

SILICATES, TITAN. \Tl.s. I.TC. 



425 



promt. Relief is high and tin- sections appear very rough. 

 Cleavage is always well developed, and in sections at right angles 

 to the vertical axis will show 

 the true prismatic angle of 

 12'. Pleochroism is not 

 ked. The cleavage angle 

 and lack of pleochroism serve 

 to distinguish the pyroxenes 

 from the amphiholos. 



Crystals may show zonal 

 structure and intcrgrowths 

 with the orthorhombic va- 

 riet ies. Extinction in sections 

 parallel to the orthoaxis is 

 either straight or bisects the 

 cleavage angle ; in all other 

 sectionsthere is inclined extinc- 

 tion, increasing to a maximum 

 in the plane of symmetry, 

 where it varies from 38 to 



FIG. 472. Augite from Egausville, Ren- 

 frew County, Canada. 



54, according to the composition of the specimen. The acute 

 bisectrix is in the large angle P; the angle between it and the 



axis d varies with the variety, as 

 is shown in the diagram, Fig. 

 473 . Double refraction is strong, 

 from .020 to .030, always yield- 

 ing interference colors of at least 

 the second order. Interference 

 figure distinct, showing an optic 

 axis in the basal section near the 

 center of the field ; the other axis 

 emerges in the orthopinacoid. 

 Optically (+). 



The pyroxenes are constituents 

 of the igneous rocks, especially the 

 ferro-magnesian varieties, erup- 

 tives and lavas. They are one of 

 the essential minerals of gabbros, 

 FIG. 473. Diagram of the Plane of where they are associated with the 



the Optic Axes 010, showing the Ex- , ' i ,-,, i i_i j 

 tin-lion of the Various Species of plfgWCfcseB, biotlte hornblende, 



Pyroxenes. olivinc, and less often, with quartz. 



