36 Buddington — Natural and Synthetic Melilites. 



akermanite. They therefore suggested that the compound 

 2CaO.Mg0.2Si0 2 found by them is the synthetic equival- 

 ent of akermanite. Later, Ferguson and Buddington 3 

 found that the compounds 2CaO.Mg0.2Si0 2 and 2CaO. 

 Al 2 3 .Si0 2 form a complete isomorphous series of solid 

 solutions. This confirmed the suggestion of Vogt 4 that 

 akermanite and gehlenite form a complete series of solid 

 solutions, and also serves to indicate the correctness of 

 the formula assigned to akermanite by Ferguson and 

 Merwin. The same series of analyses of natural min- 

 erals used by Schaller in his study has been recalculated 

 on the assumption that 2CaO.Mg0.2Si0 2 is the correct 

 formula for akermanite, and the results have been found 

 to check equally well with those of Schaller for the hum- 

 boldtilite and gehlenite varieties, but not for the ferric 

 iron rich melilites or for akermanite itself. In this 

 paper the term akermanite will be applied to the com- 

 pound 2CaO.Mg0.2Si0 2 , since the balance of the evidence 

 seems to favor this interpretation. 



Schaller has proposed that the compound 2CaO.Al 2 3 . 

 Si0 2 be called velardehite, since the natural gehlenites are 

 not definite compounds but variable solid solutions, and 

 since the gehlenite of Velardeha, Mexico, approaches 

 more nearly than any so far analyzed to the pure com- 

 pound, 2CaO.Al 2 3 .Si0 2 . The term gehlenite, however, 

 has been used by Vogt 5 to designate one of the indepen- 

 dent primary members of the melilite group, and this 

 usage is common in the literature. The pure compound 

 2CaO.Al 2 3 .Si0 2 was first prepared by Shepherd and 

 Rankin 6 and was called gehlenite. 



The term albite is used both for the pure end member 

 and also for any plagioclase between Ab and A^An^ 

 Similarly, the term gehlenite may be used for the pure 

 end member and for solid solutions within certain limits. 

 The compound 2CaO.Al 2 3 .Si0 2 is therefore called gehle- 

 nite in this paper, and is the equivalent of the velardehite 

 of Schaller. 



3 J. B. Ferguson and A. F. Buddington, this Journal, 50, 131-140, 1920. 



4 J. H. L. Vogt, Mineralbildung in Schmelzmassen, p. 96, 1892. Vogt gives 

 the formula (Ca,E3)3.(Al 2 ,Fe 2 )Si 2 O 10 for gehlenite and (Ca,E) 4 Si 3 O 10 for 

 akermanite. 



5 J. H. L. Vogt, loc. cit. 



6 E. S. Shepherd and G. A. Eankin, Preliminary report on the ternary 

 system CaO-Al 2 3 -Si0 2 ; a study of the constitution of portland cement 

 clinker, with optical study by F. E. Wright, J. Ind. Eng. Chem., 3, 224, 1911. 



