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



The system affords an excellent example of a series 

 of solid solutions with a minimum melting point. This 

 point lies at 1388 °C, a little below the melting point of 

 diopside and corresponds to the composition of about 74 

 per cent akermanite, 26 per cent gehlenite. Thus the 

 minimum is 70 degrees below the melting point of aker- 

 manite, the component with the lower melting point. On 

 the akermanite side of the minimum, the hiatus has a 

 maximum value of 8 degrees and on the gehlenite side the 

 hiatus has a maximum value of 70 degrees. 



The liquidus and solidus curves are typical in shape 

 and, were the proper thermal data available, a check upon 

 their location might be made by means of thermodynam- 



ic ,q °\ 



/.670 



O /o 

 G£//l£/Y/T£- 



ZO 3o 



40 so 6o 70 



VJ.W 



U30 



80 90 /oo 



Fig. 2. — Eefractive indices plotted against composition in weight per- 

 cent, for crystals and glasses. 



ical calculations. Such calculations would also give some 

 idea as to the molecular complexity of the components but 

 in view of the lack of such data there is little use enter- 

 ing into any discussion of this character. Those inter- 

 ested are referred to the various papers by J. J. Van 

 Laar 8 upon solid solutions in systems of two components. 

 Optical Characters. — Akermanite and gehlenite have 

 similar crystal habits. Both are tabular parallel to the 

 base and in sections appear as laths and octagonal and 

 rectangular plates. The optical characters of crystals 

 with the composition of akermanite and gehlenite and of 

 intermediate mixtures are given in Table II. In fig. 

 2, curves for the indices of refraction for sodium light of 

 both crystals and glasses plotted against composition in 

 weight percent are given. The indices of refraction are 

 correct to ± 0.002. 



8 Zs. phys. Chem., 63, 225, 1908; 64, 265, 1908. 



