132 J. B. Ferguson and A. F. Buddington — 



The optical properties given by them for the artificial 

 material are : 



Tetragonal 



positive 



co 1.631 . 



e 1.638 



The melting point is 1458° C. The density was not 

 determined by them. Schaller 3 gives 3.12 for the mineral. 

 We have determined it on the artificial material and find 

 the density to be 2.944 at 25° C. The refractive index of 

 the glass of this composition as determined by ns is 

 1.641 ± 0.002. 



Although the X-rays have not yet led to a clear defini- 

 tion of the crystalline character of solid solutions, still 

 there is a well-grounded belief that substances of similar 

 optical properties and like molecular volumes 4 are more 

 likely to form solid solutions than others. If this 

 hypothesis is any criterion of such possible behavior, one 

 would surely predict that akermanite and gehlenite 

 would form a continuous series of solid solutions, for in 

 addition to the similar optical properties which they 

 possess, their molecular volumes are nearly equal, being 

 273.06/2.944 = 92.8 and 274.64/3.038 = 90.4 respectively 

 if we assume the formulae previously given to correctly 

 interpret their molecular condition. In his study of the 

 melilite group of minerals, partly on these grounds and 

 partly for other reasons, Schaller 5 arrived at a similar 

 conclusion although he gave to akermanite a slightly 

 different composition from that assumed by us. A con- 

 sideration of his deductions led us to undertake the study 

 of this binary system since it appeared to be the most 

 effective way in which to start a comprehensive labora- 

 tory study of the minerals of the melilite group. 



Method. — The quenching method was employed 

 throughout the entire research. Details of this method 

 may be found in many of the previous publications from 

 the laboratory. The thermal apparatus was the identical 

 apparatus used by Ferguson and Merwin in their study 

 of the ternary system CaO-MgO-Si0 2 and perhaps refer- 



3 W. T. Schaller, U. S. Geol. Survey, Bull. 610, page 117, 1916. When 

 later in the paper we refer to Schaller, the reference is that given here. 



4 A. E. H. Tutton; Crystalline Structure and Chemical Constitution, page 

 128, London, 1910. 



5 Schaller has given to the compound which we call gehlenite the name 

 velardinite. We will refer to this later in the paper. 



