86 Buddington— Natural and Synthetic Melilites. 



age index of refraction of about 1.695 and more strongly 

 birefringent fibers with index about 1.680. The indices 

 for the same preparation crystallized at the solidus are 

 ?/„ =1.654 and^e =1.644. A mixture composed of 10 per- 

 cent 3Na 2 O.Al 2 3 .3Si0 2 , 45 percent 2CaO.Mg0.2Si0 2 , 

 and 45 percent of 3CaO.Fe 2 3 .3Si0 2 , heated for 1 month 

 at about 700 °, gave an aggregate of low birefringent 

 spherulites with an average index of about 1.730 ± 0.010 

 and more strongly birefringent fibres. This is con- 

 trasted with the indices n u = 1.688 and r>e = 1.658 for the 

 same preparation crystallized at the solidus. Several 

 experiments with other mixtures at the same tempera- 

 ture for the same length of time gave similar results, 

 with varying high indices of refraction. 



Summary 



The character of the work upon which this paper is 

 based and the conclusions reached are briefly summarized 

 here. 



Over 100 synthetic crystalline mixtures of 2CaO.MgO. 

 2Si0 2 , 2CaO.Al 2 8 .Si0 2 , and 3RO\R 2 3 .3Si0 2 compounds 

 were prepared from appropriate glasses at temperatures 

 above 1000° (the approximate lower limit of experi- 

 ments) ; and their homogeneity, the optical characters 

 of those forming homogeneous mixtures and of the dom- 

 inant phase of some of those forming inhomogeneous 

 mixtures, and the temperatures of complete melting, were 

 determined. 



The compounds 2CaO.Mg0.2Si0 2 and 2CaO.Al 2 3 . 

 Si0 2 have previously been shown to form a complete 

 series of solid solutions at the solidus, and are here shown 

 to agree very closely in their properties with the aker- 

 manite-gehlenite series of minerals. 



In certain homogeneous solid solutions with the other 

 compounds at certain temperatures 3CaO.Al 2 3 .3Si0 2 is 

 shown to have the properties of a negative, uniaxial, 

 moderately birefringent crystalline compound. 



Mixtures of 2CaO.Mg0.2Si0 2 , 2CaO.Al 2 3 .Si0 2 , and 

 a mixture of 90 percent 3CaO.Al 2 3 .3Si0 2 + 10 percent 

 3Na 2 O.Al 2 03.3Si0 2 form a complete series of solid solu- 

 tions, except for a trace of inhomogeneity in some pre- 

 parations high in 2CaO.Mg0.2$i0 2 . Mixtures of these 



