64 



Buddington — Natural and Synthetic Melilites. 



Fig. 10. 



1-640 



) 1-630 



1-620 



1-6/0 



O ZO 4-0 60 80 IOO 



90[2CaO-MgO-23c0 2 ] WT- PERCENT 9O^3CclO-AI z 3 -J^L0 2 ] 



J0/jNa 2 0-A A, 3 -JSl 2 J /0[3Na 2 0-AI 2 OyJSc 2 ] 



10. — Curves for refractive indices plotted against composition in weight 

 percent for mixtures crystallized between 980° and the dissociation point. 



Mixtures of 10 Percent 3Na 2 O.Al 2 3 .3Si0 2 and 20 Percent 

 2CaO.ALOo.Si0 9 with Varying Percentages of 3CaO.Fe 2 3 . 



3Si0 2 , 3Ca0.Al 2 3 .3Si0 2 , 



and 2Ca0.Mg0.2Si0 2 . 



Mixtures of 10 percent 3Na 2 O.Al 2 3 .3Si0 2 with vary- 

 ing percentages of 3CaO.Al 2 3 .3Si0 2 , 3CaO.Fe 2 3 .3Si0 2 

 and 2CaO.Mg0.2Si0 2 were found to form inhomogeneous 

 mixtures under the conditions of experiment, except for 

 a very small range of compositions near the 3CaO.Al 2 3 . 

 3Si0 2 -2CaO.Hg0.2Si0 2 line at the 3CaO.Al 2 3 .3Si0 2 end. 

 Natural melilites have a wider range of composition, and 

 some have a considerable percentage of the gehlenite 

 molecule. A series of mixtures was therefore prepared, 

 each of which contained 10 percent 3Na 2 O.Al 2 3 .3Si0 2 

 and 20 percent 2CaO.Al 2 3 .Si0 2 , but with varying 

 amounts of 3CaO.Fe 2 3 .3Si0 2 , 2CaO.Mg0.2Si0 2 and 

 3CaO.Al 2 3 .3Si0 2 . The results (fig. 11) show that the 

 presence of 20 percent 2CaO.Al 2 3 .Si0 2 has greatly 

 extended the area of solid solution (for a temperature 

 range above 980°), and includes mixtures which, are simi- 

 lar in composition to the natural melilites. Tables 

 XIII-XVI. 



The compositions of the mixtures experimented with 

 are relatively far apart, and the plotted limits of solid 

 solution are therefore only approximate. 



Comparison of Minerals with Synthetic Preparations. 



Introduction. — The data obtained from a study of some 

 synthetic mixtures of compounds, believed to be the more 

 common components of the natural melilites, have been 

 set forth in the preceding pages. It now remains to 



