72 Buddington — Natural and Synthetic Melilites. 



gehlenite mineral series. Reference is also made here 

 to the fact that Vogt 16 studied the relations of akerman- 

 ite, gehlenite, and their mixtures in artificial slags, and 

 concluded that they form an isomorphous series. His 

 materials necessarily contained iron, manganese, soda, 

 and other impurities,. and differed to that extent from the 

 artificial preparations studied in this work. 



Grossularite. 



The pure compound 3CaO.Al 2 3 .3Si0 2 is known in 

 nature only as the mineral grossularite, one of the garnet 

 group, crystallizing in the isometric system. As a com- 

 ponent of other minerals, however, it forms 90 percent of 

 sarcolite, up to 50 percent of some humboldtilites, and 

 isomorphous mixtures with other members of the garnet 

 group. 



The mineral sarcolite is positive, uniaxial; crystalliz- 

 ing in the tetragonal system, with indices of refraction 

 n u = 1.6035 and n e == 1.6147. Since sarcolite contains 90 

 percent of 3CaO.Al 2 3 .3Si0 2 , this compound may have 

 similar, though somewhat modified, optical characters in 

 the form in which it exists there. 



A series of solid solutions of 3CaO.Al 2 3 .3Si0 2 with 

 2CaO.Mg0.2Si0 2 , 2CaO.Al 2 3 .Si0 2 , 3Na 2 O.Al 2 3 .3$i0 2 

 and 3CaO.Fe 2 3 .3Si0 2 , formed within a temperature 

 range above 1000°, have been described in this paper. 

 These solid solutions crystallize in the tetragonal system 

 and the 3CaO.Al 2 3 .3Si0 2 compound has the effect of a 

 negative uniaxial material with a moderate birefringence. 

 If we project the curves of the indices of refraction for 

 complete solid solutions of 3CaO.Al 2 3 .3Si0 2 and 3Na 2 0. 

 Al 2 3 .3Si0 2 , we find that the former component will have 

 the indices n u = 1.633 ± 0.005 ; n e = 1.624 ± 0.005 ; and 

 that it is uniaxial negative with a birefringence of 

 0.011 ± 0.005. 



We therefore have evidence of at least three distinct 

 forms in which the compound 3CaO.Al 2 3 .3Si0 2 may exist 

 in solid solutions. 



Further evidence of the polymorphism of this com- 

 pound is indicated by the birefringence observed in many 



18 Loc. eit. 



