208 N. L. Boiven—The Ternary System ; 



among the solid phases, there are representatives of the import- 

 ant rock-forming mineral groups, the pyroxenes and the oliv- 

 ines. 



The compound diopside, CaMgSi 2 6 , was studied formerly 

 at this laboratory* in an investigation of the system CaSi0 3 - 

 MgSiOg. This system was treated as binary, but recent workf 

 has shown that the compound MgSi0 3 is unstable at its melt- 

 ing point and can itself be treated only as part of a two-com- 

 ponent system. It is necessary, therefore, to treat the system 

 CaSi0 3 -MgSi0 3 , or at least the portion of it CaMgSi 2 6 - 

 MgSi0 3 , as a part of the ternary system CaMgSi 2 6 -Mg 2 Si0 4 - 

 Si0 2 . The present paper, which deals with this ternary sys- 

 tem, is, therefore, essentially a revision of former work on 

 CaMgSi 2 6 and MgSi0 3 , in which it has been found necessary 

 to study completely the field of the ternary system. 



Method of Working. 



The mixtures used were made up from specially purified 

 silica, calcium carbonate, and magnesia obtained by igniting 

 the carbonate to constant weight. Equilibrium was studied by 

 the method of quenching, which consists in holding a small 

 charge of known composition at a measured temperature for a 

 period of time and then chilling instantly to room temperature. 

 The charge is then examined under the microscope and the 

 phases present are determined. 



The phases found represent equilibrium at the furnace tem- 

 perature for inert bodies like the silicates, unless there exists 

 a readily reversible inversion in a solid phase, in which case the 

 low temperature form may be found under the microscope 

 whereas the high temperature form existed in the furnace. 

 In such cases the solid phase nearly always gives evidence of 

 having passed through this inversion. 



The furnace used was a platinum resistance furnace and the 

 temperature was measured by means of a Pt : Pt-Rh thermo- 

 element which was calibrated by quenching charges of pure 

 artificial minerals of known melting-point. 



One point, however, namely the melting-point of the com- 

 pound forsterite, was determined by observing directly the 

 phenomenon of melting in an iridium furnace, the temperature 

 being measured optically. This method of determining the 

 temperature at which a mixture becomes visibly fluent is satis- 

 factory for fixing the melting temperature of a compound or a 

 eutectic when these form thin liquids, but cannot be relied 



* Allen, White, Wright and Larsen : Diopside and Its Eelation to Calcium 

 and Magnesium Metasiiicates, this Journal (4), xxvii, 1, 1909. 

 f Bowen and Andersen, this Journal (4), xxxvii, 487, 1914. 



