12 Allen, etc.— Diopside and its Relations 



problem was finally obtained in the following way: Two small 

 crucibles containing charges slightly different in composition 

 were pnt together in the furnace and brought to such a temper- 

 ature that the main eutectic melting was completed but the 

 excess component, if any, still left undissolved. Then by sud- 

 denly increasing the furnace current and making alternate 

 temperature readings of the two crucibles at intervals of 15 

 seconds, the relative heat absorptions in the region immediately 

 above the eutectic melting point could be determined, and thus 

 in a very few trials of different mixtures the eutectic composi- 

 tion could be located. The results of such a determination are 

 shown in curves I-I and II-II, fig. 3. The temperature in 

 II rises more slowly than in I, because of an absorption of 

 heat due to the melting of the excess component. The curves 

 were continued to a higher temperature to make sure that this 

 difference was really due to this cause and not to the relative 

 position of the two crucibles in the furnace or to some other 

 accidental circumstance. The curves V and IF are now paral- 

 lel, confirming the conclusion that their previous divergence 

 was due to a melting in II which is now over. It will be seen 

 from this result that the 28 per cent mixture lies nearer the 

 eutectic than the 30 per cent. A similar comparison of the 

 28 with the 26 per cent also showed that the eutectic lies 

 nearer the former. These experiments are sufficient to locate 

 the point well within 1 per cent at the 28 per cent mixture. 

 The previous work had indicated that the 28 per cent was 

 probably the eutectic composition. Calculated in terms of 

 diopside, the eutectic mixture would contain in round num- 

 bers 60 per cent diopside : 40 per cent pseudo-wollastonite. 



Diojpside. — As previously stated, the melting-point of diop- 

 side is a maximum on the melting point curve, which fact, 

 taken in connection with the microscopic homogeneity at this 

 composition and the occurrence of a eutectic point on either 

 side of it, proves that it is a compound in stable equilibrium 

 with its own liquid. Abundant confirmation of this conclu- 

 sion is found in the form of the specific-volume curve, p. 27, as 

 well as in the close agreement of the composition of natural 

 diopside with the rational formula CaSi0 3 .MgSi0 3 . As an 

 example, we give an analysis of an exceptionally pure specimen 

 from a metamorphosed limestone which occurs in Ham Island, 

 Alaska. The composition of the anhydrous substance is also 

 given for the sake of fairer comparison. This is justifiable, 

 for the water is not chemically combined, a conclusion thor- 

 oughly established by the fact that the substance remains 

 homogeneous as the water escapes. 



