to Calcium and Magnesium Metasilicates. 19 



2. Diopside in /3-MgSi0 3 . The microscope detected inho- 

 mogeneity in the 3 per cent CaSi0 3 mixture, but none in the 2 

 per cent. No eutectic was observed in the latter. We may 

 therefore put the solubility as 2 per cent CaSi0 3 , or 3*7 per 

 cent diopside. 



3. MgSi0 3 in diopside. The microscope places the limit of 

 solubility at about 66*5 per cent MgSi0 3 : 33*5 per cent CaSi0 3 , 

 i. e., the quantity of diopside in the saturated mix-crystal 

 would be 33-5/53*7 = 63*4 per cent. In other words, diopside 

 is capable of dissolving 3 T 6/62 "4 = more than 60 per cent of its 

 own weight of magnesium silicate. This remarkable series of 

 mix-crystals strongly resembles diopside. The optical work 

 described in detail in Part II shows that the extinction angle 

 and the optic axial angle both fall about \° for each 

 additional per cent of magnesium silicate. The specific 

 volume curve (fig. 9) shows that the solution is attended by 

 expansion. 



4. Calcium silicate in diopside. The microscope detected 

 inhomogeneity at 44*5 per cent MgSi0 3 while the thermoelement 

 detected a plain eutectic in the 45 per cent MgSi0 3 . The 

 limit of solubility is doubtless small, certainly less than 100 — 

 45/46-3 = 3 per cent CaSiO,. 



5. Diopside in a-CaSi0 3 (pseudo-wollastonite). Mixtures 

 containing as much as 3 per cent MgSi0 3 plainly showed 

 inhomogeneity when examined by the microscope. The 

 inhomogeneity took the form of irregular bands, irregularly 

 distributed, which showed a distinctly lower birefringence 

 than the rest. The 2 per cent MgSi0 3 solution showed traces 

 of the above structure and gave a plain eutectic when examined 

 thermally. The 1 per cent solution is microscopically homo- 

 geneous and gives no more than a suspicion of a eutectic. The 

 error will be slight if we put the limit of solubility at about 

 1-2 per cent MgSi0 3 , or in round numbers, 3-4 per cent of 

 diopside. 



6. Diopside in /3-CaSi0 3 (wollastonite). In this case a 

 thermal test is obviously useless. The microscopic analysis 

 showed that solution ceased at about 17 per cent diopside (8 per 

 cent MgSi0 3 ). Crystallization took place at about 1058°. The 

 crystals of diopside and wollastonite are both monoclinic and 

 the latter resembles diopside more closely than pseudo-wollas- 

 tonite, so that it is not surprising that wollastonite should dis- 

 solve diopside in large quantity. 



These solutions showed a very interesting behavior when 

 they were heated. To understand this clearly, it should be 

 remembered that the eutectic between pseudo-wollastonite and 

 diopside melts at about 1348°, while wollastonite has an inver- 

 sion point at about 1190°. Upon heating crystals of wollaston- 



