20 Allen , etc. — Diopside and its Relations 



ite carrying 1 per cent MgSi0 3 (2*1 per cent diopside) for one 

 hour at 1221°, no change was apparent. Heated again for two 

 hours at 1245°, the inversion was slight. The inversion point 

 of wollastonite appears, therefore, to be raised 40°-50° by the 

 solution of only 2 per cent diopside. (See p. 18.) 



Since time is an important factor in sluggish changes, a 

 direct comparison was made between pure wollastonite and two 

 mix-crystal preparations of this series by heating all three in 

 the same furnace for the same length of time, viz., 1 hour. 

 The temperature ranged from 1257 6 ~to 1263°, i. e., about 65° 

 above the inversion point of wollastonite. The wollastonite 

 was completely inverted, the 2*1 per cent solution slightly 

 inverted, while in the 4\3 per cent solution (2 per cent MgSi0 3 ) 

 it was doubtful if any change at all had taken place. The two 

 solid solutions were now returned to the furnace and held an 

 hour longer between 1273° and 1300°, about a hundred degrees 

 above the inversion point of pure calcium silicate. The weaker 

 solution was now found to be much changed, the stronger one 

 less so. The 8 per cent solution of MgSi0 3 , containing 17'3 

 per cent diopside, was heated for an hour at 1278° to 1280°, 

 90° above the inversion point of calcium silicate. A careful 

 microscopic examination of the crystals then showed a con- 

 siderable change in their appearance ; a new product had sepa- 

 rated but it did not show the characteristics of pseudo-wollas- 

 tonite. Such optical properties as could be measured in fine- 

 grained material (index of refraction, birefringence) agreed 

 with diopside. This indicates that the solubility of diopside 

 in wollastonite is greater at lower temperatures where the 

 crystallization occurred, or perhaps that the solid solutions, 

 being formed by rapid crystallization, were supersaturated. In 

 either case the excess separates when the solution is heated to 

 higher temperature. 



Heated an hour longer at a temperature of 1298°-1303°, 

 more diopside separated, but the signs of inversion were still 

 doubtful. Again, the crystals were returned to the furnace 

 and the heating continued another hour at 1327°-1343°. 

 This time inversion was evident. These experiments show 

 either a marked rise in the inversion temperature of wollas- 

 tonite or else a great increase in molecular sluggishness caused 

 by the dissolved diopside. 



A brief discussion will make it clear under what conditions 

 an inversion point may be raised and thus help to decide 

 whether we have a real rise in the inversion temperature or 

 not. By a thermodynamic method Beckman^ has shown that 

 the freezing or inversion point of a substance is changed by 



* Ostwald's Lehrbuch der Cliemie, vol. 2, pt. 2, pp. 38, 68. 



