Bowen and Andersen — Binary System MgO-SiO % . 491 



Discussion of Results. 



Magnesia. — We have found magnesia in only one form, as 

 isotropic crystals corresponding with the mineral periclase. 

 When it occurs embedded in glass, it is always in rounded 

 grains with crystal outline at best only suggested. We have 

 not tried to determine its melting point. Kanolt places it at 

 2800° 0.';* Ruff, above 2500°.f 



Silica. — Silica occurs in contact with liquid in magnesia- 

 silica mixtures only as cristobalite. The inversion temperature 

 cristobalite-tridymite, 1470°, % lies below the range of tempera- 

 tures at which any of our mixtures are liquid. We have taken 

 Fenner's value 1625° as the melting point of cristobalite. 



Compounds. — There are two binary compounds, the ortho- 

 silicate, Mg 2 Si0 4 , and the metasilicate, MgSi0 3 . We have not 

 found any compounds, corresponding with the lime-silica com- 

 pounds, akermanite and tricalcium silicate, or traces of any phase 

 other than the metasilicate, the orthosilicate and the end com- 

 ponents, occurring in contact with liquid in the binary system. 



MgJ$iO K . — Magnesium orthosilicate occurs in orthorhombic 

 crystals corresponding with the natural mineral forsterite. 

 The melting point of forsterite was determined in an iridium 

 furnace, the temperature being measured by means of an optical 

 pyrometer of the Holborn-Kurlbaum type. The pyrometer 

 was calibrated for the known points : 2050°, melting-point of 

 Al„O s ;§ 1755°, melting-point of platinum ; 1557°, dissociation 

 (with melting) of MgSi0 3 . The lamp-filament was matched 

 against a piece of thin iridium foil lying on the surface of , the 

 charge of the powdered mineral contained in an iridium boat. 

 On the occurrence of melting, movement of the foil is 

 observed. By this method the melting-point of forsterite was 

 found to be 1890°. The error of a measurement made in this 

 manner may be as much as 20° or 25°, an accuracy very much 

 less than that attained for points lying within the temperature 

 range of the platinum furnace and Pt-Pt : Rh thermoelement 

 (±2°). We have therefore distinguished on the diagram the 

 points determined by these two methods, indicating the latter 

 by small circles and the former by larger circles. 



MgSiO z . — Magnesium metasilicate has no true melting point, 

 i. e., there is no temperature at which the solid MgSiO g is in 

 equilibrium with liquid of its own composition. The thermo- 

 element records a strong absorption of heat at 1557° and this 

 temperature has heretofore been considered the melting point, 

 but quenching has shown that 1557° is the temperature of dis- 

 sociation of MgSi0 3 into forsterite and liquid in the propor- 



* Kanolt, C. W., Bureau of Standards, Reprint No. 212, 19, 1913. 

 fRuff, O., Zs. anorg. Chern., lxxxii, p. 373, 19 13. 



^Fenner, C. N., The Stability Relations of the Silica Minerals, this Journal 

 (4), xxxvi, 337, 1913. 



§ Kanolt, C. W., Bureau of Standards, Reprint No. 212, 19, 1913. 



