216 MINERALOGY 



of the other may be approximately estimated . This method depends 

 upon the principle that light passing from a rarer medium or one 

 with a lower index of refraction to one with a higher index of re- 

 fraction will pass at all angles and there will be no critical angle ; 

 but in the reverse direction there will be a critical angle and some of 

 the light is reflected to the side of the higher index of refraction. 

 The illumination on the side of the higher index of refraction will 

 be brighter, as some of the rays are totally reflected to that side. 



As an example the index of refraction of quartz and orthoclase 

 in a section of granite may be tested in this way. The effect is 

 best seen with a medium high objective and with a converging light, 

 the light being diaphragmed off so as to illuminate only a small area. 

 If the microscope is carefully focused on the boundary between 

 the two minerals, then the objective slowly lifted, a line of light 

 parallel to the line of contact will appear to move in the direc- 

 tion of the mineral with the higher index of refraction, or on the 

 side of quartz; the difference in this case on the average is .02. With 

 careful work and experience a difference of .001 may be detected by 

 this method. 



Study of minerals in rock sections. In the study of a mineral 

 section or of minerals in rock sections, an order of observation 

 should always be followed. The section should be carefully 

 studied in all parts with the low power adjective in order to de- 

 termine the relative abundance of each mineral species, their rela- 

 tions and relative size, and favorable sections should be chosen for the 

 optical observations, then the following order should be followed : 



1 . Color. When the mineral is opaque, it is observed in reflected 

 light, with the mirror under the stage turned off. Minerals which 

 are opaque in thin sections are of metallic luster, as magnetite, 

 pyrite, pyrrhotite, or chromite. Transparent sections are observed 

 in transmitted light and the color noted ; whether evenly colored or 

 irregular, caused by a difference in chemical composition or due to 

 inclusions, cavities, etc. 



2. Form. The outline of each individual species is noted if 

 bounded by straight lines, i.e. crystal faces well developed, the 

 individuals being euhedral or idiomorphic, in contrast to those 

 irregular in outline, which have no well-defined crystal faces 

 developed and are anhedral or allotriomorphic ; such irregular in- 

 dividuals usually act as a matrix, filling the cavities between the 

 minerals of earlier crystallization, which are often of larger size, 

 with distinct outlines or phenocrysts. 



