Buddington — Natural and Synthetic Melilites. 35 



Aet. II. — On Some Natural and Synthetic Melilites; by 

 A. F. Buddington. 



Introduction. 



The melilite group is usually considered to include the 

 minerals akermanite, gehlenite, humboldtilite, sarcolite, 

 fuggerite, and the ferric iron-rich melilite originally 

 called melilite. Many different theories have been pro- 

 posed to explain their composition. One of the most 

 recent is suggested by Schaller, 1 who has reconsidered 

 previous theories and all the available analyses of the 

 natural minerals and has arrived at a new theory of 

 their composition. His statement, in brief, is that all the 

 suitably described members of the melilite and gehlenite 

 group can be satisfactorily explained as isomorphous 

 mixtures of the primary compounds, sarcolite (3CaO. 

 AL0 3 .3Si0 2 ), a hypothetical soda-sarcolite (3Na 2 O.Al 2 3 . 

 3Si0 2 ), velardehite (2CaO.Al 2 3 .Si0 2 ), and akermanite 

 (4Mg0.8Ca0.9Si0 2 ). He further suggests the possibil- 

 ity that a ferric sarcolite (3CaO.Fe 2 3 .3Si0 2 ) may be 

 another component of the melilites. 



The present work was undertaken to test this hypothe- 

 sis, and it is a pleasure to state that the results of the 

 laboratory study on pure mixtures confirm the more 

 essential features of Schaller 's theory. The present 

 work, however, indicates that the melilites are too complex 

 to be wholly explained in this way and that several modi- 

 fications are necessary. These concern, in part, the com- 

 position of the mineral akermanite, and the polymorphic 

 character of the 3R0.B 2 3 .3Si0 2 compounds and certain 

 solid solutions into which they enter. 



Schaller states that the composition of akermanite is 

 4Mg0.8Ca0.9Si0 2 . Ferguson and Merwin 2 in their 

 study of the ternary system CaO-MgO-Si0 2 were unable 

 to prepare a compound or solid solution having this for- 

 mula. On the other hand, they prepared a compound 

 2CaO.Mg0.2Si0 2 whose crystal system and essential 

 optical properties corresponded to those of the mineral 



*W. T. Schaller, U. S. Geol. Surv., Bull. 610, pp. 106-128, 1916. When- 

 ever the work of Schaller is referred to in this paper, the reference is that 

 given here. 



2 J. B. Ferguson and H. E. Merwin. this Journal, 48, 118-122, 1919. 



