Diopside — Forslerite — Silica. 209 



upon in determining points on the liquidus intermediate 

 between a eutectic and a compound. Some investigators* 

 have, however, used this method on these intermediate com- 

 positions, although a little consideration would make it clear 

 that such compositions will soften either at the eutectic point 

 or at some temperature intermediate between the eutectic and 

 the liquidus point, but always well below the liquidus. 



The Binary Systems. 



The results obtained in the three binary systems will first be 

 presented. 



The System Mg 2 Si0 4 -Si0 2 . 



The system Mg 2 Si0 4 -Si0 2 has been described in a former 

 publication which deals with the system MgO-Si0 2 .f The 

 equilibrium diagram is given in fig. 1. The principal feature 

 of this system is the fact that the compound MgSi0 3 (clino- 

 enstatite) is unstable at its melting-point, breaking up at 1557° 

 into liquid and forsterite. The details of the system are given 

 in the paper referred to. 



The System CaMgSi 2 -Si0 2 . 



The system, diopside-silica, shows the simple eutectic rela- 

 tion (fig. 2). Diopside melts at 1391°. The eutectic between 

 diopside and tridymite is at 1362° and at the composition, 

 diopside 84 per cent, silica 16 per cent. 



The quenching experiments by which the figure was deter- 

 mined are tabulated below (Table I). 



The liquidus for silica was determined up to 1580°, approxi- 

 mately the limit of safety with the platinum resistance furnace. 

 Though the quenching experiments do not definitely prove its 

 existence, a break on the silica liquidus is shown at 1470°, cor- 

 responding to the inversion point tridymite-cristobalite found 

 by Fenner. 



Lime-Magnesian Amphibole. — In one of the mixtures con- 

 taining diopside 85 per cent and silica 15 per cent,' which was 

 melted and quickly cooled, crystals were found which, in 

 general appearance, were very similar to the amphibole obtained 

 when MgSi0 3 is quickly cooled. The crystals have, however, 

 considerably higher refractive indices than the magnesian 

 amphibole and probably are a lime-magnesian amphibole. The 

 exact composition cannot be decided upon, inasmuch as the 

 material was obtained only in the viscous mixture containing 

 excess silica. It is probable that the viscosity of the mixture 

 and the quick cooling condition the formation of this amphi- 



*Deleano, V. S., Zs. anorg. Chemie, lxxxiv, 401, 1914. 

 fBowen and Andersen, this Journal (4), xxxvii, 487. 



