D lops ide — Forsterite — Silica. 247 



bands, though most of them are polysynthetically twinned. In 

 45 per cent MgSi0 3 , 55 per cent CaMgSi 2 6 most of the grains 

 are twinned only twice though many show several lamellse. 

 Closer to diopside the twinning is distinctly less frequent. In 

 diopside itself most of the grains are untwinned, several show 

 two twinned bands and only rarely is repeated twinning 

 observed. This progressive increase in the tendency towards 

 twinning was observed in mixtures crystallized by heating 

 various glasses under the same conditions. It will be shown 

 later that in the same pyroxene the tendency towards twinning 

 varies when it is formed by different methods. 



Extinction Angle. — The extinction angle on 010 can be 

 measured very closely and shows the continuous variation in 

 an excellent manner. The angle was found by measuring 

 symmetrical extinctions against the trace of the twinning plane 

 in those grains which showed this trace and the prismatic 

 cleavage parallel to each other. By choosing such grains, a 

 section in the prism zone was ensured and by choosing those in 

 which there was no overlapping of the twinning lamellae a 

 section close to the clino-pinacoid was obtained. The overlap- 

 ping of the twinning lamellse is an especially delicate test in the 

 present case because the lamellse are usually much narrower than 

 the thickness of the grain and a very slight turning from the 

 clino-pinacoidal section gives a marked overlapping. For each 

 pyroxene, measurements were made on ten or more grains and 

 several measurements on each grain. An example is given 

 below of the symmetrical extinctions found on different grains 

 of composition CaMgSi 2 O c 37 per cent, MgSi0 3 63 per cent. 



30 31 j 27 ) 32 31 30 29 30 30 j 30 ) $ 27 ) 31 

 ^29 30 ( 33 j 29 30 30 28 30 30 ( 34 j \ 29 j 30 



mean, after rejecting those pairs that are somewhat unsym- 

 metrical (in brackets) = 30°. 



Some compositions did not give quite as accordant results as 

 the above. The measurements on CaMgSi 2 6 56 per cent- 

 MgSi0 3 44 per cent were, for example, as follows : 



c ^ c \' S1 \ 



j 40 1 32 34 35 33 

 " ( 27 j 36 34 34 36 ' 



(31 ) 



31 34 34 35 31 32 



C * C ~ ]37 1 



}39j 



' 35 35 34 33 37 36 



mean = 34°. 









The angle c ^ c for other compositions was found to be as 

 given below in Table VI. 



The curve indicating the change of extinction angle with 

 composition (fig. 21) is convex upwards, that is, the angle 

 increases more rapidly near the clino-enstatite end of the series. 



