to Calcium and Magnesium Metasilioates. 37 



difficulty of determining the thickness of the plates accu- 

 rately. This error was reduced so far as possible by taking 

 the average of a number of measurements of thickness 

 on the same section and by grinding the sections thicker 

 than usual. 



The optical data prove that wollastonite can take up in solid 

 solution about 17 per cent of diopside, while pseudo-wollaston- 

 ite can absorb only about 4 per cent of diopside in solid solution ; 

 that in diopside only a small amount of the calcium metasil- 

 icate, not over 5 per cent, can enter into solid solution, while 

 mixed crystals containing up to about 39 per cent of MgSi0 3 

 in diopside can exist ; the maximal solid solution of diopside 

 in /3-MgSi0 3 is not great and does not exceed 5 per cent. 

 These relations were ascertained both by observing the limit of 

 homogeneity of the preparations and by observing the changes 

 in the different optic constants of the preparations. 



In fig. 12 the optic constants are represented graphically and 

 the limits of solicl solution are indicated by the breaks in the 

 curves. The optical data from which these curves were drawn 

 are included in the following table (VIII). 



Although every effort was made to reduce the probable error 

 of the values listed in this table, the very nature of the mate- 

 rial precluded accuracy of a very high order. The refractive 

 indices given in the table may be considered exact within 

 ±'003 ; the direct determinations of birefringence within 

 ±'003, especially when checked by refractive index determi- 

 nation ; the optic axial angles are not all of the same order of 

 accuracy, — that of pseudo-wollastonite being the least satisfac- 

 tory, with /S-^IirSiOg next ; in general the probable error does 

 not exceed ±1°; the extinction-angle determinations on 110 

 probably vary less than 1°- from the true values. 



Table YIII as well as fig. 12 prove conclusively that diopside 

 is a compound, and that the limits of solid solution for 

 the different members of the series is that indicated above. 

 Beginning with wollastonite, the refractive indices, the birefrin- 

 gence and the optical angle of the pure compound increase with 

 increasing admixture of MgSi() 3 up to about 8 per cent MgSi0 3 , 

 after which the curves are practically horizontal, thus marking 

 the limit of crystal miscibility with diopside. In the sections 

 free diopside was observed first in the preparation containing 

 10 per cent MgSi0 3 . — For pseudo-wollastonite the refractive 

 indices, the birefringence and the optic axial angle increase 

 slightly only up to about 2 per cent MgSi0 3 , and the exami- 

 nation of the preparations proved that beyond this limit inho- 

 mogeneity exists and diopside is present. — The limits of solid 

 solution of CaSi0 3 in diopside were difficult to determine optic- 

 ally with any degree of certainty. The data indicate only 

 slight solid solution, probably not below 15 per cent MgSi0 3 , 

 or not over 2 per cent of calcium metasilicate. In the prepa- 



