156 



CARNEGIE INSTITUTION OF WASHINGTON. 



It will be seen from the tables that most of the stable binary com- 

 pounds are made up of the oxides in the simple ratios 1:1 or 2: 1. 

 Most of the unstable binary compounds, on the other hand, are made 

 up in the proportions 3 : 2 or 1 : 3. A similar rather crude generali- 

 zation holds for the ternary compounds, since the stable ternary com- 

 pounds can be considered as built up from the known simple binary 

 compounds in the simple ratio of 1 : 1 as follows : 



Anorthite CaSiOa . Al2Si06 (1:1) : (1 : 1) 



Gehlenite Ca2Si04 . Al2SiOt (2:1) : (1 : 1) 



Diopside CaSiOs. MgSiOa (1:1) : (1 : 1) 



Akermanite Ca2Si04 . MgSiOa (2:1) : (1 : 1) 



Two of the unstable ternary compounds, on the other hand, have 

 the irrational-appearing ratios 2:2:5 and 5:2:6, although the third 

 (monticellite) may be considered a 1 : 1 compound of the orthosilicates 

 Ca2Si04 and Mg2Si04. 



The regularities noted give the impression that the units of which 



these compounds are built are not atoms, but the atomic groups that 



we are familiar with as the oxides. These silicates seem to fall into 



the class of Werner's second and third order compounds. It is, then, 



clear why attempts to apply to the silicates the structural formulas 



and hypothetical ''silicic acids" suggested by organic chemistry have 



accomplished so little either in predicting new facts or in rationaUzing 



those already known. Future progress is to be expected rather from 



X-ray studies of the structure of these compounds, which are shown by 



accumulating evidence to be essentially different in character from 



the compounds of carbon.^ It is not too much to expect that some of 



the complex silicates will prove to be geometrical rather than chemical 



compounds. 



Phases and Phase Boundaries. 



The boundary curves of the fields of the various primary phases in 

 the three-component systems show certain regularities. The general 

 direction of the boundary curves in the system Si02-Al203-CaO is 



^ See also abstract (45) below. 



