Bowen and Andersen — Binary System MgO-SiO^. 495 



carefully heated on a small piece of platinum foil in the oxy- 

 hydrogen flame. The mixture melted about the same time as 

 the platinum and as the edge of foil melted away, the liquid 

 silicate ran back upon the still unmelted portion and was 

 thereby saved for examination. The larger of the liquid 

 globules recrystallized on removal from the flame to a mixture 

 of forsterite and periclase, both well crystallized. Some of the 

 more quickly cooled material, however, consisted of periclase 

 and glass, indicating that the composition dealt with lies on the 

 magnesia side of the eutectic. 



The behaviour of this mixture was then examined in the 

 iridium furnace, after the manner of the determination of the 

 melting-point of forsterite. The mixture became liquid at 

 1850°, 40° below the melting point of forsterite. The diagram 

 has been drawn in accordance with these results, showing a 

 eutectic between forsterite and periclase at 1850° and with the 

 composition close to forsterite (less than 14 per cent MgO). 

 The determination of the composition of the eutectic is, of 

 course, the roughest approximation. 



The Liquidus between Mg^SiO i and MgSiO t . — Points on 

 the liquidus between Mg 2 Si0 4 and MgSiO, were determined 

 up to a temperature of 1626° by means of the thermoelement, 

 Table I (a). Above this temperature no points were deter- 

 minable except the melting point of forsterite itself (1890°). 



The Invariant Point, Forsterite- Clino-enstatite- Liquid. — 

 Between forsterite and clino-enstatite there is no eutectic, for the 

 reason that clino-enstatite itself is unstable at its melting point, 

 breaking up into forsterite and liquid. The composition of 

 the liquid capable of existing in contact with both clino- 

 enstatite and forsterite (D) was fixed in the following manner: 

 Its composition was first approximately determined by holding 

 a charge of MgSi0 3 at 1560°, a little above the dissociation 

 point, and quenching. The product consists of forsterite 

 crystals and glass and the refractive index of the glass 

 was determined. The refractive index of pure MgSiO a 

 glass is l - 580, that of the glass in the quenched product as 

 obtained above was 1*575 =t -001. From the indices of refrac- 

 tion of glasses of mixtures of MgSi0 3 and Si0 2 in known 

 proportions the composition of this glass was found to corre- 

 spond approximately with MgSi0 3 97 percent, SiO„ 3 per cent. 

 Mixtures of various compositions close to this were then made 

 up and the first crystal separating from them (primary phase) 

 determined by quenching. In this manner it was found that 

 in 3 per cent Si0 2 the primary phase was clino-enstatite, in 

 2 - 5 per cent Si0 2 clino-enstatite, and in 2 per cent Si0 2 

 forsterite. Thus the composition of the point D (fig. 1) was 

 determined at 2*5 per cent Si0 2 . 



All mixtures of compositions lying between Mg„Si0 4 and the 



