34 



Allen, etc. — 1 



liopside and its 



Relation 



4>* 



E 





E, 



0° 



0° 





0° 



10° 



-13° 





-12° -5 



20° 



— 20°*9 





-21°-7 



30° 



-27°'l 





-27°-9 



40° 



-32°-2 





•— 31°-9 



43°30' 



— 32°-9 





-32°-5 



50° 



— 34°'3 





— 34°'6 



60° 



-35°-5 





-36°'4 



70° 



-37°-2 





-37°-6 



80° 



-38°-5 





— 38°*3 



90° 



-38°'5 





-38°-5 



* <p = angle of prism face with orthopinacoid 100. 



These figures indicate that for the first 40° from the ortho- 

 pinacoid the extinction angle rises very rapidly while for faces 

 near the clinopinacoid the variations are very slight. 



For the sake of comparison the theoretical values of the 

 extinction angles indicated by the Michel-Levy formula* are 

 listed under column E x . 



From the melt diopside crystallizes readily, usually in the 

 form of radiating prismatic individuals intricately intergrown 

 and overlapping. A characteristic microscopic feature is the 

 presence of fine bubble-like inclusions or cavities throughout 

 the crystallized mass. These cavities are either tubular in 

 shape and parallel in a general way the prismatic elongations 

 of the crystallites ; or they appear cutting across the sections 

 in an irregular way not unlike the cavities in a section of 

 worm-eaten wood. The cavities are probably due to the 

 shrinkage accompanying the crystallization of diopside from 

 the silicate melt. — Such air spaces in the crystals from CaCl 2 

 fluxes were only rarely observed and are not characteristic 

 of the same. 



The Intermediate Compositions. — In studying the prepara- 

 tions of this series intermediate in composition between the 

 compounds, the microscopic analysis has been directed along 

 two principal lines : (1) To ascertain whether or not the prod- 

 uct is homogeneous ; (2) to determine as accurately as pos- 

 sible the optic properties of the one or more components in 

 each preparation. Experience with both thermal and optical 

 data has shown that in certain instances limits of homogeneity 

 cannot be detected within one or two per cent optically and 

 the optical determinations of the limits of solid solution in 

 this series given below may easily be in error therefore one or 

 two per cent. 



*Les Mineraux cles Boches, p. 11, 1888. In this formula the following 

 values were used: V=29 n< 6, y=0°, ext. angle=38°-5 ; or //=8 0> 9, v=68-l ; 

 u + v- 77°. 



