5, MgO 18-5, Si0 2 55, 

 was found to contain after a 15 hour heat treatment at 

 1300°C, crystals of 2CaO.Mg0.2Si0 2 in such quantities 

 as to indicate not more than a trace of such solid solutions. 

 However, diopside forms a continuous series of solid 

 solutions with clino-enstatite MgO.Si0 2 , a thorough dis- 

 cussion of which has been given by Bowen. 



The CaO.MgO.SiOo (monticellite) solid solutions. — The 

 temperature relations along the monticellite-periclase 

 boundary line 10, 11, are such as to indicate considerable 

 solid solution between monticellite and forsterite. 43 The 

 limit thus indicated would be about ten percent forsterite, 

 or to the composition CaO 32, MgO 28-75, Si0 2 39-25. 

 Owing to the difficulties encountered here, due to the great 

 readiness with which magnesia crystallizes and the great 

 slowness with which it is resorbed, only a few confirma- 

 tory experiments were made. These indicated that the 

 solid solution extends at least to the composition CaO 33, 

 MgO 28, Si0 2 39. 



Attempts to prepare pure monticellite did not succeed. 

 Instead of a homogeneous mass of this composition a 

 mixture of crystals of a2CaO.Si0 2 and crystals of a monti- 

 cellite solid solution was always obtained even though the 

 original charge consisted of a glass in which very minute 

 crystals of magnesia were imbedded. This result may 

 be explained by reference to Hg. 14 which is an enlarge- 

 ment of this portion of the diagram given in fig. 8. The 

 arrows indicate the direction of falling temperatures. 

 The maximum C on the line 10, 11, represents the decom- 

 position temperature of the solid solution D and there 



43 The existence of such solutions was previously discovered but the some- 

 what crude apparatus used in the investigation made the interpretation of 

 the results an uncertain matter. P. Herman, Zeit., d. deutsch Geol. Ges. 

 58, 39, 1906. 



