494 Bowen and Andersen — Binary System MgO-SiO.,. 



the deficiency of silica in the forsterite there is an excess silica 

 (above the composition MgSi0 3 ) in the glass as revealed by the 

 fact that the refractive index of the glass, 1*575, is definitely 

 lower than that of MgSiO, glass, 1"5S0. If this material is re- 

 turned to the furnace at a lower temperature and crystallized, 

 the product consists of clear forsterite grains surrounded by dusty 

 material (MgSi0 3 and silica), the whole precisely the same as the 

 material obtained in one step by cooling from the liquid state. 

 On the other hand, a clear, forsterite-free, MgSiO, glass, which 

 can be obtained only by quenching from a temperature above 

 1577°, may be crystallized at any temperature between 1200° 

 and 1557° and the product is only clean clino-enstatite, with- 

 out the strongly-birefracting forsterite grains which were 

 called a-MgSi0 3 and without the dusty appearance (silica). 

 This clino-enstatite may be held for an indefinite period at any 

 temperature up to 1557° without showing any tendency to 

 invert to another form. 



It should be noted, moreover, that this method of holding 

 the mixture MgSi0 3 above 1577°, then quenching and crystal- 

 lizing the glass below 1557°, is the only dependable method of 

 obtaining pure clino-enstatite free from forsterite and silica, 

 though occasionally the melt can be cooled quickly enough in 

 the furnace, through the range 1577°-1557°, to avoid the crys- 

 tallization of forsterite. The occasional non-appearance of the 

 material called a-MgSiO, in the earlier work is due to this 

 latter fact. 



In summary, then, it may be stated that there is no enantio- 

 tropic inversion of clino-enstatite into a-MgSiO s . The sub- 

 stance which was called a-MgSi0 3 is the product, not of inver- 

 sion of clino-enstatite, but of dissociation of clino-enstatite and 

 is the mineral forsterite, Mg 2 Si0 4 . 



Since we are dealing with equilibrium, the form clino- 

 enstatite is the only form of MgSiO s which appears on our 

 diagram. It always occurs in the characteristically twinned 

 form described in former publications and in the optical part 

 of this paper. 



The Eutectic, Periclase-For sterile. — The melting tempera- 

 tures of all mixtures of MgO and Mg 2 Si0 4 are beyond the 

 range of the platinum resistance furnace used for most of the 

 work, but we studied one mixture between these two in order 

 to decide the presence or absence of a eutectic point. Silica 

 and magnesia were mixed in the proportion to give Mg 2 Si0 4 

 86 per cent, MgO 14 per cent, and ground together and the 

 material heated for half an hour at about 1600°. Grinding 

 and heating were repeated and then the material was examined. 

 It proved to be clearly a fine-grained mixture of forsterite and 

 periclase and gave no trace of any compound intermediate 

 between these two. A small charge of this mixture was then 



