The principal cleavages are parallel to the prismatic faces, and intersect 

 therefore at an angle of 87. Cross-sections are eight-sided, and the 

 boundaries are determined by the faces in the prismatic zone, (100), (010) 

 and (110). In some augites the prismatic faces and in others the pinacoidal 

 faces are the best developed ; the former can, as a rule, be readily distinguished 

 from the latter by the parallel cleavage cracks. The most common type of 

 twinning is the one in which the orthopinacoid (100) is both twin-plane and 

 face of composition. Sometimes the twinning is repeated several times in the 

 same individual, but this feature is comparatively rare. 



The double refraction of augite is strong, so that thin sections give, 

 on the average, high interference tints. It is stronger than that of 

 the rhombic pyroxenes, but not so strong as that of olivine. The mean axis 

 of elasticity always coincides with the ortho-diagonal (b=ft), so that the optic 

 axes lie in the plane of symmetry. The acute bisecti'ix is positive, and in the 

 normal augites (not akmite) always makes a considerable though variable 

 angle with the vertical axis. This angle (c : 7) is an important characteristic 

 for the minerals of this group. It is the maximum extinction in sections 

 out of the zone of the vertical axis ; that is, in sections in which the two sets 

 of cleavage cracks are parallel to each other. As already stated it varies in 

 different augites and is no doubt related to the chemical composition. 

 All that can be said, however, at present is that it is somewhat less in those 

 augites which are poor in iron and alumina than it is in those which are rich 

 in these substances. In the former it varies between 36 and 40 and in the 

 latter between 41 and 54. The optic axial angle also varies considerably in 

 different augites, but as this element cannot be determined in ordinary rock 

 sections, and with the simple appliances of an ordinary petrographical 

 microscope, it possesses less importance from our point of view than the angle 

 c : 7. Now in observing augite-sections under the microscope the zone 100 : 

 001 becomes of considerable importance because sections out of this zone are at 

 right angles to the optic axial plane, and therefore show when viewed in 

 convergent polarised light different portions of the interference figure in the 

 most favourable position for examination. Such sections may be easily 

 distinguished by the fact that the extinction position bisects the angle formed 

 by the cleavage cracks, and lies parallel to the cleavage cracks when these are 

 parallel to each other. In the latter case the section must not only be out of 

 the zone 100 : 001, but can only be cut in one direction in this zone, namely 

 parallel to 100. If examined in convergent light, such a section will show an 

 optic axis somewhat oblique to the axis of the microscope. This is often a 

 valuable means of discriminating between rhombic and monoclinic pyroxenes. 

 Longitudinal sections giving straight extinction must, if the crystal be augite, 

 show an optic axis, or at any rate an axial shadow ; whereas, if the crystal be a 

 rhombic pyroxene, all longitudinal sections will give straight extinction, and 

 in none of them will a single optic axis be visible. Cross sections of an augite 

 prism also show an optic axis somewhat oblique to the field of view, whereas 

 the corresponding sections of a rhombic pyroxene show a positive bisectrix. 



