10 



Allen, etc. — Diopside and its delations 



follows the principal melting now extends beyond the upper 

 point and completely overwhelms it. The difficulty of deter- 

 mining a small residual melting is in striking contrast to the 

 ease with which a slight absorption of heat can be detected 

 below the principal melting.* Owing to this masking of the 

 upper point, a direct determination of the eutectic composition 

 is impossible by the ordinary method; that is, we cannot dis- 

 tinguish within several per cent the mixture which melts leav- 



Fig. 2. 



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Temperature-time curves of mixtures in the vicinity of the eutectic com- 

 position (pseudo-wollastonite-diopside). 



I 24$ MgSi0 8 . II. 26% MgSi0 3 . III. 28$ MgSi0 3 . IV. 32% MgSi0 3 . 

 v . 36% MgSi0 3 . 



ing no excess component, since a number of mixtures appear 

 to do so. This is well shown by the temperature-time curves 

 in fig. 2, which were made with especial care with reference 

 to this very point. The upper melting can hardly be distin- 

 guished in the 24 per cent mixture, though this contains over 

 8 per cent of the component in excess. ' (The point is in fact 

 so faint that we could hardly distinguish it from the minute 

 irregularities in the temperature curve of the furnace were it 

 not for the fact that it lies on the curve ABC, fig. 1, which is 

 well defined in other mixtures farther removed from the 

 eutectic.) For locating the eutectic two methods still remain. 



* It was undoubtedly the difficulty of detecting the upper points in this 

 region and the failure to appreciate how easily such an experimental diffi- 

 culty might arise which led R. Freis (Neues Jahrb. Min., Beil. Bd. xxiii, 76, 

 1907) to describe a similar series of melting points as forming a curve with 

 the two inclined portions separated by a horizontal branch of considerable 

 extent. 



