A TABLE OF INDEX OF REFRACTION AND BIREFRIN- 

 GENCE OF ROCK-MAKING MINERALS 



W. O. HOTCHKISS 



With regard to determining minerals in thin sections of rocks, 

 index of refraction and birefringence are two of the most useful 

 characteristics. These constants vary through wide ranges and 

 comparatively few minerals possess the same or even nearly the same 

 index. Consequently if the index can be determined even approxi- 

 mately the student has a most valuable starting-point to aid in his 

 diagnosis of an unknown mineral. 



The methods of determining the index described in Iddings' Rock 

 Minerals furnish very delicate means of comparing the index of an 

 unknown with that of a known mineral and sometimes will serve to 

 determine the index closely enough for purposes of diagnosis, but it 

 often occurs that the student would be able to determine a mineral 

 much more readily if he had any means of actually measuring 

 the index in a rough fashion. While instructing classes in petrology 

 in the University of Wisconsin the writer spent some odd hours in 

 trying to devise such a means, but did not succeed in finding any- 

 thing simple enough for ordinary use. In connection with this work 

 it was found somewhat difficult to get students to appreciate 

 difference in index and make use of it in their studies, the tempta- 

 tion to identify a mineral simply by comparison — without a careful 

 study of its constants — being too strong to resist. In the effort to 

 overcome this tendency the accompanying table and diagram were 

 constructed, and as they proved very satisfactory they are given here 

 in the hope that others will find them as useful. 



The diagram, as will be recognized, is simply an extension of Becke's 

 diagram for the feldspars to include all minerals whose constants are 

 known. The index is indicated by a short vertical line and the birefrin- 

 gence by a fime horizontal line. By giving all three indices for the 

 optically biaxial minerals both the greatest and least possible biref rin- 



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