Morey & Bo wen — Melting Potash Feldspar. 9 



two-component system. In order to represent its behav- 

 ior graphically one must construct a two-component 

 diagram. Thus fig. 2 represents equilibrium in mixtures 

 of Mg 2 Si0 4 and Si0 2 and shows MgSi0 3 as a compound 

 of these with an incongruent melting point at 1557°. 

 From the diagram we may read off that MgSi0 3 breaks 

 up at 1557° into Mg 2 Si0 4 and liquid somewhat more sili- 

 ceous than MgSi0 3 . Above 1557° a mass of the compo- 

 sition MgSi0 3 is made up of crystals of Mg 2 Si0 4 

 (forsterite) and liquid, the forsterite gradually dissolv- 







Fig 



2. 



I860 









'-EC 



\ 







1700 



FORSTERITE v 









& \ 





y 





LIQUID . 



$*/ 





1620 



\ 



$$/ 







\ 



OS/ 



CRISTOBALITE & LIQUID 





N 











V / 





1540 







~^s/ 



FORSTERITE 









& 



CLINOENSTATITE & CRISTOSALITE 





CLIN0-ENSTAT1TE 







20 MgS.0.3 40 WT PER CENT 80 



Fk. 2. — Equilibrium diagram of the system Mg 2 Si0 4 — Si0 2 . 



ing as the temperature is raised, until at 1577° the for- 

 sterite completely disappears and only then is a liquid 

 of the composition MgSi0 3 obtained. 6 



A compound of three oxides may melt incongruently 

 in such a way that it breaks up into a liquid and two 

 new crystalline compounds. Monticellite (CaMgSiOJ 

 shows this behavior. At 1498° it breaks up into Ca 2 Si0 4 , 

 MgO, and liquid. It can be treated only as a part of a 

 three-component system. 7 However, a compound of 

 three oxides, which melts incongruently, does not neces- 

 sarily break up in such a way that it must be treated as 

 a three-component system. It may form liquid and only 



"For a full discussion see Bowen and Andersen, this Journal, 37, 487-500. 

 1914. 



' Ferguson and Merwin, this Journal, 48, 116. 1919. 



