Ferguson and Buddington — Binary System. 131 



Aet. X. — The Binary 'System Akermanite-Gehlenite ; by 

 J. B. Feeguson and A. F. Buddington. 



The assignment of formulae to minerals of variable 

 composition is a problem which often confronts and per- 

 plexes the mineralogist. In general he must rely upon 

 analyses, frequently made on impure material. The 

 divers explanations of the complex mineral groups, which 

 have resulted, bear evidence to the difficulties that hamper 

 anyone who endeavors to glean the truth from such re- 

 sults. Unfortunately, to date, the laboratory studies on 

 mineral systems have not afforded the aid that many may 

 have expected of them. This has been due, more to the 

 tedious character of the researches required and the neces-' 

 sity of building from firm foundations, rather than to any 

 inherent defects in the laboratory methods themselves. 

 In view of this situation it is with a feeling of distinct 

 pleasure that we present the results of a research which 

 owes its directional character to the statistical studies 

 of a mineralogist and its successful completion to the 

 earlier laboratory studies without which it could not have 

 been carried out. 



The compound 2CaO . A1 2 3 . Si0 2 , which will be re- 

 ferred to throughout this paper as gehlenite, was 

 thoroughly investigated by Rankin and Wright 1 in the 

 study of the ternary system CaO-Al 2 3 -Si0 2 . The opti- 

 cal properties given by them are : 



Tetragonal 



(001) distinct 



negative 



co 1.669 



e 1.658 



<0-€ 0.011 



The density is 3.038 and the melting point 1590 °C. 

 The refractive index of the glass of this composition as 

 later determined by us is 1.638 ± 0.002. 



The compound 2CaO . MgO . 2Si0 2 was discovered by 

 Ferguson and Merwin 2 in their study of the ternary sys- 

 tem CaO-MgO-Si0 2 and the mineral akermanite was 

 thought by them to consist essentially of this compound. 



1 G. A. Eankin and F. E. Wright, this Journal, 39, 26, 1915. 



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

 1919. ^ ' 



Am. Jour. Sci.— Fourth Series, Vol. L, No. 296.— August, 1920. 

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