496 Bowen and Andersen — Binary System MgO-SiO v 



point D should break up when the temperature is raised to 1557° 

 into a liquid of composition D and forsterite. We have con- 

 firmed this behaviour in several of these mixtures (see Table 

 I (b)). 



The Crystallization of Mixtures ofJIg„SiO i and MgSiO z . — 

 The crystallization of mixtures lying between Mg 2 Si0 4 and 

 MgSi0 3 takes place, when perfect equilibrium obtains, in the 

 following manner : Forsterite crystallizes out first and increases 

 in amount until the temperature 1557° is reached. At this 

 temperature the liquid has the composition D and clino-enstat- 

 ite begins to crystallize out, forsterite to redissolve, this process 

 continuing until all the liquid is used up and the whole consists 

 of forsterite and clino-enstatite. In pure MgSi0 3 the early sep- 

 aration of forsterite takes place in the same way, but at the reac- 

 tion temperature (1557°) the last of the liquid and the last of the 

 forsterite are used up at the same instant and the whole consists 

 of clino-enstatite. In compositions between MgSiO, and the 

 point D, forsterite separates first as before, at the reaction point 

 it is completely resorbed, leaving clino-enstatite and some liquid. 

 With further lowering of temperature clino-enstatite continues 

 to separate until, at 1543°, when the liquid has the composition 

 E, cristobalite separates also and the whole crystallizes at this 

 temperature, giving a mixture of clino-enstatite and cristobalite. 



Such is the behaviour of these mixtures when complete equi- 

 librium is attained. We have already seen, however, that with 

 quick cooling, equilibrium is not attained and a preparation of 

 composition MgSi0 3 will then crystallize to a mixture of clino- 

 enstatite, silica, and forsterite. The same fact is true of all 

 mixtures lying between Mg 2 Si0 4 and D, the reason being that 

 with quick cooling the liquid of composition D, instead of 

 reacting with (redissolving) forsterite at 1557°, simply crystal- 

 lizes, as the temperature falls quickly below this point, to a 

 mixture of MgSiO a and silica. Such mixtures containing both 

 forsterite and silica are unstable but will persist indefinitely. 



The Eutectic, Cristobalite- Clino-enstatite. — The eutectic point 

 between cristobalite and clino-enstatite was located by quench- 

 ing charges of various compositions to determine the primary 

 phase. The composition MgSi0 3 90 per cent, Si0 2 10 per cent 

 shows clino-enstatite as the primary phase. The composition 

 MgSiO, 87*5 per cent, Si0 2 12 - 5 per cent is completely crystal- 

 line at 1542° and completely liquid at 1544° and therefore cor- 

 responds sensibly with the eutectic composition. Cristobalite 

 is the primary phase in mixtures richer in Si0 2 (see Table I 

 (d)). The eutectic melting was observed at the same tempera- 

 ture (1543°) in mixtures on both sides of the eutectic. 



In the mixtures on the silica side we were not able to deter- 

 mine the liquidus with great assurance on account of the high 

 viscosity of the mixtures. There are, however, indications that 



