Rankin and Merwin — The Ternary System 301 



Abt. XX.— The Ternary System MgO-ALO-.-SiO,; by 

 G. A. Rankin and H. E. Merwin. 



Introduction. 

 The Crystalline Phases. 

 General Procedure. 

 Limits of the Fields. 

 Stable Compounds. 



Slow crystallization of certain glasses high in ALO ; . 

 Unstable Compounds. 

 Melting Temperatures within the Fields. 

 The Boundary Curves. 

 The Quintuple Points. 



Composition and Properties of the Ternary Compound (Cordierite), espe- 

 cially with regard to solid solution. 

 The Concentration-temperature Solid Model. 

 Natural Cordierite. 



FeO in Magnesian minerals and rocks. 

 Summary. 



Introduction. 



The investigation of the ternary system MgO-Al 2 3 - 

 Si0 2 was undertaken in order to determine all the crys- 

 talline substances formed from pure melts of these oxides, 

 and to establish the equilibrium relations of such of the 

 crystals as are stable in contact with the liquid. Stated 

 briefly, this involved a microscopical study to identify 

 the different crystalline substances, and a thermal study 

 to determine the melting temperatures of these sub- 

 stances when alone or in mixtures. 



The Crystalline Phases. 



The crystalline substances found to be stable in contact 

 with liquid are here enumerated: the three components, 

 MgO (periclase), A1 2 3 (corundum), Si0 2 (tridymite, 

 cristobalite) ; the four binary compounds, 2MgO.Si0 2 

 (forsterite), MgO.SiO ? (clinoenstatite), MgO.Al 2 3 

 (spinel), Al 2 3 .Si0 2 (sillimanite) ; and a ternary com- 

 pound, 2Mg0.2Al 2 3 .5SiOo (which may be considered an 

 end member in the series of cordierites). Since the prop- 

 erties of the components and binary compounds have 

 been described in previous papers, 1 they will be treated 



1 The following papers summarize and amplify what had been done pre- 

 viously: The Binary System MgO-SiO,, N. L. Bowen and Olaf Andersen, 

 this Journal, 37, 478, 1914; The Ternary System CaO-ALO :; -SiO, G. A. 

 Rankin and F. E. Wright, ibid., 39, 1-79, 1915; The Ternary System 

 CaO-ALA-MgO, G. A. Rankin and H. E. Merwin, J. Am. Chem. Soc, 38, 

 568, 1916 ; and The Ternary System Diopside-forsterite-silica, N. L. 

 Bowen, this Journal, 38, 207, 1914. 



