18 Allen, etc. — Diopside and its Relations 



cent it is minute and in the 98 per cent can be no longer 

 detected ; while the inversion, small in the 70 per cent mixture, 

 is much greater in the 90 per cent and 95 per cent mixtures. 

 The 98 per cent mixture, where solid solution of calcium sili- 

 cate begins, shows the inversion plainly but not so markedly as 

 the 95 per cent mixture. It evidently partakes of the char- 

 acter of the pure silicate, which is more resistant to rapid 

 change. Dissolved calcium silicate therefore facilitates the 

 inversion of magnesium silicate, while, as we shall presently 

 see (page 20), dissolved magnesium silicate hinders the inver- 

 sion of calcium silicate. It is not strange that the properties 

 of a substance should be modified by solid solution, though it is 

 evident that the direction of such a change cannot yet be pre- 

 dicted, but why an excess of foreign solid should affect the 

 inversion point of a substance is not clear. The influence of 

 solid diopside in concentrating or sharpening the inversion of 

 magnesium silicate, however, seems to be established. 



Curves I and III show the behavior of materials which con- 

 tain no free magnesium silicate and therefore show no inversion. 

 Curve IV shows what happens when mixtures are chilled to 

 glass and then heated. Crystallization then leads to enstatite, 

 which evolves heat slowly over a long range of temperature. 

 Since this evolution covers the region above 1300°, the small 

 absorption at 1365° is effectually hidden, where, as explained, 

 the inversion seems to be lengthened out. 



Solid Solutions. — The system CaSi0 3 -MgSi0 3 contains six 

 different series of solid solutions, in only two of which is there 

 more than a few per cent of the smaller constituent. The 

 limit of solubility was determined by the thermoelement, by 

 the microscope or by both when possible. 



So far as thermal tests are concerned, the results refer of 

 course to the presence or absence of a eutectic melting, i. e., 

 the solubility at the eutectic temperature is approximately 

 determined. The microscope, on the other hand, makes its 

 determinations at ordinary temperatures, where presumably the 

 solubility is generally greater. In almost all cases the solu- 

 bility as determined by the microscope is a little higher, but 

 how far this is a real difference and how far it is due to a differ- 

 ence in the delicacy of the methods, it is unsafe to say on 

 account of the uncertainty in establishing equilibrium in these 

 solid silicate solutions ; in other words, it is impossible to say 

 whether a given solution is saturated or not. In the diagram, 

 fig. 1, we have drawn the lines straight with a single exception, 

 where we have more evidence that solubility increases with 

 falling temperature. 



1. Diopside in a-MgSi0 3 . On account of the difficulties 

 involved, this series was not investigated. 



