OFFICAI. PROPERTIES "I KYSTALS 



10!) 



plane <>f symmetry; :ind these angles are of great service in the 

 identification of mineral species in rock sections. 



The plane of the optic :i\is when p:ir:illel to the plane of symnic -try 

 of the system is fixed, but the acute bisectrix may revolve in that 

 plane around the orthoaxis, and I he angle it makes with the vertical 

 axis c is the measure of the angle of extinction in the plane of sym- 

 metry and is characteristic of mineral species, hut varies with the 

 ((imposition of the specimen. Fig. 347 is a diagram of the plane of 

 symmetry of the amphiboles representing the extinction angles of 

 the common varieties. The angle is measured in the clinopina- 

 coidal section in reference to the crystalline outline or the prismatic 

 and orthopinacoidal cleavage cracks, 

 in the acute bisectrix may be the 

 orthoaxis, at right angles to the plane 

 of symmetry, when the plane of the 

 optic axis may revolve around the 

 acute bisectrix as an axis; in this 

 the extinction angle is measured 

 from the obtuse bisectrix, which will 

 lie in the plane of symmetry. 



In the triclinic system, any plane 

 may be the plane of the optic axes, and 

 there is no relation between the in- 

 dicatrix or optical symmetry and the 

 crystallographical axes, except in in- 

 dividual species, where the angles of 

 extinction are usually given in reference 

 to some well-marked cleavage plane, 

 or the acute bisectrix is oriented by 

 giving the angles it forms with the normals of common crystal faces 



of the species. 



Iii measuring the angle of extinction, at times it is quite im- 

 pov-ible to determine exactly the point at which there is no double 

 retraction or the least illumination. To the unaided eye this area 

 may seem to extend over several degrees. At such times a sensi- 

 tive plate is used, one by means of which the slightest double 

 refraction may be detected. This sensitive plate is made from a 

 cleavage piece of selenite, of such a thickness that , when mounted 

 and slipped in the tube of the microscope in the same position 

 as the quartz wedge, with its- vibration planes at 45 to those of 

 the nicols, will illuminate the field of the microscope evenly with 



IK. :U7. Diagram of the Ex- 

 tinction Angles of the Amphi- 

 bole. 



