Ferguson and Merwin — The Ternary System. 117 



4. The magnesia disappears and the charge becomes 

 completely crystalline at 10 to form a mixture of a2CaO. 

 Si0 2 and a solid solution X. The last liquid has a com- 

 position represented by the point 10. The solid solution 

 X lies on the line MD between M and D. 



Were it possible to make a reaction take place in the 

 solid state in a reasonable time, pure monticellite would 

 be formed when the mixture of a2CaO.Si0 2 and the solid 

 solution X were heated at temperatures below the decom- 

 position temperature of the pure compound. 



Akermanite. — Many attempts have been made to explain 

 the composition of the members of the melitite group of 

 minerals (tetragonal in symmetry), and most of the 

 resultant explanations have presupposed the existence of 

 a mineral akermanite. Vogt 44 assumed the formula of 

 this pure compound to be 4Ca0.3Si0 2 although usually 

 part of the lime was "replaced'' by magnesia or a like 

 base. Day and Shepherd 45 were, however, unable to 

 obtain any evidence of the existence of such a compound 

 in their investigation of the lime-silica series of minerals. 

 Later Eankin and Wright 46 noting the similarity between 

 some of the properties ascribed to akermanite and those 

 of the compound 3Ca0.2Si0 2 ( orthorhombic in sym- 

 metry) suggested that the latter might be the akermanite 

 analogue in the binary system. More recently Schaller, 47 

 recognizing the ternary nature of the melitite group, has 

 stated that the correct formula for this second compound 

 is 8Ca0.4Mg0.9Si0 2 . 



Since the compound 4Ca0.3Si0 2 has never been pre- 

 pared in the pure state and there is little real evidence of 

 its existence, this formula may be regarded as purely 

 speculative. 



The formula 8Ca0.4Mg0.9Si0 2 is based on Schaller 's 

 interpretation of two analyses 48 of a tetragonal Vesuvian 

 mineral. This interpretation would need but little modi- 

 fication if the formula were written 2CaO.Mg0.2Si0 2 , 

 corresponding to the tetragonal ternary compound of this 

 system. 



The essential optical properties of the analyzed aker- 

 manite (1) and of the ternary compound (2 ) are compared 

 as follows : 



44 T. H. L. Vogt, Mineralbildung in Schmelzmassen, 96, 1892. 



45 A. L. Day and E. S. Shepherd, this Journal, 22, 280, 1906. 



46 G. A. Rankin and F. E. Wright, this Journal 39, 1, 1915. 



47 W. T. Schaller, U. S. Geol. Survey, Bull. 610, 1916. 



48 See F. Zambonini, Mineralogia Vesuviana, 255, 1910. 



