. i'i;ori,irni.s OF CRYSTALS 217 



3. Cleavage cracks, partings, and fractures due to pressure or 

 strain should I H- noted and the angles between two well-defined 

 cleavage directions, when- they occur, measured in a number of 

 sections, in order to obtain the true angle or that measured at 90 

 to the intersection of the two cleavage planes. 



4. Note the crystal outlines where well developed and where 

 elongated; the direction of the elongation in regard to the cry 

 tallographic axes is determined, also any irregularity of the outline. 

 as that due to corrosion, resolution, alteration, or weathering, and 

 whether these changes are restricted to the surface or have fol- 

 lowed cleavage cracks and fractures ; also the nature of the alter- 

 ation product is noted. 



5. Index of refraction. Rock sections are usually mounted in 

 Canada balsam, the index of refraction of which is approximately 

 1.539, but varies slightly with the amount of solvent it contains or 

 with the age of the mounted section, as the balsam is constantly 

 hardening with age. Specimens with an index of refraction near 

 that of balsam will have little or no relief ; their surfaces will appear 

 flat and smooth. It is well to have several mineral sections mounted 

 for comparison, the index of each being known ; their relief may be 

 compared with the unknown section and the index of refraction 

 approximately determined. Sodalite, 1.483; leucite, 1.508; 

 orthoclase, 1.523; quartz, 1.547; beryl, 1.584; olivine, 1.670; 

 and rutile, 1.712, are good minerals for comparison. A specimen 

 with an index of refraction above 1.60 or below 1.50 will have a 

 rather high relief. The cracks, as cleavage, scratches on the 

 surface made in grinding the section, and the outline, all will 

 appear well marked and distinct. Whether the refraction is above 

 or below 1 .549, that of balsam, can be determined by Becke's method. 

 Minerals with a high relief seem particularly rough when the 

 section is shaded from reflected light by passing the hand up and 

 down in front of the microscope. 



6. In crossed nicols and parallel light. If the section remains 

 dark between crossed nicols when the stage is revolved, it is either 

 amorphous, as glass, isometric, or a double refracting mineral cut 

 perpendicular to an optic axis. An anisotropic section between 

 crossed nicols will yield an interference color which will be a measure 

 of its double refraction and the section will extinguish every 90, 

 that is, each time the vibration planes of the section are parallel to 

 the cross hairs of the eyepiece. 



7. The angle of extinction is measured in reference to the vertical 



