The Ternary System MgO-AL^O-SiO,. 325 



determination did not at once penetrate the pores. 12 

 Such behavior has prevented the determination of a pos- 

 sible inversion in cordierite corresponding' to that of the 

 M-form of the ternary compound into the a-form. 



FeO in Magnesian Minerals and Rocks. 

 The magnesia compounds of this ternary system are 

 represented among rock-forming minerals by compounds 

 in which ferrous oxide plays the role of magnesia to a 

 considerable extent. Although it is probable that FeO 

 can thus replace MgO without causing the formation of 

 new phases, yet the temperature relations must be so 

 different that fig. 1 cannot represent the shapes of the 

 fields of these phases. It is conceivable — but not prob- 

 able unless FeO largely predominates — that the field of 

 cordierite might disappear, and that the field of corun- 

 dum might approach or even border the field of olivine, 

 but even then corundum would not be expected as an 

 early phase in the crystallization of a dry magma near 

 olivine in composition. 



Summary. 



The temperature-concentration relations of the various 

 crystalline phases in equilibrium with liquid in the ter- 

 nary system MgO-Al 2 3 -Si0 2 have been investigated, and 

 are represented by diagrams and a model. A ternary 

 compound, 2Mg0.2Al 2 3 .5Si0 2 , unstable at its melting 

 point and considerably affected by solid solution has 

 been observed in two forms. The u-form (unstable) 

 crystallizes from glass at temperatures below about 950°, 

 and transforms at somewhat higher temperatures into 

 the a-form (stable). The properties of both forms, but 

 especially the a-form, are close to those of the mineral 

 cordierite. The effects of solid solution in flattening two 

 of the boundary curves of the field of this compound are 

 discussed. 



The preparation and properties of crystalline aggre- 

 gates of substances which disassociate when melting are 

 considered. 



Crystals of natural cordierite disassociate when melt- 

 ing. Brief consideration is given to the effects of FeO 

 in magnesian rocks and minerals. 



Geophysical Laboratory, Carnegie Institution of Washington, 

 Washington, D. C. 



12 The progressive refilling of pores of crystals from which volatile matter 

 has been expelled can often be observed thus in a viscous liquid under the 

 microscope. 



