Diopside — Forsterite — Silica. 



249 



If it is assumed that the molecular weight of clino-enstatite is 

 the formula weight, MgSi0 3 = 100*6, and that of diopside also 

 the formula weight, CaMgSi 3 6 = 216*37, and the composition 

 is plotted in mol. per cent, then the measured values of the 

 extinction angles agree closely with the values calculated by 

 means of Mallard's formula.* In fig. 22 the circles indicate 

 the measured values of the extinction angles plotted in this 



Fig. 23. 



1.690 



1.670 



1.650 



DIOPSIDE 2 ° 4 ° „ 6 ° 8 °CLINO-ENSTATlTE 



WT. PERCENT 



Fig. 23. Eefractive Indices of Pyroxene Series. 



manner and the curve ACB is calculated from the Mallard 

 equation. The equation reads 



cot£ 2a = ^& 



i 



sin 2ft - *J 



+ cotg2(i l - z 2 ) 



where {i-a) is the desired extinction angle of a pyroxene 

 mix-crystal containing n^ mols diopside to n^ mols clino- 

 enstatite ; d^ and cZ 2 the birefringence of diopside and clino- 

 enstatite respectively, and i 1 and \ their extinction angles. 



The agreement of the observed points with the calculated 

 curve is marked. It may be purely accidental, indeed the 

 assumption that the molecular weight is equal to the formula 

 weight for each of the compounds may be quite unwarranted, 

 for if a near-equality of molecular volumes is necessary for 

 complete solid-solution, the molecular weight of clino-enstatite 

 ♦Bull. Soc. Min., iv, 101, 1881. 



