140 Ferguson and Buddington — Binary System. 



We are under obligations to Dr. H. E. Merwin of this 

 Laboratory for his generous counsel in the microscopical 

 examinations. 



Conclusion. 



1. The binary system 2CaO . MgO . 2Si0 2 (Aker- 

 manite) - 2CaO . A1 2 3 . Si0 2 (Gehlenite) was studied by 

 the quenching method, the solidus and liquidus curves 

 drawn, and the system found to form a complete series 

 of solid solutions with a minimum melting point. 



2. The optical characters for crystals of several inter- 

 mediate compositions were determined and found to be 

 a continuous function of the composition. Crystals of a 

 certain intermediate mixture are isotropic for sodium 

 light, and constitute a transition phase from crystals of 

 positive optical character to those 'of negative optical 

 character. Zoned crystals showing material of both posi- 

 tive and negative optical character in the same crystal 

 were found. These are analogous, in this character, to 

 certain natural melilite crystals which have previously 

 been described. 



3. The densities of crystals and glasses of akermanite, 

 gehlenite, and several intermediate mixtures were deter- 

 mined and found to confirm the isomorphous character of 

 the system. Akermanite was found to show the unusual 

 feature of its glass having a greater density than the cor- 

 responding crystals at 25 °C. 



Geophysical Laboratory, Carnegie Institution of Washington, 

 Washington, D. C, May 25, 1920. 



