Buddington — Natural and Synthetic Melilites. 67 



by Ferguson and Buddington 13 as 2.944. The explana- 

 tion for the much lower density of the artificial akerman- 

 ite is not apparent, but the natural mineral is not pure 

 and the synthetic material may be a trifle light owing to 

 the presence of included air. Material crystallized from 

 akermanite glass at 700° has the same indices of refrac- 

 tion as that crystallized at 1450° so that there is no 

 evidence of polymorphism between these temperatures. 

 The compound 2CaO.Mg0.2Si0 2 enters into complete 

 solid solution with 2CaO.Al 2 3 .Si0 2 and into limited solid 

 solution with 3RO.R 2 3 .3Si0 2 compounds. In this prop- 

 erty it is analogous to the mineral akermanite. The two 

 analyses of natural akermanites, however, show silica in 

 excess of that called for by the formula for the synthetic 

 compound 2CaO.Mg0.2Si0 2 . This discrepancy remains 

 to be explained but the balance of evidence favors the 

 essential identity of the synthetic preparation and the 

 mineral. 







Table XVII 











Akermanite 



from Vesuvius. 









Calculation from 



Freda 's 



analysis. 







Original 

 analysis 



Analysis 

 reduced 



Mol. 



2CaO.Mg0.2Si0 2 



and 

 3CaO.AI,0 3 .3Si0 2 

 molecules 



Difference 



SiO-, . . 

 ALO, .. 

 CaO . 

 MgO . . 



. . . 46.70 

 . ... 1.09 

 . . . 39.62 

 . . . 13.38 



46.47 



.96 



39.23 



13.34 



7764 



94 



7054 



3310 



6902 



94 



6902 



3310 



—862 

 —152 





100.79 



100.00 





Table XVII gives the analysis by Freda of akermanite 

 from Vesuvius, stated in terms of its constituent mole- 

 cules and compared with the total number of molecules 

 which can be recomputed to form 2CaO.Mg0.2Si(X (90.4 

 percent) and 3CaO.Al 2 3 .3Si0 2 (4.2 percent). 



Table XVIII gives the analysis by Zambonini of aker- 

 manite from Vesuvius, stated in terms of its constituent 

 molecules and compared with the total number of mole- 

 cules which can be recomputed to form 2CaO.Mg0.2Si0 2 

 (90 percent) and 3CaO.Al 2 3 .3Si0 2 (4.8 percent). 



13 Op. cit., p. 132. 



